Make the API robust against using invalid handles. Fixes #105.

Introduces a "validationCounter" field for all handle types that gets incremented (at least) whenever an OS file descriptor/handle gets invalidated or re-allocated. This way, an old eventcore handle to a reused OS handle can always be distinguished from the current one to avoid interference.

source/eventcore/driver.d

@@ -114,7 +114,23 @@ interface EventDriverCore {
 
 	/** Executes a callback in the thread owning the driver.
 	*/
-	void runInOwnerThread(ThreadCallback del, intptr_t param) shared;
+	void runInOwnerThread(ThreadCallback2 fun, intptr_t param1, intptr_t param2) shared;
+	/// ditto
+	final void runInOwnerThread(ThreadCallback1 fun, intptr_t param1)
+	shared {
+		runInOwnerThread((p1, p2) {
+			auto f = () @trusted { return cast(ThreadCallback1)p2; } ();
+			f(p1);
+		}, param1, cast(intptr_t)fun);
+	}
+	/// ditto
+	final void runInOwnerThread(ThreadCallback0 fun)
+	shared {
+		runInOwnerThread((p1, p2) {
+			auto f = () @trusted { return cast(ThreadCallback0)p2; } ();
+			f();
+		}, 0, cast(intptr_t)fun);
+	}
 
 	/// Low-level user data access. Use `getUserData` instead.
 	protected void* rawUserData(StreamSocketFD descriptor, size_t size, DataInitializer initialize, DataInitializer destroy) @system;
@@ -341,6 +357,18 @@ interface EventDriverSockets {
 	/// Cancels an ongoing wait for an outgoing datagram.
 	void cancelSend(DatagramSocketFD socket);
 
+	/** Determines whether the given socket handle is valid.
+
+		A handle that is invalid will result in no operations being carried out
+		when used. In particular `addRef`/`releaseRef` will have no effect, but
+		can safely be called and I/O operations will result in
+		`IOStatus.invalidHandle`.
+
+		A valid handle gets invalid when either the reference count drops to
+		zero, or after the socket was explicitly closed.
+	*/
+	bool isValid(SocketFD handle) const @nogc;
+
 	/** Increments the reference count of the given socket.
 	*/
 	void addRef(SocketFD descriptor);
@@ -352,6 +380,8 @@ interface EventDriverSockets {
 
 		Returns:
 			Returns `false` $(I iff) the last reference was removed by this call.
+
+			Passing an invalid handle will result in a return value of `true`.
 	*/
 	bool releaseRef(SocketFD descriptor);
 
@@ -411,6 +441,13 @@ interface EventDriverDNS {
 
 	/// Cancels an ongoing DNS lookup.
 	void cancelLookup(DNSLookupID handle);
+
+	/** Determines whether the given DNS lookup handle is valid.
+
+		A valid handle gets invalid when the lookup has completed or got
+		cancelled.
+	*/
+	bool isValid(DNSLookupID handle) const @nogc;
 }
 
 
@@ -447,6 +484,18 @@ interface EventDriverFiles {
 	void cancelWrite(FileFD file);
 	void cancelRead(FileFD file);
 
+	/** Determines whether the given file handle is valid.
+
+		A handle that is invalid will result in no operations being carried out
+		when used. In particular `addRef`/`releaseRef` will have no effect, but
+		can safely be called and I/O operations will result in
+		`IOStatus.invalidHandle`.
+
+		A valid handle gets invalid when either the reference count drops to
+		zero, or after the file was explicitly closed.
+	*/
+	bool isValid(FileFD handle) const @nogc;
+
 	/** Increments the reference count of the given file.
 	*/
 	void addRef(FileFD descriptor);
@@ -458,6 +507,8 @@ interface EventDriverFiles {
 
 		Returns:
 			Returns `false` $(I iff) the last reference was removed by this call.
+
+			Passing an invalid handle will result in a return value of `true`.
 	*/
 	bool releaseRef(FileFD descriptor);
 
@@ -505,6 +556,16 @@ interface EventDriverEvents {
 	/// Cancels an ongoing wait operation.
 	void cancelWait(EventID event, EventCallback on_event);
 
+	/** Determines whether the given event handle is valid.
+
+		A handle that is invalid will result in no operations being carried out
+		when used. In particular `addRef`/`releaseRef` will have no effect, but
+		can safely be called.
+
+		A valid handle gets invalid when the reference count drops to zero.
+	*/
+	bool isValid(EventID handle) const @nogc;
+
 	/** Increments the reference count of the given event.
 	*/
 	void addRef(EventID descriptor);
@@ -516,6 +577,8 @@ interface EventDriverEvents {
 
 		Returns:
 			Returns `false` $(I iff) the last reference was removed by this call.
+
+			Passing an invalid handle will result in a return value of `true`.
 	*/
 	bool releaseRef(EventID descriptor);
 
@@ -558,6 +621,16 @@ interface EventDriverSignals {
 	*/
 	SignalListenID listen(int sig, SignalCallback on_signal);
 
+	/** Determines whether the given signal handle is valid.
+
+		A handle that is invalid will result in no operations being carried out
+		when used. In particular `addRef`/`releaseRef` will have no effect, but
+		can safely be called.
+
+		A valid handle gets invalid when the reference count drops to zero.
+	*/
+	bool isValid(SignalListenID handle) const @nogc;
+
 	/** Increments the reference count of the given resource.
 	*/
 	void addRef(SignalListenID descriptor);
@@ -569,6 +642,8 @@ interface EventDriverSignals {
 
 		Returns:
 			Returns `false` $(I iff) the last reference was removed by this call.
+
+			Passing an invalid handle will result in a return value of `true`.
 	*/
 	bool releaseRef(SignalListenID descriptor);
 }
@@ -588,6 +663,16 @@ interface EventDriverTimers {
 	void wait(TimerID timer, TimerCallback2 callback);
 	void cancelWait(TimerID timer);
 
+	/** Determines whether the given timer handle is valid.
+
+		A handle that is invalid will result in no operations being carried out
+		when used. In particular `addRef`/`releaseRef` will have no effect, but
+		can safely be called.
+
+		A valid handle gets invalid when the reference count drops to zero.
+	*/
+	bool isValid(TimerID handle) const @nogc;
+
 	/** Increments the reference count of the given resource.
 	*/
 	void addRef(TimerID descriptor);
@@ -599,6 +684,8 @@ interface EventDriverTimers {
 
 		Returns:
 			Returns `false` $(I iff) the last reference was removed by this call.
+
+			Passing an invalid handle will result in a return value of `true`.
 	*/
 	bool releaseRef(TimerID descriptor);
 
@@ -624,6 +711,16 @@ interface EventDriverWatchers {
 	/// Watches a directory or a directory sub tree for changes.
 	WatcherID watchDirectory(string path, bool recursive, FileChangesCallback callback);
 
+	/** Determines whether the given watcher handle is valid.
+
+		A handle that is invalid will result in no operations being carried out
+		when used. In particular `addRef`/`releaseRef` will have no effect, but
+		can safely be called.
+
+		A valid handle gets invalid when the reference count drops to zero.
+	*/
+	bool isValid(WatcherID handle) const @nogc;
+
 	/** Increments the reference count of the given resource.
 	*/
 	void addRef(WatcherID descriptor);
@@ -635,6 +732,8 @@ interface EventDriverWatchers {
 
 		Returns:
 			Returns `false` $(I iff) the last reference was removed by this call.
+
+			Passing an invalid handle will result in a return value of `true`.
 	*/
 	bool releaseRef(WatcherID descriptor);
 
@@ -688,7 +787,9 @@ interface EventDriverProcesses {
 	*/
 	void kill(ProcessID pid, int signal);
 
-	/** Wait for the process to exit. Returns an identifier that can be used to cancel the wait.
+	/** Wait for the process to exit.
+
+		Returns an identifier that can be used to cancel the wait.
 	*/
 	size_t wait(ProcessID pid, ProcessWaitCallback on_process_exit);
 
@@ -696,6 +797,16 @@ interface EventDriverProcesses {
 	*/
 	void cancelWait(ProcessID pid, size_t waitId);
 
+	/** Determines whether the given process handle is valid.
+
+		A handle that is invalid will result in no operations being carried out
+		when used. In particular `addRef`/`releaseRef` will have no effect, but
+		can safely be called.
+
+		A valid handle gets invalid when the reference count drops to zero.
+	*/
+	bool isValid(ProcessID handle) const @nogc;
+
 	/** Increments the reference count of the given resource.
 	*/
 	void addRef(ProcessID pid);
@@ -708,6 +819,8 @@ interface EventDriverProcesses {
 
 		Returns:
 			Returns `false` $(I iff) the last reference was removed by this call.
+
+			Passing an invalid handle will result in a return value of `true`.
 	*/
 	bool releaseRef(ProcessID pid);
 
@@ -772,6 +885,18 @@ interface EventDriverPipes {
 	*/
 	void close(PipeFD pipe);
 
+	/** Determines whether the given pipe handle is valid.
+
+		A handle that is invalid will result in no operations being carried out
+		when used. In particular `addRef`/`releaseRef` will have no effect, but
+		can safely be called and I/O operations will result in
+		`IOStatus.invalidHandle`.
+
+		A valid handle gets invalid when either the reference count drops to
+		zero, or the pipe is explicitly closed.
+	*/
+	bool isValid(PipeFD handle) const @nogc;
+
 	/** Increments the reference count of the given resource.
 	*/
 	void addRef(PipeFD pid);
@@ -783,6 +908,8 @@ interface EventDriverPipes {
 
 		Returns:
 			Returns `false` $(I iff) the last reference was removed by this call.
+
+			Passing an invalid handle will result in a return value of `true`.
 	*/
 	bool releaseRef(PipeFD pid);
 
@@ -919,7 +1046,8 @@ enum IOStatus {
 	ok,           /// The data has been transferred normally
 	disconnected, /// The connection was closed before all data could be transterred
 	error,        /// An error occured while transferring the data
-	wouldBlock    /// Returned for `IOMode.immediate` when no data is readily readable/writable
+	wouldBlock,    /// Returned for `IOMode.immediate` when no data is readily readable/writable
+	invalidHandle, /// The passed handle is not valid
 }
 
 enum DNSStatus {
@@ -986,9 +1114,6 @@ struct Process {
 }
 
 mixin template Handle(string NAME, T, T invalid_value = T.init) {
-	static if (is(T.BaseType)) alias BaseType = T.BaseType;
-	else alias BaseType = T;
-
 	alias name = NAME;
 
 	enum invalid = typeof(this).init;
@@ -997,23 +1122,45 @@ mixin template Handle(string NAME, T, T invalid_value = T.init) {
 
 	T value = invalid_value;
 
-	this(BaseType value) { this.value = T(value); }
+	static if (is(T.BaseType)) {
+		alias BaseType = T.BaseType;
 
-	U opCast(U : Handle!(V, M), V, int M)()
-	const {
-		// TODO: verify that U derives from typeof(this)!
-		return U(value);
+		this(BaseType value, uint validation_counter)
+		{
+			this.value = T(value, validation_counter);
+		}
+	} else {
+		alias BaseType = T;
+
+		uint validationCounter;
+
+		this(BaseType value, uint validation_counter)
+		{
+			this.value = value;
+			this.validationCounter = validation_counter;
+		}
 	}
 
-	U opCast(U : BaseType)()
-	const {
+	U opCast(U)() const
+		if (is(U.BaseType) && is(typeof(U.value)))
+	{
+		// TODO: verify that U derives from typeof(this)!
+		return U(cast(U.BaseType)value, validationCounter);
+	}
+
+	U opCast(U)() const
+		if (is(typeof(U(BaseType.init))))
+	{
 		return cast(U)value;
 	}
 
 	alias value this;
 }
 
-alias ThreadCallback = void function(intptr_t param) @safe nothrow;
+alias ThreadCallback0 = void function() @safe nothrow;
+alias ThreadCallback1 = void function(intptr_t param1) @safe nothrow;
+alias ThreadCallback2 = void function(intptr_t param1, intptr_t param2) @safe nothrow;
+alias ThreadCallback = ThreadCallback1;
 
 struct FD { mixin Handle!("fd", size_t, size_t.max); }
 struct SocketFD { mixin Handle!("socket", FD); }

source/eventcore/drivers/posix/dns.d

@@ -35,6 +35,7 @@ final class EventDriverDNS_GAI(Events : EventDriverEvents, Signals : EventDriver
 		static struct Lookup {
 			shared(bool) done;
 			DNSLookupCallback callback;
+			uint validationCounter;
 			addrinfo* result;
 			int retcode;
 			string name;
@@ -44,6 +45,7 @@ final class EventDriverDNS_GAI(Events : EventDriverEvents, Signals : EventDriver
 		Events m_events;
 		EventID m_event = EventID.invalid;
 		size_t m_maxHandle;
+		uint m_validationCounter;
 	}
 
 	this(Events events, Signals signals)
@@ -64,7 +66,7 @@ final class EventDriverDNS_GAI(Events : EventDriverEvents, Signals : EventDriver
 	override DNSLookupID lookupHost(string name, DNSLookupCallback on_lookup_finished)
 	{
 		debug (EventCoreLogDNS) print("lookup %s", name);
-		auto handle = getFreeHandle();
+		auto handle = allocateHandle();
 		if (handle > m_maxHandle) m_maxHandle = handle;
 
 		assert(on_lookup_finished !is null, "Null callback passed to lookupHost");
@@ -123,15 +125,22 @@ final class EventDriverDNS_GAI(Events : EventDriverEvents, Signals : EventDriver
 
 		debug (EventCoreLogDNS) print("lookup handle: %s", handle);
 		m_events.loop.m_waiterCount++;
-		return handle;
+		return DNSLookupID(handle, l.validationCounter);
 	}
 
 	override void cancelLookup(DNSLookupID handle)
 	{
+		if (!isValid(handle)) return;
 		m_lookups[handle].callback = null;
 		m_events.loop.m_waiterCount--;
 	}
 
+	override bool isValid(DNSLookupID handle)
+	const {
+		if (handle.value >= m_lookups.length) return false;
+		return m_lookups[handle.value].validationCounter == handle.validationCounter;
+	}
+
 	private void onDNSSignal(EventID event)
 		@trusted nothrow
 	{
@@ -171,20 +180,26 @@ final class EventDriverDNS_GAI(Events : EventDriverEvents, Signals : EventDriver
 					l.done = false;
 					if (i == m_maxHandle) m_maxHandle = lastmax;
 					m_events.loop.m_waiterCount--;
-					passToDNSCallback(cast(DNSLookupID)cast(int)i, cb, status, ai);
+					passToDNSCallback(DNSLookupID(i, l.validationCounter), cb, status, ai);
 				} else lastmax = i;
 			}
 		}
 		debug (EventCoreLogDNS) print("Max active DNS handle: %s", m_maxHandle);
 	}
 
