move stale connection check to ResetSession() (#1496)

When ResetSession was added, it was called when the connection is put into the pool.
Thet is why we had only set `mc.reset` flag on ResetSession().

In Go 1.15, this behavior was changed. (golang/go@971f8a2)

ResetSession is called when the connection is checked out from the pool.
So we can call checkConnLiveness() directly from ResetSession.

connection.go

@@ -34,7 +34,6 @@ type mysqlConn struct {
 	status           statusFlag
 	sequence         uint8
 	parseTime        bool
-	reset            bool // set when the Go SQL package calls ResetSession
 
 	// for context support (Go 1.8+)
 	watching bool
@@ -646,7 +645,31 @@ func (mc *mysqlConn) ResetSession(ctx context.Context) error {
 	if mc.closed.Load() {
 		return driver.ErrBadConn
 	}
-	mc.reset = true
+
+	// Perform a stale connection check. We only perform this check for
+	// the first query on a connection that has been checked out of the
+	// connection pool: a fresh connection from the pool is more likely
+	// to be stale, and it has not performed any previous writes that
+	// could cause data corruption, so it's safe to return ErrBadConn
+	// if the check fails.
+	if mc.cfg.CheckConnLiveness {
+		conn := mc.netConn
+		if mc.rawConn != nil {
+			conn = mc.rawConn
+		}
+		var err error
+		if mc.cfg.ReadTimeout != 0 {
+			err = conn.SetReadDeadline(time.Now().Add(mc.cfg.ReadTimeout))
+		}
+		if err == nil {
+			err = connCheck(conn)
+		}
+		if err != nil {
+			mc.cfg.Logger.Print("closing bad idle connection: ", err)
+			return driver.ErrBadConn
+		}
+	}
+
 	return nil
 }
 

packets.go

@@ -98,34 +98,6 @@ func (mc *mysqlConn) writePacket(data []byte) error {
 		return ErrPktTooLarge
 	}
 
-	// Perform a stale connection check. We only perform this check for
-	// the first query on a connection that has been checked out of the
-	// connection pool: a fresh connection from the pool is more likely
-	// to be stale, and it has not performed any previous writes that
-	// could cause data corruption, so it's safe to return ErrBadConn
-	// if the check fails.
-	if mc.reset {
-		mc.reset = false
-		conn := mc.netConn
-		if mc.rawConn != nil {
-			conn = mc.rawConn
-		}
-		var err error
-		if mc.cfg.CheckConnLiveness {
-			if mc.cfg.ReadTimeout != 0 {
-				err = conn.SetReadDeadline(time.Now().Add(mc.cfg.ReadTimeout))
-			}
-			if err == nil {
-				err = connCheck(conn)
-			}
-		}
-		if err != nil {
-			mc.cfg.Logger.Print("closing bad idle connection: ", err)
-			mc.Close()
-			return driver.ErrBadConn
-		}
-	}
-
 	for {
 		var size int
 		if pktLen >= maxPacketSize {