buffer: Delay the swapping of the background buffer

Author: vmg

buffer.go

@@ -44,38 +44,35 @@ func newBuffer(nc net.Conn) buffer {
 }
 
 // swap replaces the active buffer with the background buffer
+// this is a delayed swap that simply increases the buffer counter;
+// the actual swap will be performed the next time we call `buffer.fill`
 func (b *buffer) swap() {
 	b.dbufn += 1
-	dest := b.dbuf[b.dbufn&1]
-	if b.length > 0 {
-		// existing buffer is too large for double-buffering;
-		// we must allocate a temporary buffer that will be discarded
-		if b.length > len(dest) {
-			dest = make([]byte, b.length)
-		}
-		copy(dest[0:b.length], b.buf[b.idx:])
-	}
-	b.buf = dest
-	b.idx = 0
 }
 
 // fill reads into the buffer until at least _need_ bytes are in it
 func (b *buffer) fill(need int) error {
 	n := b.length
+	// fill data into its double-buffering target: if we've called
+	// swap on this buffer, we'll be copying to the background buffer,
+	// and then filling it with network data; otherwise we'll just move
+	// the contents of the current buffer to the front before filling it
+	dest := b.dbuf[b.dbufn&1]
 
-	// move existing data to the beginning
-	if n > 0 && b.idx > 0 {
-		copy(b.buf[0:n], b.buf[b.idx:])
+	// grow buffer if necessary to fit the whole packet.
+	if need > len(dest) {
+		// Round up to the next multiple of the default size;
+		// this buffer will be discarded the next time we swap buffers
+		dest = make([]byte, ((need/defaultBufSize)+1)*defaultBufSize)
 	}
 
-	// grow buffer if necessary
-	if need > len(b.buf) {
-		// Round up to the next multiple of the default size
-		newBuf := make([]byte, ((need/defaultBufSize)+1)*defaultBufSize)
-		copy(newBuf, b.buf)
-		b.buf = newBuf
+	// if we're filling the fg buffer, move the existing data to the start of it.
+	// if we're filling the bg buffer, copy over the data
+	if n > 0 {
+		copy(dest[0:n], b.buf[b.idx:])
 	}
 
+	b.buf = dest
 	b.idx = 0
 
 	for {