region: Use bufferPool for read buffers

Reuse buffers for reading responses from HBase, both for the
compressed response and decompressed response. This should reduce
memory pressure when reading lots of data.

region/client.go

@@ -103,12 +103,8 @@ const (
 var bufferPool sync.Pool
 
 func newBuffer(size int) []byte {
-	v := bufferPool.Get()
-	var b []byte
-	if v != nil {
-		b = v.([]byte)
-	}
-	return append(b[:0], make([]byte, size)...)
+	b, _ := bufferPool.Get().([]byte)
+	return append(b, make([]byte, size)...)
 }
 
 func freeBuffer(b []byte) {
@@ -477,7 +473,8 @@ func (c *client) receive(r io.Reader) (err error) {
 	}
 
 	size := binary.BigEndian.Uint32(sz[:])
-	b := make([]byte, size)
+	b := newBuffer(int(size))
+	defer freeBuffer(b)
 
 	_, err = io.ReadFull(r, b)
 	if err != nil {
@@ -546,6 +543,7 @@ func (c *client) receive(r io.Reader) (err error) {
 		b := b[size-cellsLen:]
 		if c.compressor != nil {
 			b, err = c.compressor.decompressCellblocks(b)
+			defer freeBuffer(b)
 			if err != nil {
 				err = RetryableError{fmt.Errorf("failed to decompress the response: %s", err)}
 				return

region/compressor.go

@@ -92,7 +92,7 @@ func readUint32(b []byte) (uint32, []byte, error) {
 func (c *compressor) decompressCellblocks(b []byte) ([]byte, error) {
 	var (
 		err                  error
-		out                  []byte
+		out                  = newBuffer(0) // gets freed in receive
 		compressedChunk      []byte
 		compressedChunkLen   uint32
 		uncompressedBlockLen uint32