-	private DNSLookupID getFreeHandle()
+	private DNSLookupID allocateHandle()
 	@safe nothrow {
 		assert(m_lookups.length <= int.max);
+		int id = cast(int)m_lookups.length;
 		foreach (i, ref l; m_lookups)
-			if (!l.callback)
-				return cast(DNSLookupID)cast(int)i;
-		return cast(DNSLookupID)cast(int)m_lookups.length;
+			if (!l.callback) {
+				id = cast(int)i;
+				break;
+			}
+
+		auto vc = ++m_validationCounter;
+		m_lookups[id].validationCounter = vc;
+		return DNSLookupID(cast(int)id, vc);
 	}
 
 	private void setupEvent()
@@ -204,12 +219,14 @@ final class EventDriverDNS_GAIA(Events : EventDriverEvents, Signals : EventDrive
 	private {
 		static struct Lookup {
 			gaicb ctx;
+			uint validationCounter;
 			DNSLookupCallback callback;
 		}
 		ChoppedVector!Lookup m_lookups;
 		Events m_events;
 		Signals m_signals;
 		int m_dnsSignal;
+		uint m_validationCounter;
 		SignalListenID m_sighandle;
 	}
 
@@ -232,7 +249,7 @@ final class EventDriverDNS_GAIA(Events : EventDriverEvents, Signals : EventDrive
 	{
 		import std.string : toStringz;
 
-		auto handle = getFreeHandle();
+		auto handle = allocateHandle();
 
 		sigevent evt;
 		evt.sigev_notify = SIGEV_SIGNAL;
@@ -260,6 +277,13 @@ final class EventDriverDNS_GAIA(Events : EventDriverEvents, Signals : EventDrive
 		m_events.loop.m_waiterCount--;
 	}
 
+	override bool isValid(DNSLookupID handle)
+	{
+		if (handle.value >= m_lookups.length)
+			return false;
+		return m_lookups[handle.value].validationCounter == handle.validationCounter;
+	}
+
 	private void onDNSSignal(SignalListenID, SignalStatus status, int signal)
 		@safe nothrow
 	{
@@ -284,11 +308,14 @@ final class EventDriverDNS_GAIA(Events : EventDriverEvents, Signals : EventDrive
 		}
 	}
 
-	private DNSLookupID getFreeHandle()
+	private DNSLookupID allocateHandle()
 	{
 		foreach (i, ref l; m_lookups)
-			if (!l.callback)
+			if (!l.callback) {
+				m_lookups[i].validationCounter = ++m_validationCounter;
 				return cast(DNSLookupID)cast(int)i;
+			}
+		m_lookups[m_lookups.length].validationCounter = ++m_validationCounter;
 		return cast(DNSLookupID)cast(int)m_lookups.length;
 	}
 }
@@ -344,7 +371,7 @@ final class EventDriverDNS_GHBN(Events : EventDriverEvents, Signals : EventDrive
 	{
 		import std.string : toStringz;
 
-		auto handle = DNSLookupID(m_maxHandle++);
+		auto handle = DNSLookupID(m_maxHandle++, 0);
 
 		auto he = () @trusted { return gethostbyname(name.toStringz); } ();
 		if (he is null) {
@@ -377,6 +404,11 @@ final class EventDriverDNS_GHBN(Events : EventDriverEvents, Signals : EventDrive
 	}
 
 	override void cancelLookup(DNSLookupID) {}
+
+	override bool isValid(DNSLookupID)
+	const {
+		return true;
+	}
 }
 
 package struct DNSSlot {

source/eventcore/drivers/posix/driver.d

@@ -160,6 +160,8 @@ final class PosixEventDriverCore(Loop : PosixEventLoop, Timers : EventDriverTime
 
 	protected alias ExtraEventsCallback = bool delegate(long);
 
+	private alias ThreadCallbackEntry = Tuple!(ThreadCallback2, intptr_t, intptr_t);
+
 	private {
 		Loop m_loop;
 		Timers m_timers;
@@ -169,7 +171,7 @@ final class PosixEventDriverCore(Loop : PosixEventLoop, Timers : EventDriverTime
 		EventID m_wakeupEvent;
 
 		shared Mutex m_threadCallbackMutex;
-		ConsumableQueue!(Tuple!(ThreadCallback, intptr_t)) m_threadCallbacks;
+		ConsumableQueue!ThreadCallbackEntry m_threadCallbacks;
 	}
 
 	this(Loop loop, Timers timers, Events events, Processes processes)
@@ -186,7 +188,7 @@ final class PosixEventDriverCore(Loop : PosixEventLoop, Timers : EventDriverTime
 			() @trusted { m_threadCallbackMutex = cast(shared)mallocT!Mutex; } ();
 		}
 
-		m_threadCallbacks = mallocT!(ConsumableQueue!(Tuple!(ThreadCallback, intptr_t)));
+		m_threadCallbacks = mallocT!(ConsumableQueue!ThreadCallbackEntry);
 		m_threadCallbacks.reserve(1000);
 	}
 
@@ -260,7 +262,7 @@ final class PosixEventDriverCore(Loop : PosixEventLoop, Timers : EventDriverTime
 		m_exit = false;
 	}
 
-	final override void runInOwnerThread(ThreadCallback del, intptr_t param)
+	final override void runInOwnerThread(ThreadCallback2 del, intptr_t param1, intptr_t param2)
 	shared {
 		auto m = atomicLoad(m_threadCallbackMutex);
 		auto evt = atomicLoad(m_wakeupEvent);
@@ -273,12 +275,14 @@ final class PosixEventDriverCore(Loop : PosixEventLoop, Timers : EventDriverTime
 		try {
 			synchronized (m)
 				() @trusted { return (cast()this).m_threadCallbacks; } ()
-					.put(tuple(del, param));
+					.put(ThreadCallbackEntry(del, param1, param2));
 		} catch (Exception e) assert(false, e.msg);
 
 		m_events.trigger(m_wakeupEvent, false);
 	}
 
+	alias runInOwnerThread = EventDriverCore.runInOwnerThread;
+
 
 	final protected override void* rawUserData(StreamSocketFD descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
@@ -300,14 +304,14 @@ final class PosixEventDriverCore(Loop : PosixEventLoop, Timers : EventDriverTime
 		import std.stdint : intptr_t;
 
 		while (true) {
-			Tuple!(ThreadCallback, intptr_t) del;
+			ThreadCallbackEntry del;
 			try {
 				synchronized (m_threadCallbackMutex) {
 					if (m_threadCallbacks.empty) break;
 					del = m_threadCallbacks.consumeOne;
 				}
 			} catch (Exception e) assert(false, e.msg);
-			del[0](del[1]);
+			del[0](del[1], del[2]);
 		}
 	}
 }
@@ -336,11 +340,13 @@ package class PosixEventLoop {
 	/// Updates the event mask to use for listening for notifications.
 	protected abstract void updateFD(FD fd, EventMask old_mask, EventMask new_mask, bool edge_triggered = true) @nogc;
 
-	final protected void notify(EventType evt)(FD fd)
+	final protected void notify(EventType evt)(size_t fd)
 	{
 		//assert(m_fds[fd].callback[evt] !is null, "Notifying FD which is not listening for event.");
-		if (m_fds[fd.value].common.callback[evt])
-			m_fds[fd.value].common.callback[evt](fd);
+		if (m_fds[fd].common.callback[evt]) {
+			auto vc = m_fds[fd].common.validationCounter;
+			m_fds[fd].common.callback[evt](FD(fd, vc));
+		}
 	}
 
 	final protected void enumerateFDs(EventType evt)(scope FDEnumerateCallback del)
@@ -348,7 +354,7 @@ package class PosixEventLoop {
 		// TODO: optimize!
 		foreach (i; 0 .. cast(int)m_fds.length)
 			if (m_fds[i].common.callback[evt])
-				del(cast(FD)i);
+				del(FD(i, m_fds[i].common.validationCounter));
 	}
 
 	package void setNotifyCallback(EventType evt)(FD fd, FDSlotCallback callback)
@@ -370,17 +376,23 @@ package class PosixEventLoop {
 		}
 	}
 
-	package void initFD(T)(FD fd, FDFlags flags, auto ref T slot_init)
+	package FDType initFD(FDType, T)(size_t fd, FDFlags flags, auto ref T slot_init)
 	{
-		with (m_fds[fd.value]) {
+		uint vc;
+
+		with (m_fds[fd]) {
 			assert(common.refCount == 0, "Initializing referenced file descriptor slot.");
 			assert(specific.kind == typeof(specific).Kind.none, "Initializing slot that has not been cleared.");
 			common.refCount = 1;
 			common.flags = flags;
 			specific = slot_init;
+			vc = common.validationCounter;
 		}
+
 		if (!(flags & FDFlags.internal))
 			m_handleCount++;
+
+		return FDType(fd, vc);
 	}
 
 	package void clearFD(T)(FD fd)
@@ -388,6 +400,7 @@ package class PosixEventLoop {
 		import taggedalgebraic : hasType;
 
 		auto slot = () @trusted { return &m_fds[fd.value]; } ();
+		assert(slot.common.validationCounter == fd.validationCounter, "Clearing FD slot for invalid FD");
 		assert(slot.common.refCount == 0, "Clearing referenced file descriptor slot.");
 		assert(slot.specific.hasType!T, "Clearing file descriptor slot with unmatched type.");
 
@@ -400,7 +413,10 @@ package class PosixEventLoop {
 			foreach (cb; slot.common.callback)
 				if (cb !is null)
 					m_waiterCount--;
+
+		auto vc = slot.common.validationCounter;
 		*slot = m_fds.FullField.init;
+		slot.common.validationCounter = vc + 1;
 	}
 
 	package final void* rawUserDataImpl(size_t descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
@@ -426,6 +442,7 @@ alias FDSlotCallback = void delegate(FD);
 private struct FDSlot {
 	FDSlotCallback[EventType.max+1] callback;
 	uint refCount;
+	uint validationCounter;
 	FDFlags flags;
 
 	DataInitializer userDataDestructor;

source/eventcore/drivers/posix/epoll.d

@@ -51,7 +51,7 @@ final class EpollEventLoop : PosixEventLoop {
 		if (ret > 0) {
 			foreach (ref evt; m_events[0 .. ret]) {
 				debug (EventCoreEpollDebug) print("Epoll event on %s: %s", evt.data.fd, evt.events);
-				auto fd = cast(FD)evt.data.fd;
+				auto fd = cast(size_t)evt.data.fd;
 				if (evt.events & (EPOLLERR|EPOLLHUP|EPOLLRDHUP)) notify!(EventType.status)(fd);
 				if (evt.events & EPOLLIN) notify!(EventType.read)(fd);
 				if (evt.events & EPOLLOUT) notify!(EventType.write)(fd);

source/eventcore/drivers/posix/events.d

@@ -45,9 +45,8 @@ final class PosixEventDriverEvents(Loop : PosixEventLoop, Sockets : EventDriverS
 		version (linux) {
 			auto eid = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
 			if (eid == -1) return EventID.invalid;
-			auto id = cast(EventID)eid;
 			// FIXME: avoid dynamic memory allocation for the queue
-			m_loop.initFD(id, FDFlags.internal,
+			auto id = m_loop.initFD!EventID(eid, FDFlags.internal,
 				EventSlot(mallocT!(ConsumableQueue!EventCallback), is_internal));
 			m_loop.registerFD(id, EventMask.read);
 			m_loop.setNotifyCallback!(EventType.read)(id, &onEvent);
@@ -101,19 +100,20 @@ final class PosixEventDriverEvents(Loop : PosixEventLoop, Sockets : EventDriverS
 
 			// use the second socket as the event ID and as the sending end for
 			// other threads
-			auto id = cast(EventID)fd[1];
-			try m_sockets.userData!EventID(s) = id;
-			catch (Exception e) assert(false, e.msg);
 			// FIXME: avoid dynamic memory allocation for the queue
-			m_loop.initFD(id, FDFlags.internal,
+			auto id = m_loop.initFD!EventID(fd[1], FDFlags.internal,
 				EventSlot(mallocT!(ConsumableQueue!EventCallback), is_internal, s));
 			assert(getRC(id) == 1);
+			try m_sockets.userData!EventID(s) = id;
+			catch (Exception e) assert(false, e.msg);
 			return id;
 		}
 	}
 
 	final override void trigger(EventID event, bool notify_all)
 	{
+		if (!isValid(event)) return;
+
 		auto slot = getSlot(event);
 		if (notify_all) {
 			//log("emitting only for this thread (%s waiters)", m_fds[event].waiters.length);
@@ -141,6 +141,8 @@ final class PosixEventDriverEvents(Loop : PosixEventLoop, Sockets : EventDriverS
 
 	final override void wait(EventID event, EventCallback on_event)
 	@nogc {
+		if (!isValid(event)) return;
+
 		if (!isInternal(event)) m_loop.m_waiterCount++;
 		getSlot(event).waiters.put(on_event);
 	}
@@ -150,6 +152,8 @@ final class PosixEventDriverEvents(Loop : PosixEventLoop, Sockets : EventDriverS
 		import std.algorithm.searching : countUntil;
 		import std.algorithm.mutation : remove;
 
+		if (!isValid(event)) return;
+
 		if (!isInternal(event)) m_loop.m_waiterCount--;
 		getSlot(event).waiters.removePending(on_event);
 	}
@@ -176,14 +180,24 @@ final class PosixEventDriverEvents(Loop : PosixEventLoop, Sockets : EventDriverS
 		}
 	}
 
