buffer: Use a double-buffering scheme to prevent data races

Author: vmg

buffer.go

@@ -21,41 +21,58 @@ const defaultBufSize = 4096
 // In other words, we can't write and read simultaneously on the same connection.
 // The buffer is similar to bufio.Reader / Writer but zero-copy-ish
 // Also highly optimized for this particular use case.
+// This buffer is backed by two byte slices in a double-buffering scheme
 type buffer struct {
 	buf     []byte // buf is a byte buffer who's length and capacity are equal.
 	nc      net.Conn
 	idx     int
 	length  int
 	timeout time.Duration
+	dbuf    [2][]byte // dbuf is an array with the two byte slices that back this buffer
+	flipcnt int       // flipccnt is the current buffer counter for double-buffering
 }
 
 // newBuffer allocates and returns a new buffer.
 func newBuffer(nc net.Conn) buffer {
+	fg := make([]byte, defaultBufSize)
+	bg := make([]byte, defaultBufSize)
 	return buffer{
-		buf: make([]byte, defaultBufSize),
-		nc:  nc,
+		buf:  fg,
+		nc:   nc,
+		dbuf: [2][]byte{fg, bg},
 	}
 }
 
+// flip replaces the active buffer with the background buffer
+// this is a delayed flip that simply increases the buffer counter;
+// the actual flip will be performed the next time we call `buffer.fill`
+func (b *buffer) flip() {
+	b.flipcnt += 1
+}
+
 // fill reads into the buffer until at least _need_ bytes are in it
 func (b *buffer) fill(need int) error {
 	n := b.length
-
-	// move existing data to the beginning
-	if n > 0 && b.idx > 0 {
-		copy(b.buf[0:n], b.buf[b.idx:])
+	// fill data into its double-buffering target: if we've called
+	// flip 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.flipcnt&1]
+
+	// 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 flip buffers
+		dest = make([]byte, ((need/defaultBufSize)+1)*defaultBufSize)
 	}
 
-	// grow buffer if necessary
-	// TODO: let the buffer shrink again at some point
-	//       Maybe keep the org buf slice and swap back?
-	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 {

rows.go

@@ -111,6 +111,8 @@ func (rows *mysqlRows) Close() (err error) {
 		return err
 	}
 
+	mc.buf.flip()
+
 	// Remove unread packets from stream
 	if !rows.rs.done {
 		err = mc.readUntilEOF()