Implement a concurrent mode for pipe().

This maximizes throughput for typical disk I/O loads.

source/vibe/core/stream.d

@@ -35,17 +35,23 @@ public import eventcore.driver : IOMode;
 		The actual number of bytes written is returned. If `nbytes` is given
 		and not equal to `ulong.max`, íts value will be returned.
 */
-ulong pipe(InputStream, OutputStream)(InputStream source, OutputStream sink, ulong nbytes)
-	@blocking @trusted
+ulong pipe(InputStream, OutputStream)(InputStream source, OutputStream sink,
+	ulong nbytes, PipeMode mode = PipeMode.sequential) @blocking @trusted
 	if (isOutputStream!OutputStream && isInputStream!InputStream)
 {
 	import vibe.internal.allocator : theAllocator, makeArray, dispose;
+	import vibe.core.core : runTask;
+	import vibe.core.sync : LocalManualEvent, createManualEvent;
+	import vibe.core.task : InterruptException;
 
+	final switch (mode) {
+		case PipeMode.sequential:
+			{
 				scope buffer = cast(ubyte[]) theAllocator.allocate(64*1024);
 				scope (exit) theAllocator.dispose(buffer);
 
-	//logTrace("default write %d bytes, empty=%s", nbytes, stream.empty);
 				ulong ret = 0;
+
 				if (nbytes == ulong.max) {
 					while (!source.empty) {
 						size_t chunk = min(source.leastSize, buffer.length);
@@ -65,14 +71,137 @@ ulong pipe(InputStream, OutputStream)(InputStream source, OutputStream sink, ulo
 						ret += chunk;
 					}
 				}
+
 				return ret;
+			}
+		case PipeMode.concurrent:
+			{
+				enum bufcount = 4;
+				enum bufsize = 64*1024;
+
+				static struct ConcurrentPipeState {
+					InputStream source;
+					OutputStream sink;
+					ulong nbytes;
+					ubyte[][bufcount] buffers;
+					size_t[bufcount] buffer_fill;
+					// buffer index that is being read/written
+					size_t read_idx = 0, write_idx = 0;
+					Exception readex;
+					bool done = false;
+					LocalManualEvent evt;
+					size_t bytesWritten;
+
+					void readLoop()
+					{
+						if (nbytes == ulong.max) {
+							while (!source.empty) {
+								while (read_idx >= write_idx + buffers.length) {
+									evt.wait();
+								}
+
+								size_t chunk = min(source.leastSize, bufsize);
+								auto bi = read_idx % bufcount;
+								source.read(buffers[bi][0 .. chunk]);
+								bytesWritten += chunk;
+								buffer_fill[bi] = chunk;
+								if (write_idx >= read_idx++)
+									evt.emit();
+							}
+						} else {
+							while (nbytes > 0) {
+								while (read_idx >= write_idx + buffers.length)
+									evt.wait();
+
+								size_t chunk = min(nbytes, bufsize);
+								auto bi = read_idx % bufcount;
+								source.read(buffers[bi][0 .. chunk]);
+								nbytes -= chunk;
+								bytesWritten += chunk;
+								buffer_fill[bi] = chunk;
+								if (write_idx >= read_idx++)
+									evt.emit();
+							}
+						}
+					}
+
+					void writeLoop()
+					{
+						while (read_idx > write_idx || !done) {
+							while (read_idx <= write_idx) {
+								if (done) return;
+								evt.wait();
+							}
+
+							auto bi = write_idx % bufcount;
+							sink.write(buffers[bi][0 .. buffer_fill[bi]]);
+
+							// notify reader that we just freed a buffer
+							if (write_idx++ <= read_idx - buffers.length)
+								evt.emit();
+						}
+					}
+				}
+
+				scope buffer = cast(ubyte[]) theAllocator.allocate(bufcount * bufsize);
+				scope (exit) theAllocator.dispose(buffer);
+
+				ConcurrentPipeState state;
+				foreach (i; 0 .. bufcount)
+					state.buffers[i] = buffer[i*($/bufcount) .. (i+1)*($/bufcount)];
+				swap(state.source, source);
+				swap(state.sink, sink);
+				state.nbytes = nbytes;
+				state.evt = createManualEvent();
+
+				auto reader = runTask(function(ConcurrentPipeState* state) nothrow {
+						try state.readLoop();
+						catch (InterruptException e) {}
+						catch (Exception e) state.readex = e;
+						state.done = true;
+						state.evt.emit();
+					}, &state);
+
+				scope (failure) {
+					reader.interrupt();
+					reader.joinUninterruptible();
+				}
+
+				// write loop
+				state.writeLoop();
+
+				reader.join();
+
+				if (state.readex) throw state.readex;
+
+				return state.bytesWritten;
+			}
+	}
 }
 /// ditto
-ulong pipe(InputStream, OutputStream)(InputStream source, OutputStream sink)
-	@blocking
+ulong pipe(InputStream, OutputStream)(InputStream source, OutputStream sink,
+	PipeMode mode = PipeMode.sequential) @blocking
 	if (isOutputStream!OutputStream && isInputStream!InputStream)
 {
-	return pipe(source, sink, ulong.max);
+	return pipe(source, sink, ulong.max, mode);
+}
+
+enum PipeMode {
+	/** Sequentially reads into a buffer and writes it out to the sink.
+
+		This mode reads and writes to the same buffer in a ping-pong fashion.
+		The memory overhead is low, but if the source does not support
+		read-ahead buffering, or the sink does not have an internal buffer that
+		is drained asynchronously, the total throghput will be reduced.
+	*/
+	sequential,
+
+	/** Uses a task to concurrently read and write.
+
+		This mode maximizes throughput at the expense of setting up a task and
+		associated sycnronization.
+	*/
+	concurrent
 }