+	override bool isValid(EventID handle)
+	const {
+		if (handle.value >= m_loop.m_fds.length) return false;
+		return m_loop.m_fds[handle.value].common.validationCounter == handle.validationCounter;
+	}
+
 	final override void addRef(EventID descriptor)
 	{
+		if (!isValid(descriptor)) return;
+
 		assert(getRC(descriptor) > 0, "Adding reference to unreferenced event FD.");
 		getRC(descriptor)++;
 	}
 
 	final override bool releaseRef(EventID descriptor)
 	@nogc {
+		if (!isValid(descriptor)) return true;
+
 		nogc_assert(getRC(descriptor) > 0, "Releasing reference to unreferenced event FD.");
 		if (--getRC(descriptor) == 0) {
 			if (!isInternal(descriptor))
@@ -209,6 +223,7 @@ final class PosixEventDriverEvents(Loop : PosixEventLoop, Sockets : EventDriverS
 
 	final protected override void* rawUserData(EventID descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
+		if (!isValid(descriptor)) return null;
 		return m_loop.rawUserDataImpl(descriptor, size, initialize, destroy);
 	}
 

source/eventcore/drivers/posix/kqueue.d

@@ -66,7 +66,7 @@ final class KqueueEventLoop : PosixEventLoop {
 			foreach (ref evt; m_events[0 .. ret]) {
 				//print("event %s %s", evt.ident, evt.filter, evt.flags);
 				assert(evt.ident <= uint.max);
-				auto fd = cast(FD)cast(int)evt.ident;
+				auto fd = cast(size_t)evt.ident;
 				if (evt.flags & (EV_EOF|EV_ERROR))
 					notify!(EventType.status)(fd);
 				switch (evt.filter) {

source/eventcore/drivers/posix/pipes.d

@@ -24,20 +24,24 @@ final class PosixEventDriverPipes(Loop : PosixEventLoop) : EventDriverPipes {
 
 	final override PipeFD adopt(int system_fd)
 	{
-        auto fd = PipeFD(system_fd);
-        if (m_loop.m_fds[fd].common.refCount) // FD already in use?
+		if (m_loop.m_fds[system_fd].common.refCount) // FD already in use?
 			return PipeFD.invalid;
 
         // Suprisingly cannot use O_CLOEXEC here, so use FD_CLOEXEC instead.
         () @trusted { fcntl(system_fd, F_SETFL, fcntl(system_fd, F_GETFL) | O_NONBLOCK | FD_CLOEXEC); } ();
 
-        m_loop.initFD(fd, FDFlags.none, PipeSlot.init);
+		auto fd = m_loop.initFD!PipeFD(system_fd, FDFlags.none, PipeSlot.init);
 		m_loop.registerFD(fd, EventMask.read|EventMask.write|EventMask.status);
 		return fd;
 	}
 
 	final override void read(PipeFD pipe, ubyte[] buffer, IOMode mode, PipeIOCallback on_read_finish)
 	{
+		if (!isValid(pipe)) {
+			on_read_finish(pipe, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		auto ret = () @trusted { return read(cast(int)pipe, buffer.ptr, min(buffer.length, int.max)); } ();
 
 		// Read failed
@@ -137,6 +141,8 @@ final class PosixEventDriverPipes(Loop : PosixEventLoop) : EventDriverPipes {
 
 	final override void cancelRead(PipeFD pipe)
 	{
+		if (!isValid(pipe)) return;
+
 		auto slot = () @trusted { return &m_loop.m_fds[pipe].pipe(); } ();
 		assert(slot.readCallback !is null, "Cancelling read when there is no read in progress.");
 		m_loop.setNotifyCallback!(EventType.read)(pipe, null);
@@ -145,6 +151,11 @@ final class PosixEventDriverPipes(Loop : PosixEventLoop) : EventDriverPipes {
 
 	final override void write(PipeFD pipe, const(ubyte)[] buffer, IOMode mode, PipeIOCallback on_write_finish)
 	{
+		if (!isValid(pipe)) {
+			on_write_finish(pipe, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		if (buffer.length == 0) {
 			on_write_finish(pipe, IOStatus.ok, 0);
 			return;
@@ -219,6 +230,8 @@ final class PosixEventDriverPipes(Loop : PosixEventLoop) : EventDriverPipes {
 
 	final override void cancelWrite(PipeFD pipe)
 	{
+		if (!isValid(pipe)) return;
+
 		auto slot = () @trusted { return &m_loop.m_fds[pipe].pipe(); } ();
 		assert(slot.writeCallback !is null, "Cancelling write when there is no write in progress.");
 		m_loop.setNotifyCallback!(EventType.write)(pipe, null);
@@ -228,6 +241,11 @@ final class PosixEventDriverPipes(Loop : PosixEventLoop) : EventDriverPipes {
 
 	final override void waitForData(PipeFD pipe, PipeIOCallback on_data_available)
 	{
+		if (!isValid(pipe)) {
+			on_data_available(pipe, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		if (pollPipe(pipe, on_data_available))
 		{
 			return;
@@ -288,12 +306,22 @@ final class PosixEventDriverPipes(Loop : PosixEventLoop) : EventDriverPipes {
 
 	final override void close(PipeFD pipe)
 	{
+		if (!isValid(pipe)) return;
+
 		// TODO: Maybe actually close here instead of waiting for releaseRef?
 		close(cast(int)pipe);
 	}
 
+	override bool isValid(PipeFD handle)
+	const {
+		if (handle.value >= m_loop.m_fds.length) return false;
+		return m_loop.m_fds[handle.value].common.validationCounter == handle.validationCounter;
+	}
+
 	final override void addRef(PipeFD pipe)
 	{
+		if (!isValid(pipe)) return;
+
 		auto slot = () @trusted { return &m_loop.m_fds[pipe]; } ();
 		assert(slot.common.refCount > 0, "Adding reference to unreferenced pipe FD.");
 		slot.common.refCount++;
@@ -302,6 +330,9 @@ final class PosixEventDriverPipes(Loop : PosixEventLoop) : EventDriverPipes {
 	final override bool releaseRef(PipeFD pipe)
 	{
 		import taggedalgebraic : hasType;
+
+		if (!isValid(pipe)) return true;
+
 		auto slot = () @trusted { return &m_loop.m_fds[pipe]; } ();
 		nogc_assert(slot.common.refCount > 0, "Releasing reference to unreferenced pipe FD.");
 
@@ -319,6 +350,7 @@ final class PosixEventDriverPipes(Loop : PosixEventLoop) : EventDriverPipes {
 	@system {
 		return m_loop.rawUserDataImpl(fd, size, initialize, destroy);
 	}
+>>>>>>> 568465d... Make the API robust against using invalid handles. Fixes #105.
 }
 
 final class DummyEventDriverPipes(Loop : PosixEventLoop) : EventDriverPipes {
@@ -360,6 +392,11 @@ final class DummyEventDriverPipes(Loop : PosixEventLoop) : EventDriverPipes {
 		assert(false, "TODO!");
 	}
 
+	override bool isValid(PipeFD handle)
+	const {
+		return false;
+	}
+
 	override void addRef(PipeFD pid)
 	{
 		assert(false, "TODO!");

source/eventcore/drivers/posix/processes.d

@@ -22,12 +22,13 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 
 	private {
 		static shared Mutex s_mutex;
-		static __gshared ProcessInfo[ProcessID] s_processes;
+		static __gshared ProcessInfo[int] s_processes;
 		static __gshared Thread s_waitThread;
 
 		Loop m_loop;
 		// FIXME: avoid virtual funciton calls and use the final type instead
 		EventDriver m_driver;
+		uint m_validationCounter;
 	}
 
 	this(Loop loop, EventDriver driver)
@@ -42,13 +43,14 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 
 	final override ProcessID adopt(int system_pid)
 	{
-		auto pid = cast(ProcessID)system_pid;
-
 		ProcessInfo info;
 		info.exited = false;
 		info.refCount = 1;
+		info.validationCounter = ++m_validationCounter;
 		info.driver = this;
-		add(pid, info);
+
+		auto pid = ProcessID(system_pid, info.validationCounter);
+		add(system_pid, info);
 		return pid;
 	}
 
@@ -154,7 +156,7 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 	@trusted {
 		import core.sys.posix.signal : pkill = kill;
 
-		assert(cast(int)pid > 0, "Invalid PID passed to kill.");
+		if (!isValid(pid)) return;
 
 		if (cast(int)pid > 0)
 			pkill(cast(int)pid, signal);
@@ -167,7 +169,7 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 
 		size_t id = size_t.max;
 		lockedProcessInfo(pid, (info) {
-			assert(info !is null, "Unknown process ID");
+			if (!info) return;
 
 			if (info.exited) {
 				exited = true;
@@ -190,7 +192,8 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 		if (wait_id == size_t.max) return;
 
 		lockedProcessInfo(pid, (info) {
-			assert(info !is null, "Unknown process ID");
+			if (!info) return;
+
 			assert(!info.exited, "Cannot cancel wait when none are pending");
 			assert(info.callbacks.length > wait_id, "Invalid process wait ID");
 
@@ -205,18 +208,18 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 
 	private static void onLocalProcessExit(intptr_t system_pid)
 	{
-		auto pid = cast(ProcessID)system_pid;
-
 		int exitCode;
 		ProcessWaitCallback[] callbacks;
 
+		ProcessID pid;
+
 		PosixEventDriverProcesses driver;
-		lockedProcessInfo(pid, (info) {
+		lockedProcessInfoPlain(cast(int)system_pid, (info) {
 			assert(info !is null);
 
 			exitCode = info.exitCode;
-
 			callbacks = info.callbacks;
+			pid = ProcessID(cast(int)system_pid, info.validationCounter);
 			info.callbacks = null;
 
 			driver = info.driver;
@@ -234,15 +237,25 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 	{
 		bool ret;
 		lockedProcessInfo(pid, (info) {
-			assert(info !is null, "Unknown process ID");
-			ret = info.exited;
+			if (info) ret = info.exited;
+			else ret = true;
 		});
 		return ret;
 	}
 
+	override bool isValid(ProcessID handle)
+	const {
+		s_mutex.lock_nothrow();
+		scope (exit) s_mutex.unlock_nothrow();
+		auto info = () @trusted { return cast(int)handle.value in s_processes; } ();
+		return info && info.validationCounter == handle.validationCounter;
+	}
+
 	final override void addRef(ProcessID pid)
 	{
 		lockedProcessInfo(pid, (info) {
+			if (!info) return;
+
 			nogc_assert(info.refCount > 0, "Adding reference to unreferenced process FD.");
 			info.refCount++;
 		});
@@ -252,13 +265,18 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 	{
 		bool ret;
 		lockedProcessInfo(pid, (info) {
+			if (!info) {
+				ret = true;
+				return;
+			}
+
 			nogc_assert(info.refCount > 0, "Releasing reference to unreferenced process FD.");
 			if (--info.refCount == 0) {
 				// Remove/deallocate process
 				if (info.userDataDestructor)
 					() @trusted { info.userDataDestructor(info.userData.ptr); } ();
 
-				() @trusted { s_processes.remove(pid); } ();
+				() @trusted { s_processes.remove(cast(int)pid.value); } ();
 				ret = false;
 			} else ret = true;
 		});
@@ -290,7 +308,7 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 		s_mutex = new shared Mutex;
 	}
 
-	private static void lockedProcessInfo(ProcessID pid, scope void delegate(ProcessInfo*) nothrow @safe fn)
+	private static void lockedProcessInfoPlain(int pid, scope void delegate(ProcessInfo*) nothrow @safe fn)
 	{
 		s_mutex.lock_nothrow();
 		scope (exit) s_mutex.unlock_nothrow();
@@ -298,7 +316,14 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 		fn(info);
 	}
 
-	private static void add(ProcessID pid, ProcessInfo info) @trusted {
+	private static void lockedProcessInfo(ProcessID pid, scope void delegate(ProcessInfo*) nothrow @safe fn)
+	{
+		lockedProcessInfoPlain(cast(int)pid.value, (pi) {
+			fn(pi.validationCounter == pid.validationCounter ? pi : null);
+		});
+	}
+
+	private static void add(int pid, ProcessInfo info) @trusted {
 		s_mutex.lock_nothrow();
 		scope (exit) s_mutex.unlock_nothrow();
 
@@ -328,7 +353,7 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 				break;
 			}
 
-			ProcessID[] allprocs;
+			int[] allprocs;
 
 			{
 				s_mutex.lock_nothrow();
@@ -345,8 +370,8 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 
 			foreach (pid; allprocs) {
 				int status;
-				ret = () @trusted { return waitpid(cast(int)pid, &status, WNOHANG); } ();
-				if (ret == cast(int)pid) {
+				ret = () @trusted { return waitpid(pid, &status, WNOHANG); } ();
+				if (ret == pid) {
 					int exitstatus = WIFEXITED(status) ? WEXITSTATUS(status) : -WTERMSIG(status);
 					onProcessExitStatic(ret, exitstatus);
 				}
@@ -356,24 +381,21 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 
 	private static void onProcessExitStatic(int system_pid, int exit_status)
 	{
-		auto pid = cast(ProcessID)system_pid;
-
 		PosixEventDriverProcesses driver;
-		lockedProcessInfo(pid, (ProcessInfo* info) @safe {
+		lockedProcessInfoPlain(system_pid, (ProcessInfo* info) @safe {
 			// We get notified of any child exiting, so ignore the ones we're
 			// not aware of
 			if (info is null) return;
 
 			// Increment the ref count to make sure it doesn't get removed
 			info.refCount++;
-
 			info.exited = true;
 			info.exitCode = exit_status;
 			driver = info.driver;
 		});
 
 		if (driver)
-			() @trusted { return cast(shared)driver; } ().onProcessExit(cast(int)pid);
+			() @trusted { return cast(shared)driver; } ().onProcessExit(system_pid);
 	}
 
 	private static struct ProcessInfo {
@@ -381,6 +403,7 @@ final class PosixEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 		int exitCode;
 		ProcessWaitCallback[] callbacks;
 		size_t refCount = 0;
+		uint validationCounter;
 		PosixEventDriverProcesses driver;
 
 		DataInitializer userDataDestructor;
@@ -425,6 +448,11 @@ final class DummyEventDriverProcesses(Loop : PosixEventLoop) : EventDriverProces
 		assert(false, "TODO!");
 	}
 
+	override bool isValid(ProcessID handle)
+	const {
+		return false;
+	}
+
 	override void addRef(ProcessID pid)
 	{
 		assert(false, "TODO!");

source/eventcore/drivers/posix/signals.d

@@ -26,19 +26,19 @@ final class SignalFDEventDriverSignals(Loop : PosixEventLoop) : EventDriverSigna
 
 	package SignalListenID listenInternal(int sig, SignalCallback on_signal, bool is_internal = true)
 	{
-		auto fd = () @trusted {
+		auto sigfd = () @trusted {
 			sigset_t sset;
 			sigemptyset(&sset);
 			sigaddset(&sset, sig);
 
 			if (sigprocmask(SIG_BLOCK, &sset, null) != 0)
-				return SignalListenID.invalid;
+				return -1;
 
