fix spurious errors when closing inactive connections (#408)

When a connection is closed or reset before it is properly established, the
existing ::getSession will log a spurious error since the connection has no
attributes to extract for the pool type.

Fix this by refactoring so that when closing a connection we only act on the
session object if one already existed, i.e. the conn was used before.

go/vt/vtgateproxy/mysql_server.go

@@ -99,17 +99,7 @@ func (ph *proxyHandler) NewConnection(c *mysql.Conn) {
 
 func (ph *proxyHandler) ComResetConnection(c *mysql.Conn) {
 	ctx := context.Background()
-	session, err := ph.getSession(ctx, c)
-	if err != nil {
-		return
-	}
-	if session.SessionPb().InTransaction {
-		defer atomic.AddInt32(&busyConnections, -1)
-	}
-	err = ph.proxy.CloseSession(ctx, session)
-	if err != nil {
-		log.Errorf("Error happened in transaction rollback: %v", err)
-	}
+	ph.closeSession(ctx, c)
 }
 
 func (ph *proxyHandler) ConnectionClosed(c *mysql.Conn) {
@@ -127,14 +117,7 @@ func (ph *proxyHandler) ConnectionClosed(c *mysql.Conn) {
 	} else {
 		ctx = context.Background()
 	}
-	session, err := ph.getSession(ctx, c)
-	if err != nil {
-		return
-	}
-	if session.SessionPb().InTransaction {
-		defer atomic.AddInt32(&busyConnections, -1)
-	}
-	_ = ph.proxy.CloseSession(ctx, session)
+	ph.closeSession(ctx, c)
 }
 
 // Regexp to extract parent span id over the sql query
@@ -377,6 +360,23 @@ func (ph *proxyHandler) getSession(ctx context.Context, c *mysql.Conn) (*vtgatec
 	return session, nil
 }
 
+func (ph *proxyHandler) closeSession(ctx context.Context, c *mysql.Conn) {
+	session, _ := c.ClientData.(*vtgateconn.VTGateSession)
+	if session == nil {
+		return // no active session
+	}
+
+	if session.SessionPb().InTransaction {
+		defer atomic.AddInt32(&busyConnections, -1)
+	}
+	err := ph.proxy.CloseSession(ctx, session)
+	if err != nil {
+		log.Errorf("Error happened in transaction rollback: %v", err)
+	}
+
+	c.ClientData = nil
+}
+
 var mysqlListener *mysql.Listener
 var mysqlUnixListener *mysql.Listener
 var sigChan chan os.Signal