address the comment

pkg/server/conn.go

@@ -169,7 +169,7 @@ type clientConn struct {
 	lastActive    time.Time             // last active time
 	authPlugin    string                // default authentication plugin
 	isUnixSocket  bool                  // connection is Unix Socket file
-	isClosed      int32                 // atomic variable to track whether the connection is closed
+	closeOnce     sync.Once             // closeOnce is used to make sure clientConn closes only once
 	rsEncoder     *column.ResultEncoder // rsEncoder is used to encode the string result to different charsets
 	inputDecoder  *util2.InputDecoder   // inputDecoder is used to decode the different charsets of incoming strings to utf-8
 	socketCredUID uint32                // UID from the other end of the Unix Socket
@@ -353,29 +353,28 @@ func (cc *clientConn) Close() error {
 // closeConn is idempotent and thread-safe.
 // It will be called on the same `clientConn` more than once to avoid connection leak.
 func closeConn(cc *clientConn, connections int) error {
-	if !atomic.CompareAndSwapInt32(&cc.isClosed, 0, 1) {
-		return nil
-	}
-
-	metrics.ConnGauge.Set(float64(connections))
-	if cc.connectionID > 0 {
-		cc.server.dom.ReleaseConnID(cc.connectionID)
-		cc.connectionID = 0
-	}
-	if cc.bufReadConn != nil {
-		err := cc.bufReadConn.Close()
-		if err != nil {
-			// We need to expect connection might have already disconnected.
-			// This is because closeConn() might be called after a connection read-timeout.
-			logutil.Logger(context.Background()).Debug("could not close connection", zap.Error(err))
+	var err error
+	cc.closeOnce.Do(func() {
+		metrics.ConnGauge.Set(float64(connections))
+		if cc.connectionID > 0 {
+			cc.server.dom.ReleaseConnID(cc.connectionID)
+			cc.connectionID = 0
+		}
+		if cc.bufReadConn != nil {
+			err := cc.bufReadConn.Close()
+			if err != nil {
+				// We need to expect connection might have already disconnected.
+				// This is because closeConn() might be called after a connection read-timeout.
+				logutil.Logger(context.Background()).Debug("could not close connection", zap.Error(err))
+			}
 		}
-	}
-	// Close statements and session
-	// This will release advisory locks, row locks, etc.
-	if ctx := cc.getCtx(); ctx != nil {
-		return ctx.Close()
-	}
-	return nil
+		// Close statements and session
+		// This will release advisory locks, row locks, etc.
+		if ctx := cc.getCtx(); ctx != nil {
+			err = ctx.Close()
+		}
+	})
+	return err
 }
 
 func (cc *clientConn) closeWithoutLock() error {

pkg/server/conn_test.go

@@ -2091,5 +2091,4 @@ func TestCloseConn(t *testing.T) {
 		}()
 	}
 	wg.Wait()
-	require.Equalf(t, int32(1), cc.isClosed, "Expected isClosed to be 1, got 0")
 }