-			return SignalListenID(signalfd(-1, &sset, SFD_NONBLOCK | SFD_CLOEXEC));
+			return signalfd(-1, &sset, SFD_NONBLOCK | SFD_CLOEXEC);
 		} ();
 
 
-		m_loop.initFD(cast(FD)fd, is_internal ? FDFlags.internal : FDFlags.none, SignalSlot(on_signal));
+		auto fd = m_loop.initFD!SignalListenID(sigfd, is_internal ? FDFlags.internal : FDFlags.none, SignalSlot(on_signal));
 		m_loop.registerFD(cast(FD)fd, EventMask.read);
 		m_loop.setNotifyCallback!(EventType.read)(cast(FD)fd, &onSignal);
 
@@ -47,14 +47,24 @@ final class SignalFDEventDriverSignals(Loop : PosixEventLoop) : EventDriverSigna
 		return fd;
 	}
 
+	override bool isValid(SignalListenID handle)
+	const {
+		if (handle.value >= m_loop.m_fds.length) return false;
+		return m_loop.m_fds[handle.value].common.validationCounter == handle.validationCounter;
+	}
+
 	override void addRef(SignalListenID descriptor)
 	{
+		if (!isValid(descriptor)) return;
+
 		assert(m_loop.m_fds[descriptor].common.refCount > 0, "Adding reference to unreferenced event FD.");
 		m_loop.m_fds[descriptor].common.refCount++;
 	}
 
 	override bool releaseRef(SignalListenID descriptor)
 	{
+		if (!isValid(descriptor)) return true;
+
 		FD fd = cast(FD)descriptor;
 		nogc_assert(m_loop.m_fds[fd].common.refCount > 0, "Releasing reference to unreferenced event FD.");
 		if (--m_loop.m_fds[fd].common.refCount == 1) { // NOTE: 1 because setNotifyCallback adds a second reference
@@ -104,6 +114,11 @@ final class DummyEventDriverSignals(Loop : PosixEventLoop) : EventDriverSignals
 		assert(false);
 	}
 
+	override bool isValid(SignalListenID handle)
+	const {
+		return false;
+	}
+
 	override void addRef(SignalListenID descriptor)
 	{
 		assert(false);

source/eventcore/drivers/posix/sockets.d

@@ -135,21 +135,17 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 			return StreamSocketFD.invalid;
 		}
 
-		auto sock = cast(StreamSocketFD)sockfd;
-
-		void invalidateSocket() @nogc @trusted nothrow { closeSocket(sockfd); sock = StreamSocketFD.invalid; }
-
 		int bret;
 		if (bind_address !is null)
-			() @trusted { bret = bind(cast(sock_t)sock, bind_address.name, bind_address.nameLen); } ();
+			() @trusted { bret = bind(sockfd, bind_address.name, bind_address.nameLen); } ();
 
 		if (bret != 0) {
-			invalidateSocket();
-			on_connect(sock, ConnectStatus.bindFailure);
-			return sock;
+			closeSocket(sockfd);
+			on_connect(StreamSocketFD.invalid, ConnectStatus.bindFailure);
+			return StreamSocketFD.invalid;
 		}
 
-		m_loop.initFD(sock, FDFlags.none, StreamSocketSlot.init);
+		auto sock = m_loop.initFD!StreamSocketFD(sockfd, FDFlags.none, StreamSocketSlot.init);
 		m_loop.registerFD(sock, EventMask.read|EventMask.write);
 
 		auto ret = () @trusted { return connect(cast(sock_t)sock, address.name, address.nameLen); } ();
@@ -167,7 +163,7 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 			} else {
 				m_loop.unregisterFD(sock, EventMask.read|EventMask.write);
 				m_loop.clearFD!StreamSocketSlot(sock);
-				invalidateSocket();
+				closeSocket(sockfd);
 				on_connect(StreamSocketFD.invalid, determineConnectStatus(err));
 				return StreamSocketFD.invalid;
 			}
@@ -178,7 +174,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override void cancelConnectStream(StreamSocketFD sock)
 	{
-		assert(sock != StreamSocketFD.invalid, "Invalid socket descriptor");
+		if (!isValid(sock)) return;
+
 		with (m_loop.m_fds[sock].streamSocket)
 		{
 			assert(state == ConnectionState.connecting,
@@ -191,11 +188,10 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override StreamSocketFD adoptStream(int socket)
 	{
-		auto fd = StreamSocketFD(socket);
-		if (m_loop.m_fds[fd].common.refCount) // FD already in use?
+		if (m_loop.m_fds[socket].common.refCount) // FD already in use?
 			return StreamSocketFD.invalid;
-		setSocketNonBlocking(fd);
-		m_loop.initFD(fd, FDFlags.none, StreamSocketSlot.init);
+		setSocketNonBlocking(socket);
+		auto fd = m_loop.initFD!StreamSocketFD(socket, FDFlags.none, StreamSocketSlot.init);
 		m_loop.registerFD(fd, EventMask.read|EventMask.write);
 		return fd;
 	}
@@ -240,39 +236,35 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 		auto sockfd = createSocket(address.addressFamily, SOCK_STREAM);
 		if (sockfd == -1) return StreamListenSocketFD.invalid;
 
-		auto sock = cast(StreamListenSocketFD)sockfd;
-
-		void invalidateSocket() @nogc @trusted nothrow { closeSocket(sockfd); sock = StreamSocketFD.invalid; }
-
-		() @trusted {
+		auto succ = () @trusted {
 			int tmp_reuse = 1;
 			// FIXME: error handling!
 			if (options & StreamListenOptions.reuseAddress) {
-				if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &tmp_reuse, tmp_reuse.sizeof) != 0) {
-					invalidateSocket();
-					return;
+				if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &tmp_reuse, tmp_reuse.sizeof) != 0)
+					return false;
 			}
+
+			version (Windows) {}
+			else {
+				if ((options & StreamListenOptions.reusePort) && setsockopt(sockfd, SOL_SOCKET, SO_REUSEPORT, &tmp_reuse, tmp_reuse.sizeof) != 0)
+					return false;
 			}
-			version (Windows) {} else {
-				if ((options & StreamListenOptions.reusePort) && setsockopt(sockfd, SOL_SOCKET, SO_REUSEPORT, &tmp_reuse, tmp_reuse.sizeof) != 0) {
-					invalidateSocket();
-					return;
-				}
-			}
-			if (bind(sockfd, address.name, address.nameLen) != 0) {
-				invalidateSocket();
-				return;
-			}
-			if (listen(sockfd, getBacklogSize()) != 0) {
-				invalidateSocket();
-				return;
-			}
+
+			if (bind(sockfd, address.name, address.nameLen) != 0)
+				return false;
+
+			if (listen(sockfd, getBacklogSize()) != 0)
+				return false;
+
+			return true;
 		} ();
 
-		if (sock == StreamListenSocketFD.invalid)
-			return sock;
+		if (!succ) {
+			closeSocket(sockfd);
+			return StreamListenSocketFD.invalid;
+		}
 
-		m_loop.initFD(sock, FDFlags.none, StreamListenSocketSlot.init);
+		auto sock = m_loop.initFD!StreamListenSocketFD(sockfd, FDFlags.none, StreamListenSocketSlot.init);
 
 		if (on_accept) waitForConnections(sock, on_accept);
 
@@ -281,6 +273,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override void waitForConnections(StreamListenSocketFD sock, AcceptCallback on_accept)
 	{
+		if (!isValid(sock)) return;
+
 		m_loop.registerFD(sock, EventMask.read, false);
 		m_loop.m_fds[sock].streamListen.acceptCallback = on_accept;
 		m_loop.setNotifyCallback!(EventType.read)(sock, &onAccept);
@@ -298,10 +292,9 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 		} else {
 			() @trusted { sockfd = accept(cast(sock_t)listenfd, () @trusted { return cast(sockaddr*)&addr; } (), &addr_len); } ();
 			if (sockfd == -1) return;
-			setSocketNonBlocking(cast(SocketFD)sockfd, true);
+			setSocketNonBlocking(sockfd, true);
 		}
-		auto fd = cast(StreamSocketFD)sockfd;
-		m_loop.initFD(fd, FDFlags.none, StreamSocketSlot.init);
+		auto fd = m_loop.initFD!StreamSocketFD(sockfd, FDFlags.none, StreamSocketSlot.init);
 		m_loop.m_fds[fd].streamSocket.state = ConnectionState.connected;
 		m_loop.registerFD(fd, EventMask.read|EventMask.write);
 		//print("accept %d", sockfd);
@@ -311,11 +304,14 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	ConnectionState getConnectionState(StreamSocketFD sock)
 	{
+		if (!isValid(sock)) return ConnectionState.closed;
 		return m_loop.m_fds[sock].streamSocket.state;
 	}
 
 	final override bool getLocalAddress(SocketFD sock, scope RefAddress dst)
 	{
+		if (!isValid(sock)) return false;
+
 		socklen_t addr_len = dst.nameLen;
 		if (() @trusted { return getsockname(cast(sock_t)sock, dst.name, &addr_len); } () != 0)
 			return false;
@@ -325,6 +321,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override bool getRemoteAddress(SocketFD sock, scope RefAddress dst)
 	{
+		if (!isValid(sock)) return false;
+
 		socklen_t addr_len = dst.nameLen;
 		if (() @trusted { return getpeername(cast(sock_t)sock, dst.name, &addr_len); } () != 0)
 			return false;
@@ -334,12 +332,16 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override void setTCPNoDelay(StreamSocketFD socket, bool enable)
 	{
+		if (!isValid(socket)) return;
+
 		int opt = enable;
 		() @trusted { setsockopt(cast(sock_t)socket, IPPROTO_TCP, TCP_NODELAY, cast(char*)&opt, opt.sizeof); } ();
 	}
 
 	override void setKeepAlive(StreamSocketFD socket, bool enable) @trusted
 	{
+		if (!isValid(socket)) return;
+
 		int opt = enable;
 		int err = setsockopt(cast(sock_t)socket, SOL_SOCKET, SO_KEEPALIVE, &opt, int.sizeof);
 		if (err != 0)
@@ -348,6 +350,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	override void setKeepAliveParams(StreamSocketFD socket, Duration idle, Duration interval, int probeCount) @trusted
 	{
+		if (!isValid(socket)) return;
+
 		// dunnno about BSD\OSX, maybe someone should fix it for them later
 		version (linux) {
 			setKeepAlive(socket, true);
@@ -371,6 +375,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	override void setUserTimeout(StreamSocketFD socket, Duration timeout) @trusted
 	{
+		if (!isValid(socket)) return;
+
 		version (linux) {
 			uint tmsecs = cast(uint) timeout.total!"msecs";
 			int err = setsockopt(cast(sock_t)socket, SOL_TCP, TCP_USER_TIMEOUT, &tmsecs, uint.sizeof);
@@ -381,6 +387,11 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override void read(StreamSocketFD socket, ubyte[] buffer, IOMode mode, IOCallback on_read_finish)
 	{
+		if (!isValid(socket)) {
+			on_read_finish(socket, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		/*if (buffer.length == 0) {
 			on_read_finish(socket, IOStatus.ok, 0);
 			return;
@@ -435,6 +446,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	override void cancelRead(StreamSocketFD socket)
 	{
+		if (!isValid(socket)) return;
+
 		assert(m_loop.m_fds[socket].streamSocket.readCallback !is null, "Cancelling read when there is no read in progress.");
 		m_loop.setNotifyCallback!(EventType.read)(socket, null);
 		with (m_loop.m_fds[socket].streamSocket) {
@@ -511,6 +524,11 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override void write(StreamSocketFD socket, const(ubyte)[] buffer, IOMode mode, IOCallback on_write_finish)
 	{
+		if (!isValid(socket)) {
+			on_write_finish(socket, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		if (buffer.length == 0) {
 			on_write_finish(socket, IOStatus.ok, 0);
 			return;
@@ -560,6 +578,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	override void cancelWrite(StreamSocketFD socket)
 	{
+		if (!isValid(socket)) return;
+
 		assert(m_loop.m_fds[socket].streamSocket.writeCallback !is null, "Cancelling write when there is no write in progress.");
 		m_loop.setNotifyCallback!(EventType.write)(socket, null);
 		m_loop.m_fds[socket].streamSocket.writeBuffer = null;
@@ -616,6 +636,11 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override void waitForData(StreamSocketFD socket, IOCallback on_data_available)
 	{
+		if (!isValid(socket)) {
+			on_data_available(socket, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		sizediff_t ret;
 		ubyte dummy;
 		() @trusted { ret = recv(cast(sock_t)socket, &dummy, 1, MSG_PEEK); } ();
@@ -674,6 +699,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override void shutdown(StreamSocketFD socket, bool shut_read, bool shut_write)
 	{
+		if (!isValid(socket)) return;
+
 		auto st = m_loop.m_fds[socket].streamSocket.state;
 		() @trusted { .shutdown(cast(sock_t)socket, shut_read ? shut_write ? SHUT_RDWR : SHUT_RD : shut_write ? SHUT_WR : 0); } ();
 		if (st == ConnectionState.passiveClose) shut_read = true;
@@ -690,7 +717,6 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 	{
 		auto sockfd = createSocket(bind_address.addressFamily, SOCK_DGRAM);
 		if (sockfd == -1) return DatagramSocketFD.invalid;
-		auto sock = cast(DatagramSocketFD)sockfd;
 
 		if (bind_address && () @trusted { return bind(sockfd, bind_address.name, bind_address.nameLen); } () != 0) {
 			closeSocket(sockfd);
@@ -718,9 +744,9 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 			}
 		}
 
-		m_loop.initFD(sock, is_internal ? FDFlags.internal : FDFlags.none, DgramSocketSlot.init);
+		auto flags = is_internal ? FDFlags.internal : FDFlags.none;
+		auto sock = m_loop.initFD!DatagramSocketFD(sockfd, flags, DgramSocketSlot.init);
 		m_loop.registerFD(sock, EventMask.read|EventMask.write);
-
 		return sock;
 	}
 
