Reduce "busy buffer" logs (go-sql-driver#1641)

Reduce the use of `errBadConnNoWrite` to improve maintainability.

ResetSession() and IsValid() checks if the buffer is busy. This reduces
the risk of busy buffer error during connection in use. In principle,
the risk of this is zero. So I removed errBadConnNoWrite when checking
the busy buffer.

After this change, only `writePacke()` returns errBadConnNoWrite.

Additionally, I do not send COM_QUIT when readPacket() encounter read error.
It caused "busy buffer" error too and hide real errors.

buffer.go

@@ -43,6 +43,11 @@ func newBuffer(nc net.Conn) buffer {
 	}
 }
 
+// busy returns true if the buffer contains some read data.
+func (b *buffer) busy() bool {
+	return b.length > 0
+}
+
 // 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`

connection.go

@@ -121,10 +121,14 @@ func (mc *mysqlConn) Close() (err error) {
 	if !mc.closed.Load() {
 		err = mc.writeCommandPacket(comQuit)
 	}
+	mc.close()
+	return
+}
 
+// close closes the network connection and clear results without sending COM_QUIT.
+func (mc *mysqlConn) close() {
 	mc.cleanup()
 	mc.clearResult()
-	return
 }
 
 // Closes the network connection and unsets internal variables. Do not call this
@@ -637,7 +641,7 @@ func (mc *mysqlConn) CheckNamedValue(nv *driver.NamedValue) (err error) {
 // ResetSession implements driver.SessionResetter.
 // (From Go 1.10)
 func (mc *mysqlConn) ResetSession(ctx context.Context) error {
-	if mc.closed.Load() {
+	if mc.closed.Load() || mc.buf.busy() {
 		return driver.ErrBadConn
 	}
 
@@ -671,7 +675,7 @@ func (mc *mysqlConn) ResetSession(ctx context.Context) error {
 // IsValid implements driver.Validator interface
 // (From Go 1.15)
 func (mc *mysqlConn) IsValid() bool {
-	return !mc.closed.Load()
+	return !mc.closed.Load() && !mc.buf.busy()
 }
 
 var _ driver.SessionResetter = &mysqlConn{}

packets.go

@@ -32,11 +32,11 @@ func (mc *mysqlConn) readPacket() ([]byte, error) {
 		// read packet header
 		data, err := mc.buf.readNext(4)
 		if err != nil {
+			mc.close()
 			if cerr := mc.canceled.Value(); cerr != nil {
 				return nil, cerr
 			}
 			mc.log(err)
-			mc.Close()
 			return nil, ErrInvalidConn
 		}
 
@@ -45,7 +45,7 @@ func (mc *mysqlConn) readPacket() ([]byte, error) {
 
 		// check packet sync [8 bit]
 		if data[3] != mc.sequence {
-			mc.Close()
+			mc.close()
 			if data[3] > mc.sequence {
 				return nil, ErrPktSyncMul
 			}
@@ -59,7 +59,7 @@ func (mc *mysqlConn) readPacket() ([]byte, error) {
 			// there was no previous packet
 			if prevData == nil {
 				mc.log(ErrMalformPkt)
-				mc.Close()
+				mc.close()
 				return nil, ErrInvalidConn
 			}
 
@@ -69,11 +69,11 @@ func (mc *mysqlConn) readPacket() ([]byte, error) {
 		// read packet body [pktLen bytes]
 		data, err = mc.buf.readNext(pktLen)
 		if err != nil {
+			mc.close()
 			if cerr := mc.canceled.Value(); cerr != nil {
 				return nil, cerr
 			}
 			mc.log(err)
-			mc.Close()
 			return nil, ErrInvalidConn
 		}
 
@@ -125,10 +125,10 @@ func (mc *mysqlConn) writePacket(data []byte) error {
 
 		n, err := mc.netConn.Write(data[:4+size])
 		if err != nil {
+			mc.cleanup()
 			if cerr := mc.canceled.Value(); cerr != nil {
 				return cerr
 			}
-			mc.cleanup()
 			if n == 0 && pktLen == len(data)-4 {
 				// only for the first loop iteration when nothing was written yet
 				mc.log(err)
@@ -162,11 +162,6 @@ func (mc *mysqlConn) writePacket(data []byte) error {
 func (mc *mysqlConn) readHandshakePacket() (data []byte, plugin string, err error) {
 	data, err = mc.readPacket()
 	if err != nil {
-		// for init we can rewrite this to ErrBadConn for sql.Driver to retry, since
-		// in connection initialization we don't risk retrying non-idempotent actions.
-		if err == ErrInvalidConn {
-			return nil, "", driver.ErrBadConn
-		}
 		return
 	}
 
@@ -312,9 +307,8 @@ func (mc *mysqlConn) writeHandshakeResponsePacket(authResp []byte, plugin string
 	// Calculate packet length and get buffer with that size
 	data, err := mc.buf.takeBuffer(pktLen + 4)
 	if err != nil {
-		// cannot take the buffer. Something must be wrong with the connection
-		mc.log(err)
-		return errBadConnNoWrite
+		mc.cleanup()
+		return err
 	}
 
 	// ClientFlags [32 bit]
@@ -404,9 +398,8 @@ func (mc *mysqlConn) writeAuthSwitchPacket(authData []byte) error {
 	pktLen := 4 + len(authData)
 	data, err := mc.buf.takeBuffer(pktLen)
 	if err != nil {
-		// cannot take the buffer. Something must be wrong with the connection
-		mc.log(err)
-		return errBadConnNoWrite
+		mc.cleanup()
+		return err
 	}
 
 	// Add the auth data [EOF]
@@ -424,9 +417,7 @@ func (mc *mysqlConn) writeCommandPacket(command byte) error {
 
 	data, err := mc.buf.takeSmallBuffer(4 + 1)
 	if err != nil {
-		// cannot take the buffer. Something must be wrong with the connection
-		mc.log(err)
-		return errBadConnNoWrite
+		return err
 	}
 
 	// Add command byte
@@ -443,9 +434,7 @@ func (mc *mysqlConn) writeCommandPacketStr(command byte, arg string) error {
 	pktLen := 1 + len(arg)
 	data, err := mc.buf.takeBuffer(pktLen + 4)
 	if err != nil {
-		// cannot take the buffer. Something must be wrong with the connection
-		mc.log(err)
-		return errBadConnNoWrite
+		return err
 	}
 
 	// Add command byte
@@ -464,9 +453,7 @@ func (mc *mysqlConn) writeCommandPacketUint32(command byte, arg uint32) error {
 
 	data, err := mc.buf.takeSmallBuffer(4 + 1 + 4)
 	if err != nil {
-		// cannot take the buffer. Something must be wrong with the connection
-		mc.log(err)
-		return errBadConnNoWrite
+		return err
 	}
 
 	// Add command byte
@@ -1007,9 +994,7 @@ func (stmt *mysqlStmt) writeExecutePacket(args []driver.Value) error {
 		// In this case the len(data) == cap(data) which is used to optimise the flow below.
 	}
 	if err != nil {
-		// cannot take the buffer. Something must be wrong with the connection
-		mc.log(err)
-		return errBadConnNoWrite
+		return err
 	}
 
 	// command [1 byte]
@@ -1207,8 +1192,7 @@ func (stmt *mysqlStmt) writeExecutePacket(args []driver.Value) error {
 		if valuesCap != cap(paramValues) {
 			data = append(data[:pos], paramValues...)
 			if err = mc.buf.store(data); err != nil {
-				mc.log(err)
-				return errBadConnNoWrite
+				return err
 			}
 		}