optimize readPacket (go-sql-driver#1707)

Avoid unnecessary allocation.
Although this is not a bug, this is a regression in 1.9 branch.
This PR fixes performance regression when compression is not used.

Author: methane

buffer.go

@@ -42,6 +42,11 @@ func (b *buffer) busy() bool {
 	return len(b.buf) > 0
 }
 
+// len returns how many bytes in the read buffer.
+func (b *buffer) len() int {
+	return len(b.buf)
+}
+
 // fill reads into the read buffer until at least _need_ bytes are in it.
 func (b *buffer) fill(need int, r readerFunc) error {
 	// we'll move the contents of the current buffer to dest before filling it.
@@ -86,17 +91,10 @@ func (b *buffer) fill(need int, r readerFunc) error {
 
 // returns next N bytes from buffer.
 // The returned slice is only guaranteed to be valid until the next read
-func (b *buffer) readNext(need int, r readerFunc) ([]byte, error) {
-	if len(b.buf) < need {
-		// refill
-		if err := b.fill(need, r); err != nil {
-			return nil, err
-		}
-	}
-
-	data := b.buf[:need]
+func (b *buffer) readNext(need int) []byte {
+	data := b.buf[:need:need]
 	b.buf = b.buf[need:]
-	return data, nil
+	return data
 }
 
 // takeBuffer returns a buffer with the requested size.

compress.go

@@ -84,26 +84,26 @@ func (c *compIO) reset() {
 	c.buff.Reset()
 }
 
-func (c *compIO) readNext(need int, r readerFunc) ([]byte, error) {
+func (c *compIO) readNext(need int) ([]byte, error) {
 	for c.buff.Len() < need {
-		if err := c.readCompressedPacket(r); err != nil {
+		if err := c.readCompressedPacket(); err != nil {
 			return nil, err
 		}
 	}
 	data := c.buff.Next(need)
 	return data[:need:need], nil // prevent caller writes into c.buff
 }
 
-func (c *compIO) readCompressedPacket(r readerFunc) error {
-	header, err := c.mc.buf.readNext(7, r) // size of compressed header
+func (c *compIO) readCompressedPacket() error {
+	header, err := c.mc.readNext(7)
 	if err != nil {
 		return err
 	}
 	_ = header[6] // bounds check hint to compiler; guaranteed by readNext
 
 	// compressed header structure
 	comprLength := getUint24(header[0:3])
-	compressionSequence := uint8(header[3])
+	compressionSequence := header[3]
 	uncompressedLength := getUint24(header[4:7])
 	if debug {
 		fmt.Printf("uncompress cmplen=%v uncomplen=%v pkt_cmp_seq=%v expected_cmp_seq=%v\n",
@@ -120,7 +120,7 @@ func (c *compIO) readCompressedPacket(r readerFunc) error {
 	c.mc.sequence = compressionSequence + 1
 	c.mc.compressSequence = c.mc.sequence
 
-	comprData, err := c.mc.buf.readNext(comprLength, r)
+	comprData, err := c.mc.readNext(comprLength)
 	if err != nil {
 		return err
 	}

packets.go

@@ -25,19 +25,30 @@ import (
 // https://dev.mysql.com/doc/dev/mysql-server/latest/PAGE_PROTOCOL.html
 // https://mariadb.com/kb/en/clientserver-protocol/
 
+// read n bytes from mc.buf
+func (mc *mysqlConn) readNext(n int) ([]byte, error) {
+	if mc.buf.len() < n {
+		err := mc.buf.fill(n, mc.readWithTimeout)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return mc.buf.readNext(n), nil
+}
+
 // Read packet to buffer 'data'
 func (mc *mysqlConn) readPacket() ([]byte, error) {
 	var prevData []byte
 	invalidSequence := false
 
-	readNext := mc.buf.readNext
+	readNext := mc.readNext
 	if mc.compress {
 		readNext = mc.compIO.readNext
 	}
 
 	for {
 		// read packet header
-		data, err := readNext(4, mc.readWithTimeout)
+		data, err := readNext(4)
 		if err != nil {
 			mc.close()
 			if cerr := mc.canceled.Value(); cerr != nil {
@@ -85,7 +96,7 @@ func (mc *mysqlConn) readPacket() ([]byte, error) {
 		}
 
 		// read packet body [pktLen bytes]
-		data, err = readNext(pktLen, mc.readWithTimeout)
+		data, err = readNext(pktLen)
 		if err != nil {
 			mc.close()
 			if cerr := mc.canceled.Value(); cerr != nil {