@@ -731,28 +757,34 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	package DatagramSocketFD adoptDatagramSocketInternal(int socket, bool is_internal = true, bool close_on_exec = false)
 	@nogc {
-		auto fd = DatagramSocketFD(socket);
-		if (m_loop.m_fds[fd].common.refCount) // FD already in use?
+		if (m_loop.m_fds[socket].common.refCount) // FD already in use?
 			return DatagramSocketFD.init;
-		setSocketNonBlocking(fd, close_on_exec);
-		m_loop.initFD(fd, is_internal ? FDFlags.internal : FDFlags.none, DgramSocketSlot.init);
+		setSocketNonBlocking(socket, close_on_exec);
+		auto flags = is_internal ? FDFlags.internal : FDFlags.none;
+		auto fd = m_loop.initFD!DatagramSocketFD(socket, flags, DgramSocketSlot.init);
 		m_loop.registerFD(fd, EventMask.read|EventMask.write);
 		return fd;
 	}
 
 	final override void setTargetAddress(DatagramSocketFD socket, scope Address target_address)
 	{
+		if (!isValid(socket)) return;
+
 		() @trusted { connect(cast(sock_t)socket, target_address.name, target_address.nameLen); } ();
 	}
 
 	final override bool setBroadcast(DatagramSocketFD socket, bool enable)
 	{
+		if (!isValid(socket)) return false;
+
 		int tmp_broad = enable;
 		return () @trusted { return setsockopt(cast(sock_t)socket, SOL_SOCKET, SO_BROADCAST, &tmp_broad, tmp_broad.sizeof); } () == 0;
 	}
 
 	final override bool joinMulticastGroup(DatagramSocketFD socket, scope Address multicast_address, uint interface_index = 0)
 	{
+		if (!isValid(socket)) return false;
+
 		switch (multicast_address.addressFamily) {
 			default: assert(false, "Multicast only supported for IPv4/IPv6 sockets.");
 			case AddressFamily.INET:
@@ -784,6 +816,12 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 	@safe {
 		assert(mode != IOMode.all, "Only IOMode.immediate and IOMode.once allowed for datagram sockets.");
 
+		if (!isValid(socket)) {
+			RefAddress addr;
+			on_receive_finish(socket, IOStatus.invalidHandle, 0, addr);
+			return;
+		}
+
 		sizediff_t ret;
 		sockaddr_storage src_addr;
 		socklen_t src_addr_len = src_addr.sizeof;
@@ -826,6 +864,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	void cancelReceive(DatagramSocketFD socket)
 	@nogc {
+		if (!isValid(socket)) return;
+
 		assert(m_loop.m_fds[socket].datagramSocket.readCallback !is null, "Cancelling read when there is no read in progress.");
 		m_loop.setNotifyCallback!(EventType.read)(socket, null);
 		m_loop.m_fds[socket].datagramSocket.readBuffer = null;
@@ -861,6 +901,12 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 	{
 		assert(mode != IOMode.all, "Only IOMode.immediate and IOMode.once allowed for datagram sockets.");
 
+		if (!isValid(socket)) {
+			RefAddress addr;
+			on_send_finish(socket, IOStatus.invalidHandle, 0, addr);
+			return;
+		}
+
 		sizediff_t ret;
 		if (target_address) {
 			() @trusted { ret = .sendto(cast(sock_t)socket, buffer.ptr, min(buffer.length, int.max), SEND_FLAGS, target_address.name, target_address.nameLen); } ();
@@ -897,6 +943,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	void cancelSend(DatagramSocketFD socket)
 	{
+		if (!isValid(socket)) return;
+
 		assert(m_loop.m_fds[socket].datagramSocket.writeCallback !is null, "Cancelling write when there is no write in progress.");
 		m_loop.setNotifyCallback!(EventType.write)(socket, null);
 		m_loop.m_fds[socket].datagramSocket.writeBuffer = null;
@@ -929,8 +977,16 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 		() @trusted { return cast(DatagramIOCallback)slot.writeCallback; } ()(socket, IOStatus.ok, ret, null);
 	}
 
+	final override bool isValid(SocketFD handle)
+	const {
+		if (handle.value > m_loop.m_fds.length) return false;
+		return m_loop.m_fds[handle.value].common.validationCounter == handle.validationCounter;
+	}
+
 	final override void addRef(SocketFD fd)
 	{
+		if (!isValid(fd)) return;
+
 		auto slot = () @trusted { return &m_loop.m_fds[fd]; } ();
 		assert(slot.common.refCount > 0, "Adding reference to unreferenced socket FD.");
 		slot.common.refCount++;
@@ -939,6 +995,9 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 	final override bool releaseRef(SocketFD fd)
 	@nogc {
 		import taggedalgebraic : hasType;
+
+		if (!isValid(fd)) return true;
+
 		auto slot = () @trusted { return &m_loop.m_fds[fd]; } ();
 		nogc_assert(slot.common.refCount > 0, "Releasing reference to unreferenced socket FD.");
 		// listening sockets have an incremented the reference count because of setNotifyCallback
@@ -966,6 +1025,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override bool setOption(DatagramSocketFD socket, DatagramSocketOption option, bool enable)
 	{
+		if (!isValid(socket)) return false;
+
 		int proto, opt;
 		final switch (option) {
 			case DatagramSocketOption.broadcast: proto = SOL_SOCKET; opt = SO_BROADCAST; break;
@@ -977,6 +1038,8 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final override bool setOption(StreamSocketFD socket, StreamSocketOption option, bool enable)
 	{
+		if (!isValid(socket)) return false;
+
 		int proto, opt;
 		final switch (option) {
 			case StreamSocketOption.noDelay: proto = IPPROTO_TCP; opt = TCP_NODELAY; break;
@@ -988,16 +1051,19 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 
 	final protected override void* rawUserData(StreamSocketFD descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
+		if (!isValid(descriptor)) return null;
 		return m_loop.rawUserDataImpl(descriptor, size, initialize, destroy);
 	}
 
 	final protected override void* rawUserData(StreamListenSocketFD descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
+		if (!isValid(descriptor)) return null;
 		return m_loop.rawUserDataImpl(descriptor, size, initialize, destroy);
 	}
 
 	final protected override void* rawUserData(DatagramSocketFD descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
+		if (!isValid(descriptor)) return null;
 		return m_loop.rawUserDataImpl(descriptor, size, initialize, destroy);
 	}
 
@@ -1010,7 +1076,7 @@ final class PosixEventDriverSockets(Loop : PosixEventLoop) : EventDriverSockets
 		} else {
 			() @trusted { sock = socket(family, type, 0); } ();
 			if (sock == -1) return -1;
-			setSocketNonBlocking(cast(SocketFD)sock, true);
+			setSocketNonBlocking(sock, true);
 
 			// Prevent SIGPIPE on failed send
 			version (OSX) {
@@ -1078,7 +1144,7 @@ private void closeSocket(sock_t sockfd)
 	else close(sockfd);
 }
 
-private void setSocketNonBlocking(SocketFD sockfd, bool close_on_exec = false)
+private void setSocketNonBlocking(SocketFD.BaseType sockfd, bool close_on_exec = false)
 @nogc nothrow {
 	version (Windows) {
 		uint enable = 1;

source/eventcore/drivers/posix/watchers.d

@@ -38,7 +38,9 @@ final class InotifyEventDriverWatchers(Events : EventDriverEvents) : EventDriver
 		auto handle = () @trusted { return inotify_init1(IN_NONBLOCK | IN_CLOEXEC); } ();
 		if (handle == -1) return WatcherID.invalid;
 
-		auto ret = WatcherID(handle);
+		auto ret = m_loop.initFD!WatcherID(handle, FDFlags.none, WatcherSlot(callback));
+		m_loop.registerFD(cast(FD)ret, EventMask.read);
+		m_loop.setNotifyCallback!(EventType.read)(cast(FD)ret, &onChanges);
 
 		m_watches[ret] = WatchState(null, path, recursive);
 
@@ -46,23 +48,28 @@ final class InotifyEventDriverWatchers(Events : EventDriverEvents) : EventDriver
 		if (recursive)
 			addSubWatches(ret, path, "");
 
-		m_loop.initFD(FD(handle), FDFlags.none, WatcherSlot(callback));
-		m_loop.registerFD(FD(handle), EventMask.read);
-		m_loop.setNotifyCallback!(EventType.read)(FD(handle), &onChanges);
-
-		processEvents(WatcherID(handle));
+		processEvents(ret);
 
 		return ret;
 	}
 
+	final override bool isValid(WatcherID handle)
+	const {
+		if (handle.value >= m_loop.m_fds.length) return false;
+		return m_loop.m_fds[handle.value].common.validationCounter == handle.validationCounter;
+	}
+
 	final override void addRef(WatcherID descriptor)
 	{
+		if (!isValid(descriptor)) return;
 		assert(m_loop.m_fds[descriptor].common.refCount > 0, "Adding reference to unreferenced event FD.");
 		m_loop.m_fds[descriptor].common.refCount++;
 	}
 
 	final override bool releaseRef(WatcherID descriptor)
 	{
+		if (!isValid(descriptor)) return true;
+
 		FD fd = cast(FD)descriptor;
 		auto slot = () @trusted { return &m_loop.m_fds[fd]; } ();
 		nogc_assert(slot.common.refCount > 0, "Releasing reference to unreferenced event FD.");
@@ -80,6 +87,7 @@ final class InotifyEventDriverWatchers(Events : EventDriverEvents) : EventDriver
 
 	final protected override void* rawUserData(WatcherID descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
+		if (!isValid(descriptor)) return null;
 		return m_loop.rawUserDataImpl(descriptor, size, initialize, destroy);
 	}
 
@@ -204,13 +212,22 @@ final class FSEventsEventDriverWatchers(Events : EventDriverEvents) : EventDrive
 		assert(false, "TODO!");
 	}
 
+	final override bool isValid(WatcherID handle)
+	const {
+		return false;
+	}
+
 	final override void addRef(WatcherID descriptor)
 	{
+		if (!isValid(descriptor)) return;
+
 		assert(false, "TODO!");
 	}
 
 	final override bool releaseRef(WatcherID descriptor)
 	{
+		if (!isValid(descriptor)) return true;
+
 		/*FSEventStreamStop
 		FSEventStreamUnscheduleFromRunLoop
 		FSEventStreamInvalidate
@@ -220,6 +237,8 @@ final class FSEventsEventDriverWatchers(Events : EventDriverEvents) : EventDrive
 
 	final protected override void* rawUserData(WatcherID descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
+		if (!isValid(descriptor)) return null;
+
 		return m_loop.rawUserDataImpl(descriptor, size, initialize, destroy);
 	}
 
@@ -283,9 +302,15 @@ final class PollEventDriverWatchers(Events : EventDriverEvents) : EventDriverWat
 		return cast(WatcherID)evt;
 	}
 
+	final override bool isValid(WatcherID handle)
+	const {
+		return m_events.isValid(cast(EventID)handle);
+	}
+
 	final override void addRef(WatcherID descriptor)
 	{
-		assert(descriptor != WatcherID.invalid);
+		if (!isValid(descriptor)) return;
+
 		auto evt = cast(EventID)descriptor;
 		auto pt = evt in m_pollers;
 		assert(pt !is null);
@@ -294,7 +319,8 @@ final class PollEventDriverWatchers(Events : EventDriverEvents) : EventDriverWat
 
 	final override bool releaseRef(WatcherID descriptor)
 	{
-		nogc_assert(descriptor != WatcherID.invalid, "Invalid directory watcher ID released");
+		if (!isValid(descriptor)) return true;
+
 		auto evt = cast(EventID)descriptor;
 		auto pt = evt in m_pollers;
 		nogc_assert(pt !is null, "Directory watcher polling thread does not exist");

source/eventcore/drivers/threadedfile.d

@@ -103,6 +103,8 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 			IOInfo write;
 			bool open = true;
 
+			uint validationCounter;
+
 			int refCount;
 			DataInitializer userDataDestructor;
 			ubyte[16*size_t.sizeof] userData;
@@ -110,8 +112,8 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 
 		TaskPool m_fileThreadPool;
 		ChoppedVector!FileInfo m_files; // TODO: use the one from the posix loop
-		SmallIntegerSet!FileFD m_activeReads;
-		SmallIntegerSet!FileFD m_activeWrites;
+		SmallIntegerSet!size_t m_activeReads;
+		SmallIntegerSet!size_t m_activeWrites;
 		EventID m_readyEvent = EventID.invalid;
 		bool m_waiting;
 		Events m_events;
@@ -170,13 +172,17 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 		}
 
 		if (m_files[system_file_handle].refCount > 0) return FileFD.invalid;
+		auto vc = m_files[system_file_handle].validationCounter;
 		m_files[system_file_handle] = FileInfo.init;
 		m_files[system_file_handle].refCount = 1;
-		return FileFD(system_file_handle);
+		m_files[system_file_handle].validationCounter = vc + 1;
+		return FileFD(system_file_handle, vc + 1);
 	}
 
 	void close(FileFD file)
 	{
+		if (!isValid(file)) return;
+
 		// NOTE: The file descriptor itself must stay open until the reference
 		//       count drops to zero, or this would result in dangling handles.
 		//       In case of an exclusive file lock, the lock should be lifted
@@ -186,6 +192,8 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 
 	ulong getSize(FileFD file)
 	{
+		if (!isValid(file)) return ulong.max;
+
 		version (linux) {
 			// stat_t seems to be defined wrong on linux/64
 			return .lseek(cast(int)file, 0, SEEK_END);
@@ -198,6 +206,8 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 
 	override void truncate(FileFD file, ulong size, FileIOCallback on_finish)
 	{
+		if (!isValid(file)) return;
+
 		version (Posix) {
 			// FIXME: do this in the thread pool
 
@@ -214,6 +224,11 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 
 	final override void write(FileFD file, ulong offset, const(ubyte)[] buffer, IOMode, FileIOCallback on_write_finish)
 	{
+		if (!isValid(file)) {
+			on_write_finish(file, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		//assert(this.writable);
 		auto f = () @trusted { return &m_files[file]; } ();
 
