buffer: Improve len(buf)==cap(buf) consistency

buffer.go

@@ -30,9 +30,8 @@ type buffer struct {
 }
 
 func newBuffer(nc net.Conn) buffer {
-	var b [defaultBufSize]byte
 	return buffer{
-		buf: b[:],
+		buf: make([]byte, defaultBufSize),
 		nc:  nc,
 	}
 }
@@ -114,8 +113,7 @@ func (b *buffer) takeBuffer(length int) []byte {
 		return nil
 	}
 
-	// test (cheap) general case first
-	if length <= defaultBufSize || length <= cap(b.buf) {
+	if length <= len(b.buf) {
 		return b.buf[:length]
 	}
 
@@ -145,3 +143,14 @@ func (b *buffer) takeCompleteBuffer() []byte {
 	}
 	return b.buf
 }
+
+// setGrownBuffer set buf as internal buffer if cap(buf) is larger
+// than len(b.buf).  It can be used when you took buffer by
+// takeCompleteBuffer and append some data to it.
+func (b *buffer) setGrownBuffer(buf []byte) {
+	// buf may be grown by `buf = append(buf, ...)`.  So set len=cap explicitly.
+	buf = buf[:cap(buf)]
+	if len(buf) > len(b.buf) {
+		b.buf = buf
+	}
+}

packets.go

@@ -1076,7 +1076,7 @@ func (stmt *mysqlStmt) writeExecutePacket(args []driver.Value) error {
 		// In that case we must build the data packet with the new values buffer
 		if valuesCap != cap(paramValues) {
 			data = append(data[:pos], paramValues...)
-			mc.buf.buf = data
+			mc.buf.setGrownBuffer(data)
 		}
 
 		pos += len(paramValues)