@@ -226,7 +241,7 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 			assert(false, "Concurrent file writes are not allowed.");
 		assert(f.write.callback is null, "Concurrent file writes are not allowed.");
 		f.write.callback = on_write_finish;
-		m_activeWrites.insert(file);
+		m_activeWrites.insert(file.value);
 		threadSetup();
 		log("start write task");
 		try {
@@ -235,7 +250,7 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 			m_fileThreadPool.put(task!(taskFun!("write", const(ubyte)))(thiss, fs, file, offset, buffer));
 			startWaiting();
 		} catch (Exception e) {
-			m_activeWrites.remove(file);
+			m_activeWrites.remove(file.value);
 			on_write_finish(file, IOStatus.error, 0);
 			return;
 		}
@@ -243,17 +258,24 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 
 	final override void cancelWrite(FileFD file)
 	{
-		assert(m_activeWrites.contains(file), "Cancelling write when no write is in progress.");
+		if (!isValid(file)) return;
+
+		assert(m_activeWrites.contains(file.value), "Cancelling write when no write is in progress.");
 
 		auto f = &m_files[file].write;
 		f.callback = null;
-		m_activeWrites.remove(file);
+		m_activeWrites.remove(file.value);
 		m_events.trigger(m_readyEvent, true); // ensure that no stale wait operation is left behind
 		safeCAS(f.status, ThreadedFileStatus.processing, ThreadedFileStatus.cancelling);
 	}
 
 	final override void read(FileFD file, ulong offset, ubyte[] buffer, IOMode, FileIOCallback on_read_finish)
 	{
+		if (!isValid(file)) {
+			on_read_finish(file, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		auto f = () @trusted { return &m_files[file]; } ();
 
 		if (!f.open) {
@@ -265,7 +287,7 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 			assert(false, "Concurrent file reads are not allowed.");
 		assert(f.read.callback is null, "Concurrent file reads are not allowed.");
 		f.read.callback = on_read_finish;
-		m_activeReads.insert(file);
+		m_activeReads.insert(file.value);
 		threadSetup();
 		log("start read task");
 		try {
@@ -274,7 +296,7 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 			m_fileThreadPool.put(task!(taskFun!("read", ubyte))(thiss, fs, file, offset, buffer));
 			startWaiting();
 		} catch (Exception e) {
-			m_activeReads.remove(file);
+			m_activeReads.remove(file.value);
 			on_read_finish(file, IOStatus.error, 0);
 			return;
 		}
@@ -282,28 +304,40 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 
 	final override void cancelRead(FileFD file)
 	{
-		assert(m_activeReads.contains(file), "Cancelling read when no read is in progress.");
+		if (!isValid(file)) return;
+
+		assert(m_activeReads.contains(file.value), "Cancelling read when no read is in progress.");
 
 		auto f = &m_files[file].read;
 		f.callback = null;
-		m_activeReads.remove(file);
+		m_activeReads.remove(file.value);
 		m_events.trigger(m_readyEvent, true); // ensure that no stale wait operation is left behind
 		safeCAS(f.status, ThreadedFileStatus.processing, ThreadedFileStatus.cancelling);
 	}
 
+	final override bool isValid(FileFD handle)
+	const {
+		if (handle.value >= m_files.length) return false;
+		return m_files[handle.value].validationCounter == handle.validationCounter;
+	}
+
 	final override void addRef(FileFD descriptor)
 	{
+		if (!isValid(descriptor)) return;
+
 		m_files[descriptor].refCount++;
 	}
 
 	final override bool releaseRef(FileFD descriptor)
 	{
+		if (!isValid(descriptor)) return true;
+
 		auto f = () @trusted { return &m_files[descriptor]; } ();
 		if (!--f.refCount) {
 			.close(cast(int)descriptor);
 			*f = FileInfo.init;
-			assert(!m_activeReads.contains(descriptor));
-			assert(!m_activeWrites.contains(descriptor));
+			assert(!m_activeReads.contains(descriptor.value));
+			assert(!m_activeWrites.contains(descriptor.value));
 			return false;
 		}
 		return true;
@@ -311,6 +345,8 @@ final class ThreadedFileEventDriver(Events : EventDriverEvents) : EventDriverFil
 
 	protected final override void* rawUserData(FileFD descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
+		if (!isValid(descriptor)) return null;
+
 		FileInfo* fds = &m_files[descriptor];
 		assert(fds.userDataDestructor is null || fds.userDataDestructor is destroy,
 			"Requesting user data with differing type (destructor).");
@@ -370,11 +406,15 @@ log("wait for status set");
 	private void onReady(EventID)
 	{
 log("ready event");
-		foreach (f; m_activeReads)
-			m_files[f].read.finalize(f, { m_activeReads.remove(f); });
+		foreach (f; m_activeReads) {
+			auto fd = FileFD(f, m_files[f].validationCounter);
+			m_files[f].read.finalize(fd, { m_activeReads.remove(f); });
+		}
 
-		foreach (f; m_activeWrites)
-			m_files[f].write.finalize(f, { m_activeWrites.remove(f); });
+		foreach (f; m_activeWrites) {
+			auto fd = FileFD(f, m_files[f].validationCounter);
+			m_files[f].write.finalize(fd, { m_activeWrites.remove(f); });
+		}
 
 		m_waiting = false;
 		startWaiting();

source/eventcore/drivers/timer.d

@@ -87,7 +87,7 @@ final class LoopTimeoutTimerDriver : EventDriverTimers {
 
 	final override TimerID create()
 	@trusted {
-		auto id = cast(TimerID)(++m_lastTimerID);
+		auto id = TimerID(++m_lastTimerID, 0);
 		TimerSlot* tm;
 		try tm = ms_allocator.make!TimerSlot;
 		catch (Exception e) return TimerID.invalid;
@@ -102,6 +102,8 @@ final class LoopTimeoutTimerDriver : EventDriverTimers {
 
 	final override void set(TimerID timer, Duration timeout, Duration repeat)
 	@trusted {
+		if (!isValid(timer)) return;
+
 		scope (failure) assert(false);
 		auto tm = m_timers[timer];
 		if (tm.pending) stop(timer);
@@ -115,6 +117,8 @@ final class LoopTimeoutTimerDriver : EventDriverTimers {
 	@trusted {
 		import std.algorithm.mutation : swap;
 
+		if (!isValid(timer)) return;
+
 		auto tm = m_timers[timer];
 		if (!tm.pending) return;
 		TimerCallback2 cb;
@@ -129,16 +133,22 @@ final class LoopTimeoutTimerDriver : EventDriverTimers {
 
 	final override bool isPending(TimerID descriptor)
 	{
+		if (!isValid(descriptor)) return false;
+
 		return m_timers[descriptor].pending;
 	}
 
 	final override bool isPeriodic(TimerID descriptor)
 	{
+		if (!isValid(descriptor)) return false;
+
 		return m_timers[descriptor].repeatDuration > Duration.zero;
 	}
 
 	final override void wait(TimerID timer, TimerCallback2 callback)
 	{
+		if (!isValid(timer)) return;
+
 		assert(!m_timers[timer].callback, "Calling wait() on a timer that is already waiting.");
 		m_timers[timer].callback = callback;
 		addRef(timer);
@@ -147,26 +157,29 @@ final class LoopTimeoutTimerDriver : EventDriverTimers {
 
 	final override void cancelWait(TimerID timer)
 	{
+		if (!isValid(timer)) return;
+
 		auto pt = m_timers[timer];
 		assert(pt.callback);
 		pt.callback = null;
 		releaseRef(timer);
 	}
 
+	override bool isValid(TimerID handle)
+	const {
+		return (handle in m_timers) !is null;
+	}
+
 	final override void addRef(TimerID descriptor)
 	{
-		assert(descriptor != TimerID.init, "Invalid timer ID.");
-		assert(descriptor in m_timers, "Unknown timer ID.");
-		if (descriptor !in m_timers) return;
+		if (!isValid(descriptor)) return;
 
 		m_timers[descriptor].refCount++;
 	}
 
 	final override bool releaseRef(TimerID descriptor)
 	{
-		nogc_assert(descriptor != TimerID.init, "Invalid timer ID.");
-		nogc_assert((descriptor in m_timers) !is null, "Unknown timer ID.");
-		if (descriptor !in m_timers) return true;
+		if (!isValid(descriptor)) return true;
 
 		auto tm = m_timers[descriptor];
 		tm.refCount--;
@@ -199,12 +212,15 @@ final class LoopTimeoutTimerDriver : EventDriverTimers {
 
 	final bool isUnique(TimerID descriptor)
 	const {
-		if (descriptor == TimerID.init) return false;
+		if (!isValid(descriptor)) return false;
+
 		return m_timers[descriptor].refCount == 1;
 	}
 
 	protected final override void* rawUserData(TimerID descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
+		if (!isValid(descriptor)) return null;
+
 		TimerSlot* fds = m_timers[descriptor];
 		assert(fds.userDataDestructor is null || fds.userDataDestructor is destroy,
 			"Requesting user data with differing type (destructor).");

source/eventcore/drivers/winapi/core.d

@@ -16,6 +16,8 @@ import std.typecons : Tuple, tuple;
 
 final class WinAPIEventDriverCore : EventDriverCore {
 @safe: /*@nogc:*/ nothrow:
+	private alias ThreadCallbackEntry = Tuple!(ThreadCallback2, intptr_t, intptr_t);
+
 	private {
 		bool m_exit;
 		size_t m_waiterCount;
@@ -25,10 +27,11 @@ final class WinAPIEventDriverCore : EventDriverCore {
 		void delegate() @safe nothrow[MAXIMUM_WAIT_OBJECTS] m_registeredEventCallbacks;
 		DWORD m_registeredEventCount = 0;
 		HANDLE m_fileCompletionEvent;
+		uint m_validationCounter;
 		ConsumableQueue!IOEvent m_ioEvents;
 
 		shared Mutex m_threadCallbackMutex;
-		ConsumableQueue!(Tuple!(ThreadCallback, intptr_t)) m_threadCallbacks;
+		ConsumableQueue!ThreadCallbackEntry m_threadCallbacks;
 	}
 
 	package {
@@ -48,7 +51,7 @@ final class WinAPIEventDriverCore : EventDriverCore {
 		else {
 			() @trusted { m_threadCallbackMutex = cast(shared)mallocT!Mutex; } ();
 		}
-		m_threadCallbacks = mallocT!(ConsumableQueue!(Tuple!(ThreadCallback, intptr_t)));
+		m_threadCallbacks = mallocT!(ConsumableQueue!ThreadCallbackEntry);
 		m_threadCallbacks.reserve(1000);
 	}
 
@@ -138,7 +141,7 @@ final class WinAPIEventDriverCore : EventDriverCore {
 		m_exit = false;
 	}
 
-	override void runInOwnerThread(ThreadCallback del, intptr_t param)
+	override void runInOwnerThread(ThreadCallback2 del, intptr_t param1, intptr_t param2)
 	shared {
 		import core.atomic : atomicLoad;
 
@@ -152,12 +155,14 @@ final class WinAPIEventDriverCore : EventDriverCore {
 		try {
 			synchronized (m)
 				() @trusted { return (cast()this).m_threadCallbacks; } ()
-					.put(tuple(del, param));
+					.put(ThreadCallbackEntry(del, param1, param2));
 		} catch (Exception e) assert(false, e.msg);
 
 		() @trusted { PostThreadMessageW(m_tid, WM_APP, 0, 0); } ();
 	}
 
+	alias runInOwnerThread = EventDriverCore.runInOwnerThread;
+
 	package void* rawUserDataImpl(HANDLE handle, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
 		HandleSlot* fds = &m_handles[handle];
@@ -254,6 +259,7 @@ final class WinAPIEventDriverCore : EventDriverCore {
 		assert(h !in m_handles, "Handle already in use.");
 		HandleSlot s;
 		s.refCount = 1;
+		s.validationCounter = ++m_validationCounter;
 		s.specific = SlotType.init;
 		m_handles[h] = s;
 		return () @trusted { return &m_handles[h].specific.get!SlotType(); } ();
@@ -276,14 +282,14 @@ final class WinAPIEventDriverCore : EventDriverCore {
 		import std.stdint : intptr_t;
 
 		while (true) {
-			Tuple!(ThreadCallback, intptr_t) del;
+			ThreadCallbackEntry del;
 			try {
 				synchronized (m_threadCallbackMutex) {
 					if (m_threadCallbacks.empty) break;
 					del = m_threadCallbacks.consumeOne;
 				}
 			} catch (Exception e) assert(false, e.msg);
-			del[0](del[1]);
+			del[0](del[1], del[2]);
 		}
 	}
 }
@@ -302,6 +308,7 @@ private struct HandleSlot {
 		WatcherSlot watcher;
 	}
 	int refCount;
+	uint validationCounter;
 	TaggedAlgebraic!SpecificTypes specific;
 
 	DataInitializer userDataDestructor;
@@ -344,7 +351,11 @@ package struct FileSlot {
 			auto cb = this.callback;
 			this.callback = null;
 			assert(cb !is null);
-			cb(cast(FileFD)cast(size_t)overlapped.hEvent, status, bytes_transferred);
+			if (auto ps = overlapped.hEvent in overlapped.driver.m_handles) {
+				auto vc = ps.validationCounter;
+				auto fd = FileFD(cast(size_t)overlapped.hEvent, vc);
+				cb(fd, status, bytes_transferred);
+			}
 		}
 	}
 	Direction!false read;

source/eventcore/drivers/winapi/dns.d

@@ -14,7 +14,7 @@ final class WinAPIEventDriverDNS : EventDriverDNS {
 		import std.typecons : scoped;
 		import std.utf : toUTF16z;
 
-		auto id = DNSLookupID(0);
+		auto id = DNSLookupID(0, 0);
 
 		static immutable ushort[] addrfamilies = [AF_INET, AF_INET6];
 
@@ -100,6 +100,13 @@ final class WinAPIEventDriverDNS : EventDriverDNS {
 
 	void cancelLookup(DNSLookupID handle)
 	{
+		if (!isValid(handle)) return;
+
 		assert(false, "TODO!");
 	}
+
+	override bool isValid(DNSLookupID handle)
+	const {
+		return handle == DNSLookupID(0, 0);
+	}
 }

source/eventcore/drivers/winapi/driver.d

@@ -24,7 +24,7 @@ import eventcore.internal.utils : mallocT, freeT;
 import core.sys.windows.windows;
 
 static assert(HANDLE.sizeof <= FD.BaseType.sizeof);
-static assert(FD(cast(size_t)INVALID_HANDLE_VALUE) == FD.init);
+static assert(FD(cast(size_t)INVALID_HANDLE_VALUE, 0) == FD.init);
 
 
 final class WinAPIEventDriver : EventDriver {

source/eventcore/drivers/winapi/events.d

@@ -56,14 +56,16 @@ final class WinAPIEventDriverEvents : EventDriverEvents {
 
 	override EventID create()
 	{
-		auto id = EventID(m_idCounter++);
-		if (id == EventID.invalid) id = EventID(m_idCounter++);
+		auto id = EventID(m_idCounter++, 0);
+		if (id == EventID.invalid) id = EventID(m_idCounter++, 0);
 		m_events[id] = EventSlot(1, new ConsumableQueue!EventCallback); // FIXME: avoid GC allocation
 		return id;
 	}
 
 	override void trigger(EventID event, bool notify_all = true)
 	{
+		if (!isValid(event)) return;
+
 		auto pe = event in m_events;
 		assert(pe !is null, "Invalid event ID passed to triggerEvent.");
 		if (notify_all) {
@@ -82,12 +84,11 @@ final class WinAPIEventDriverEvents : EventDriverEvents {
 	override void trigger(EventID event, bool notify_all = true) shared
 	{
 		import core.atomic : atomicStore;
-		auto pe = event in m_events;
-		assert(pe !is null, "Invalid event ID passed to shared triggerEvent.");
 
 		() @trusted {
 			auto thisus = cast(WinAPIEventDriverEvents)this;
 			EnterCriticalSection(&thisus.m_mutex);
+			if (thisus.isValid(event))
 				thisus.m_pending.put(Trigger(event, notify_all));
 			LeaveCriticalSection(&thisus.m_mutex);
 			SetEvent(thisus.m_event);
@@ -96,6 +97,8 @@ final class WinAPIEventDriverEvents : EventDriverEvents {
 
 	override void wait(EventID event, EventCallback on_event)
 	{
+		if (!isValid(event)) return;
+
 		m_core.addWaiter();
 		return m_events[event].waiters.put(on_event);
 	}
@@ -105,18 +108,29 @@ final class WinAPIEventDriverEvents : EventDriverEvents {
 		import std.algorithm.searching : countUntil;
 		import std.algorithm.mutation : remove;
 
+		if (!isValid(event)) return;
+
 		m_events[event].waiters.removePending(on_event);
 		m_core.removeWaiter();
 	}
 
+	override bool isValid(EventID handle)
+	const {
+		return (handle in m_events) !is null;
+	}
+
 	override void addRef(EventID descriptor)
 	{
+		if (!isValid(descriptor)) return;
+
 		assert(m_events[descriptor].refCount > 0);
 		m_events[descriptor].refCount++;
 	}
 
 	override bool releaseRef(EventID descriptor)
 	{
+		if (!isValid(descriptor)) return true;
+
 		auto pe = descriptor in m_events;
 		nogc_assert(pe.refCount > 0, "Releasing unreference event.");
 		if (--pe.refCount == 0) {
@@ -156,7 +170,7 @@ final class WinAPIEventDriverEvents : EventDriverEvents {
 	}
 
 	private static HANDLE idToHandle(EventID event)
-	@trusted {
+	@trusted @nogc {
 		return cast(HANDLE)cast(size_t)event;
 	}
 }

source/eventcore/drivers/winapi/files.d

@@ -68,19 +68,24 @@ final class WinAPIEventDriverFiles : EventDriverFiles {
 		s.write.overlapped.driver = m_core;
 		s.write.overlapped.hEvent = handle;
 
-		return FileFD(cast(size_t)handle);
+		return FileFD(cast(size_t)handle, m_core.m_handles[handle].validationCounter);
 	}
 
 	override void close(FileFD file)
 	{
+		if (!isValid(file)) return;
+
 		auto h = idToHandle(file);
-		auto slot = () @trusted { return &m_core.m_handles[h].file(); } ();
-		if (slot.read.overlapped.hEvent != INVALID_HANDLE_VALUE)
-			slot.read.overlapped.hEvent = slot.write.overlapped.hEvent = INVALID_HANDLE_VALUE;
+		auto slot = () @trusted { return &m_core.m_handles[h]; } ();
+		if (slot.validationCounter != file.validationCounter) return;
+		if (slot.file.read.overlapped.hEvent != INVALID_HANDLE_VALUE)
+			slot.file.read.overlapped.hEvent = slot.file.write.overlapped.hEvent = INVALID_HANDLE_VALUE;
 	}
 
 	override ulong getSize(FileFD file)
 	{
+		if (!isValid(file)) return ulong.max;
+
 		LARGE_INTEGER size;
 		auto succeeded = () @trusted { return GetFileSizeEx(idToHandle(file), &size); } ();
 		if (!succeeded || size.QuadPart < 0)
@@ -90,6 +95,11 @@ final class WinAPIEventDriverFiles : EventDriverFiles {
 
 	override void truncate(FileFD file, ulong size, FileIOCallback on_finish)
 	@trusted {
+		if (!isValid(file)) {
+			on_finish(file, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		auto h = idToHandle(file);
 
 		// FIXME: do this in a separate thread
@@ -110,6 +120,11 @@ final class WinAPIEventDriverFiles : EventDriverFiles {
 
 	override void write(FileFD file, ulong offset, const(ubyte)[] buffer, IOMode mode, FileIOCallback on_write_finish)
 	{
+		if (!isValid(file)) {
+			on_write_finish(file, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		auto h = idToHandle(file);
 		auto slot = &m_core.m_handles[h].file.write;
 
@@ -134,6 +149,11 @@ final class WinAPIEventDriverFiles : EventDriverFiles {
 
 	override void read(FileFD file, ulong offset, ubyte[] buffer, IOMode mode, FileIOCallback on_read_finish)
 	{
+		if (!isValid(file)) {
+			on_read_finish(file, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		auto h = idToHandle(file);
 		auto slot = &m_core.m_handles[h].file.read;
 
@@ -158,23 +178,39 @@ final class WinAPIEventDriverFiles : EventDriverFiles {
 
 	override void cancelWrite(FileFD file)
 	{
+		if (!isValid(file)) return;
+
 		auto h = idToHandle(file);
 		cancelIO!true(h, m_core.m_handles[h].file.write);
 	}
 
 	override void cancelRead(FileFD file)
 	{
+		if (!isValid(file)) return;
+
 		auto h = idToHandle(file);
 		cancelIO!false(h, m_core.m_handles[h].file.read);
 	}
 
+	override bool isValid(FileFD handle)
+	const {
+		auto h = idToHandle(handle);
+		if (auto ps = h in m_core.m_handles)
+			return ps.validationCounter == handle.validationCounter;
+		return false;
+	}
+
 	override void addRef(FileFD descriptor)
 	{
+		if (!isValid(descriptor)) return;
+
 		m_core.m_handles[idToHandle(descriptor)].addRef();
 	}
 
 	override bool releaseRef(FileFD descriptor)
 	{
+		if (!isValid(descriptor)) return true;
+
 		auto h = idToHandle(descriptor);
 		auto slot = &m_core.m_handles[h];
 		return slot.releaseRef({
@@ -222,13 +258,15 @@ final class WinAPIEventDriverFiles : EventDriverFiles {
 	private static nothrow
 	void onIOFinished(alias fun, bool RO)(DWORD error, DWORD bytes_transferred, OVERLAPPED_CORE* overlapped)
 	{
-		FileFD id = cast(FileFD)cast(size_t)overlapped.hEvent;
-		auto handle = idToHandle(id);
+		HANDLE handle = overlapped.hEvent;
 		if (handle == INVALID_HANDLE_VALUE) return;
+		auto cslot = () @trusted { return &overlapped.driver.m_handles[handle]; } ();
+		FileFD id = FileFD(cast(size_t)overlapped.hEvent, cslot.validationCounter);
+
 		static if (RO)
-			auto slot = () @trusted { return &overlapped.driver.m_handles[handle].file.write; } ();
+			auto slot = () @trusted { return &cslot.file.write; } ();
 		else
-			auto slot = () @trusted { return &overlapped.driver.m_handles[handle].file.read; } ();
+			auto slot = () @trusted { return &cslot.file.read; } ();
 		assert(slot !is null);
 
 		if (!slot.callback) {
@@ -254,7 +292,7 @@ final class WinAPIEventDriverFiles : EventDriverFiles {
 	}
 
 	private static HANDLE idToHandle(FileFD id)
-	@trusted {
+	@trusted @nogc {
 		return cast(HANDLE)cast(size_t)id;
 	}
 }

source/eventcore/drivers/winapi/pipes.d

@@ -14,41 +14,68 @@ final class WinAPIEventDriverPipes : EventDriverPipes {
 
 	override void read(PipeFD pipe, ubyte[] buffer, IOMode mode, PipeIOCallback on_read_finish)
 	{
+		if (!isValid(pipe)) {
+			on_read_finish(pipe, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		assert(false, "TODO!");
 	}
 
 	override void cancelRead(PipeFD pipe)
 	{
+		if (!isValid(pipe)) return;
+
 		assert(false, "TODO!");
 	}
 
 	override void write(PipeFD pipe, const(ubyte)[] buffer, IOMode mode, PipeIOCallback on_write_finish)
 	{
+		if (!isValid(pipe)) {
+			on_write_finish(pipe, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		assert(false, "TODO!");
 	}
 
 	override void cancelWrite(PipeFD pipe)
 	{
+		if (!isValid(pipe)) return;
+
 		assert(false, "TODO!");
 	}
 
 	override void waitForData(PipeFD pipe, PipeIOCallback on_data_available)
 	{
+		if (!isValid(pipe)) return;
+
 		assert(false, "TODO!");
 	}
 
 	override void close(PipeFD pipe)
 	{
+		if (!isValid(pipe)) return;
+
 		assert(false, "TODO!");
 	}
 
-    override void addRef(PipeFD pid)
+	override bool isValid(PipeFD handle)
+	const {
+		return false;
+	}
+
+	override void addRef(PipeFD pipe)
 	{
+    	if (!isValid(pipe)) return;
+
 		assert(false, "TODO!");
 	}
 
-    override bool releaseRef(PipeFD pid)
+	override bool releaseRef(PipeFD pipe)
 	{
+    	if (!isValid(pipe)) return true;
+
 		assert(false, "TODO!");
 	}
 

source/eventcore/drivers/winapi/processes.d

@@ -19,31 +19,48 @@ final class WinAPIEventDriverProcesses : EventDriverProcesses {
 
 	override bool hasExited(ProcessID pid)
 	{
+		if (!isValid(pid)) return true;
+
 		assert(false, "TODO!");
 	}
 
 	override void kill(ProcessID pid, int signal)
 	{
+		if (!isValid(pid)) return;
+
 		assert(false, "TODO!");
 	}
 
 	override size_t wait(ProcessID pid, ProcessWaitCallback on_process_exit)
 	{
+		if (!isValid(pid)) return size_t.max;
+
 		assert(false, "TODO!");
 	}
 
 	override void cancelWait(ProcessID pid, size_t waitId)
 	{
+		if (!isValid(pid)) return;
+
 		assert(false, "TODO!");
 	}
 
+	override bool isValid(ProcessID handle)
+	const {
+		return false;
+	}
+
 	override void addRef(ProcessID pid)
 	{
+		if (!isValid(pid)) return;
+
 		assert(false, "TODO!");
 	}
 
 	override bool releaseRef(ProcessID pid)
 	{
+		if (!isValid(pid)) return true;
+
 		assert(false, "TODO!");
 	}
 

source/eventcore/drivers/winapi/signals.d

@@ -13,13 +13,22 @@ final class WinAPIEventDriverSignals : EventDriverSignals {
 		assert(false, "TODO!");
 	}
 
+	override bool isValid(SignalListenID handle)
+	const {
+		return false;
+	}
+
 	override void addRef(SignalListenID descriptor)
 	{
+		if (!isValid(descriptor)) return;
+
 		assert(false, "TODO!");
 	}
 
 	override bool releaseRef(SignalListenID descriptor)
 	{
+		if (!isValid(descriptor)) return true;
+
 		assert(false, "TODO!");
 	}
 }

source/eventcore/drivers/winapi/sockets.d

@@ -96,7 +96,7 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	final override void cancelConnectStream(StreamSocketFD sock)
 	{
-		assert(sock != StreamSocketFD.invalid, "Invalid socket descriptor");
+		if (!isValid(sock)) return;
 
 		with (m_sockets[sock].streamSocket) {
 			assert(state == ConnectionState.connecting,
@@ -115,8 +115,7 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	private StreamSocketFD adoptStreamInternal(SOCKET socket, ConnectionState state)
 	{
-		auto fd = StreamSocketFD(socket);
-		if (m_sockets[fd].common.refCount) // FD already in use?
+		if (m_sockets[socket].common.refCount) // FD already in use?
 			return StreamSocketFD.invalid;
 
 		// done by wsaasyncselect
@@ -132,7 +131,7 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 			overlapped.driver = m_core;
 		}
 
-		initSocketSlot(fd);
+		auto fd = initSocketSlot!StreamSocketFD(socket);
 		with (m_sockets[socket]) {
 			specific = StreamSocketSlot.init;
 			streamSocket.state = state;
@@ -178,8 +177,7 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 		if (fd == INVALID_SOCKET)
 			return StreamListenSocketFD.invalid;
 
-		auto sock = cast(StreamListenSocketFD)fd;
-		initSocketSlot(sock);
+		auto sock = initSocketSlot!StreamListenSocketFD(fd);
 		m_sockets[sock].specific = StreamListenSocketSlot.init;
 
 		if (on_accept) waitForConnections(sock, on_accept);
@@ -189,6 +187,8 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override void waitForConnections(StreamListenSocketFD sock, AcceptCallback on_accept)
 	{
+		if (!isValid(sock)) return;
+
 		assert(!m_sockets[sock].streamListen.acceptCallback);
 		m_sockets[sock].streamListen.acceptCallback = on_accept;
 		() @trusted { WSAAsyncSelect(sock, m_hwnd, WM_USER_SOCKET, FD_ACCEPT); } ();
@@ -197,13 +197,15 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override ConnectionState getConnectionState(StreamSocketFD sock)
 	{
-		assert(sock != StreamSocketFD.invalid, "Invalid socket handle");
+		if (!isValid(sock)) return ConnectionState.closed;
+
 		return m_sockets[sock].streamSocket.state;
 	}
 
 	override bool getLocalAddress(SocketFD sock, scope RefAddress dst)
 	{
-		assert(sock != StreamSocketFD.invalid, "Invalid socket handle");
+		if (!isValid(sock)) return false;
+
 		socklen_t addr_len = dst.nameLen;
 		if (() @trusted { return getsockname(sock, dst.name, &addr_len); } () != 0)
 			return false;
@@ -213,7 +215,8 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override bool getRemoteAddress(SocketFD sock, scope RefAddress dst)
 	{
-		assert(sock != StreamSocketFD.invalid, "Invalid socket handle");
+		if (!isValid(sock)) return false;
+
 		socklen_t addr_len = dst.nameLen;
 		if (() @trusted { return getpeername(sock, dst.name, &addr_len); } () != 0)
 			return false;
@@ -223,18 +226,24 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override void setTCPNoDelay(StreamSocketFD socket, bool enable)
 	@trusted {
+		if (!isValid(socket)) return;
+
 		BOOL eni = enable;
 		setsockopt(socket, IPPROTO_TCP, TCP_NODELAY, &eni, eni.sizeof);
 	}
 
 	override void setKeepAlive(StreamSocketFD socket, bool enable)
 	@trusted {
+		if (!isValid(socket)) return;
+
 		BOOL eni = enable;
 		setsockopt(socket, SOL_SOCKET, SO_KEEPALIVE, &eni, eni.sizeof);
 	}
 
 	override void setKeepAliveParams(StreamSocketFD socket, Duration idle, Duration interval, int probeCount) @trusted
 	{
+		if (!isValid(socket)) return;
+
 		tcp_keepalive opts = tcp_keepalive(1, cast(c_ulong) idle.total!"msecs"(),
 			cast(c_ulong) interval.total!"msecs");
 		int result = WSAIoctl(socket, SIO_KEEPALIVE_VALS, &opts, cast(DWORD) tcp_keepalive.sizeof,
@@ -248,6 +257,11 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override void read(StreamSocketFD socket, ubyte[] buffer, IOMode mode, IOCallback on_read_finish)
 	{
+		if (!isValid(socket)) {
+			on_read_finish(socket, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		auto slot = () @trusted { return &m_sockets[socket].streamSocket(); } ();
 		slot.read.buffer = buffer;
 		slot.read.bytesTransferred = 0;
@@ -358,6 +372,11 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override void write(StreamSocketFD socket, const(ubyte)[] buffer, IOMode mode, IOCallback on_write_finish)
 	{
+		if (!isValid(socket)) {
+			on_write_finish(socket, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		auto slot = () @trusted { return &m_sockets[socket].streamSocket(); } ();
 		slot.write.buffer = buffer;
 		slot.write.bytesTransferred = 0;
@@ -454,11 +473,18 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override void waitForData(StreamSocketFD socket, IOCallback on_data_available)
 	{
+		if (!isValid(socket)) {
+			on_data_available(socket, IOStatus.invalidHandle, 0);
+			return;
+		}
+
 		assert(false, "TODO!");
 	}
 
 	override void shutdown(StreamSocketFD socket, bool shut_read = true, bool shut_write = true)
 	{
+		if (!isValid(socket)) return;
+
 		() @trusted { WSASendDisconnect(socket, null); } ();
 		with (m_sockets[socket].streamSocket) {
 			if (state == ConnectionState.passiveClose)
@@ -469,6 +495,7 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override void cancelRead(StreamSocketFD socket)
 	@trusted @nogc {
+		if (!isValid(socket)) return;
 		if (!m_sockets[socket].streamSocket.read.callback) return;
 		CancelIoEx(cast(HANDLE)cast(SOCKET)socket, cast(LPOVERLAPPED)&m_sockets[socket].streamSocket.read.overlapped);
 		m_sockets[socket].streamSocket.read.callback = null;
@@ -477,6 +504,7 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override void cancelWrite(StreamSocketFD socket)
 	@trusted @nogc {
+		if (!isValid(socket)) return;
 		if (!m_sockets[socket].streamSocket.write.callback) return;
 		CancelIoEx(cast(HANDLE)cast(SOCKET)socket, cast(LPOVERLAPPED)&m_sockets[socket].streamSocket.write.overlapped);
 		m_sockets[socket].streamSocket.write.callback = null;
@@ -516,8 +544,7 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	private DatagramSocketFD adoptDatagramSocketInternal(SOCKET socket)
 	{
-		auto fd = DatagramSocketFD(socket);
-		if (m_sockets[fd].common.refCount) // FD already in use?
+		if (m_sockets[socket].common.refCount) // FD already in use?
 			return DatagramSocketFD.invalid;
 
 		void setupOverlapped(ref OVERLAPPED_CORE overlapped) @trusted @nogc nothrow {
@@ -529,7 +556,7 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 			overlapped.driver = m_core;
 		}
 
-		initSocketSlot(fd);
+		auto fd = initSocketSlot!DatagramSocketFD(socket);
 		with (m_sockets[socket]) {
 			specific = DatagramSocketSlot.init;
 			setupOverlapped(datagramSocket.write.overlapped);
@@ -543,11 +570,15 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	final override void setTargetAddress(DatagramSocketFD socket, scope Address target_address)
 	{
+		if (!isValid(socket)) return;
+
 		() @trusted { connect(cast(SOCKET)socket, target_address.name, target_address.nameLen); } ();
 	}
 
 	final override bool setBroadcast(DatagramSocketFD socket, bool enable)
 	{
+		if (!isValid(socket)) return false;
+
 		int tmp_broad = enable;
 		return () @trusted { return setsockopt(cast(SOCKET)socket, SOL_SOCKET, SO_BROADCAST, &tmp_broad, tmp_broad.sizeof); } () == 0;
 	}
@@ -556,6 +587,8 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 	{
 		import std.socket : AddressFamily;
 
+		if (!isValid(socket)) return false;
+
 		switch (multicast_address.addressFamily) {
 			default: assert(false, "Multicast only supported for IPv4/IPv6 sockets.");
 			case AddressFamily.INET:
@@ -583,6 +616,12 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override void receive(DatagramSocketFD socket, ubyte[] buffer, IOMode mode, DatagramIOCallback on_read_finish)
 	{
+		if (!isValid(socket)) {
+			RefAddress addr;
+			on_read_finish(socket, IOStatus.invalidHandle, 0, addr);
+			return;
+		}
+
 		auto slot = () @trusted { return &m_sockets[socket].datagramSocket(); } ();
 		slot.read.buffer = buffer;
 		slot.read.wsabuf[0].buf = () @trusted { return buffer.ptr; } ();
@@ -678,6 +717,12 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	override void send(DatagramSocketFD socket, const(ubyte)[] buffer, IOMode mode, Address target_address, DatagramIOCallback on_write_finish)
 	{
+		if (!isValid(socket)) {
+			RefAddress addr;
+			on_write_finish(socket, IOStatus.invalidHandle, 0, addr);
+			return;
+		}
+
 		auto slot = () @trusted { return &m_sockets[socket].datagramSocket(); } ();
 		slot.write.buffer = buffer;
 		slot.write.wsabuf[0].len = buffer.length;
@@ -780,8 +825,16 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 		}
 	}
 
+	override bool isValid(SocketFD handle)
+	const {
+		if (handle.value >= m_sockets.length) return false;
+		return handle.validationCounter == m_sockets[handle].common.validationCounter;
+	}
+
 	override void addRef(SocketFD fd)
 	{
+		if (!isValid(fd)) return;
+
 		assert(m_sockets[fd].common.refCount > 0, "Adding reference to unreferenced socket FD.");
 		m_sockets[fd].common.refCount++;
 	}
@@ -789,6 +842,9 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 	override bool releaseRef(SocketFD fd)
 	@nogc {
 		import taggedalgebraic : hasType;
+
+		if (!isValid(fd)) return true;
+
 		auto slot = () @trusted { return &m_sockets[fd]; } ();
 		nogc_assert(slot.common.refCount > 0, "Releasing reference to unreferenced socket FD.");
 		if (--slot.common.refCount == 0) {
@@ -819,6 +875,8 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	final override bool setOption(DatagramSocketFD socket, DatagramSocketOption option, bool enable)
 	{
+		if (!isValid(socket)) return false;
+
 		int proto, opt;
 		final switch (option) {
 			case DatagramSocketOption.broadcast: proto = SOL_SOCKET; opt = SO_BROADCAST; break;
@@ -830,6 +888,8 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 
 	final override bool setOption(StreamSocketFD socket, StreamSocketOption option, bool enable)
 	{
+		if (!isValid(socket)) return false;
+
 		int proto, opt;
 		final switch (option) {
 			case StreamSocketOption.noDelay: proto = IPPROTO_TCP; opt = TCP_NODELAY; break;
@@ -854,8 +914,10 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 		return rawUserDataImpl(descriptor, size, initialize, destroy);
 	}
 
-	private void* rawUserDataImpl(FD descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
+	private void* rawUserDataImpl(SocketFD descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system @nogc {
+		if (!isValid(descriptor)) return null;
+
 		SocketSlot* fds = &m_sockets[descriptor].common;
 		assert(fds.userDataDestructor is null || fds.userDataDestructor is destroy,
 			"Requesting user data with differing type (destructor).");
@@ -868,12 +930,16 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 		return fds.userData.ptr;
 	}
 
-	private void initSocketSlot(SocketFD fd)
+	private FDType initSocketSlot(FDType)(SOCKET socket)
 	{
 		m_socketCount++;
-		m_sockets[fd.value].common.refCount = 1;
-		m_sockets[fd.value].common.fd = fd;
-		m_sockets[fd.value].common.driver = this;
+		m_sockets[socket].common.refCount = 1;
+		auto vc = ++m_sockets[socket].common.validationCounter;
+		auto fd = FDType(socket, vc);
+		m_sockets[socket].common.fd = fd;
+		m_sockets[socket].common.driver = this;
+
+		return fd;
 	}
 
 	package void clearSocketSlot(FD fd)
@@ -905,14 +971,16 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 								auto cb = slot.streamSocket.connectCallback;
 								if (!cb) break; // cancelled connect?
 
+								auto fd = StreamSocketFD(sock, slot.common.validationCounter);
+
 								slot.streamSocket.connectCallback = null;
 								slot.common.driver.m_core.removeWaiter();
 								if (err) {
 									slot.streamSocket.state = ConnectionState.closed;
-									cb(cast(StreamSocketFD)sock, ConnectStatus.refused);
+									cb(fd, ConnectStatus.refused);
 								} else {
 									slot.streamSocket.state = ConnectionState.connected;
-									cb(cast(StreamSocketFD)sock, ConnectStatus.connected);
+									cb(fd, ConnectStatus.connected);
 								}
 								break;
 							case FD_READ:
@@ -948,7 +1016,8 @@ final class WinAPIEventDriverSockets : EventDriverSockets {
 							if (clientsockfd == INVALID_SOCKET) return 0;
 							auto clientsock = driver.adoptStreamInternal(clientsockfd, ConnectionState.connected);
 							scope RefAddress addrc = new RefAddress(() @trusted { return cast(sockaddr*)&addr; } (), addr_len);
-							slot.streamListen.acceptCallback(cast(StreamListenSocketFD)sock, clientsock, addrc);
+							auto fd = StreamListenSocketFD(sock, slot.common.validationCounter);
+							slot.streamListen.acceptCallback(fd, clientsock, addrc);
 						}
 						break;
 					case Kind.datagramSocket:
@@ -978,6 +1047,7 @@ static struct SocketSlot {
 	WinAPIEventDriverSockets driver; // redundant, but needed by the current IO Completion Routines based approach
 	@property inout(WinAPIEventDriverCore) core() @safe nothrow inout { return driver.m_core; }
 	int refCount;
+	uint validationCounter;
 	DataInitializer userDataDestructor;
 	ubyte[16*size_t.sizeof] userData;
 }

source/eventcore/drivers/winapi/watchers.d

@@ -37,8 +37,6 @@ final class WinAPIEventDriverWatchers : EventDriverWatchers {
 		if (handle == INVALID_HANDLE_VALUE)
 			return WatcherID.invalid;
 
-		auto id = WatcherID(cast(size_t)handle);
-
 		auto slot = m_core.setupSlot!WatcherSlot(handle);
 		slot.directory = path;
 		slot.recursive = recursive;
@@ -48,6 +46,9 @@ final class WinAPIEventDriverWatchers : EventDriverWatchers {
 			try return Mallocator.instance.makeArray!ubyte(16384);
 			catch (Exception e) assert(false, "Failed to allocate directory watcher buffer.");
 		} ();
+
+		auto id = WatcherID(cast(size_t)handle, m_core.m_handles[handle].validationCounter);
+
 		if (!triggerRead(handle, *slot)) {
 			releaseRef(id);
 			return WatcherID.invalid;
@@ -61,18 +62,31 @@ final class WinAPIEventDriverWatchers : EventDriverWatchers {
 		return id;
 	}
 
+	override bool isValid(WatcherID handle)
+	const {
+		if (auto ph = idToHandle(handle) in m_core.m_handles)
+			return ph.validationCounter == handle.validationCounter;
+		return false;
+	}
+
 	override void addRef(WatcherID descriptor)
 	{
+		if (!isValid(descriptor)) return;
+
 		m_core.m_handles[idToHandle(descriptor)].addRef();
 	}
 
 	override bool releaseRef(WatcherID descriptor)
 	{
+		if (!isValid(descriptor)) return true;
+
 		return doReleaseRef(idToHandle(descriptor));
 	}
 
 	protected override void* rawUserData(WatcherID descriptor, size_t size, DataInitializer initialize, DataInitializer destroy)
 	@system {
+		if (!isValid(descriptor)) return null;
+
 		return m_core.rawUserDataImpl(idToHandle(descriptor), size, initialize, destroy);
 	}
 
@@ -117,10 +131,9 @@ final class WinAPIEventDriverWatchers : EventDriverWatchers {
 		import std.path : dirName, baseName, buildPath;
 
 		auto handle = overlapped.hEvent; // *file* handle
-		auto id = WatcherID(cast(size_t)handle);
-
 		auto gslot = () @trusted { return &WinAPIEventDriver.threadInstance.core.m_handles[handle]; } ();
 		auto slot = () @trusted { return &gslot.watcher(); } ();
+		auto id = WatcherID(cast(size_t)handle, gslot.validationCounter);
 
 		if (dwError != 0 || gslot.refCount == 1) {
 			// FIXME: error must be propagated to the caller (except for ABORTED
@@ -220,5 +233,5 @@ final class WinAPIEventDriverWatchers : EventDriverWatchers {
 		return true;
 	}
 
-	static private HANDLE idToHandle(WatcherID id) @trusted { return cast(HANDLE)cast(size_t)id; }
+	static private HANDLE idToHandle(WatcherID id) @trusted @nogc { return cast(HANDLE)cast(size_t)id.value; }
 }

source/eventcore/internal/utils.d

@@ -172,6 +172,18 @@ struct ChoppedVector(T, size_t CHUNK_SIZE = 16*64*1024/nextPOT(T.sizeof)) {
 		return (*m_chunks[chunk])[subidx];
 	}
 
+	ref const(T) opIndex(size_t index)
+	const @nogc {
+		static immutable T emptySlot;
+
+		auto chunk = index / chunkSize;
+		auto subidx = index % chunkSize;
+		if (index >= m_length) return emptySlot;
+		auto c = m_chunks[chunk];
+		if (!c) return emptySlot;
+		return (*c)[subidx];
+	}
+
 	int opApply(scope int delegate(size_t idx, ref T) @safe nothrow del)
 	{
 		size_t idx = 0;