server: make clientConn() thread-safe (#49073) (#49105)

ref #48224

server/conn.go

@@ -152,6 +152,7 @@ type clientConn struct {
 	lastActive    time.Time         // last active time
 	authPlugin    string            // default authentication plugin
 	isUnixSocket  bool              // connection is Unix Socket file
+	closeOnce     sync.Once         // closeOnce is used to make sure clientConn closes only once
 	rsEncoder     *resultEncoder    // rsEncoder is used to encode the string result to different charsets.
 	inputDecoder  *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
@@ -320,21 +321,33 @@ func (cc *clientConn) Close() error {
 }
 
 func closeConn(cc *clientConn, connections int) error {
-	metrics.ConnGauge.Set(float64(connections))
-	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.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))
+			}
+			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 {
+				err = ctx.Close()
+			}
 		}
-	}
-	// Close statements and session
-	// This will release advisory locks, row locks, etc.
-	if ctx := cc.getCtx(); ctx != nil {
-		return ctx.Close()
-	}
-	return nil
+	})
+	return err
 }
 
 func (cc *clientConn) closeWithoutLock() error {

server/conn_test.go

@@ -25,6 +25,7 @@ import (
 	"io"
 	"path/filepath"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -2025,3 +2026,44 @@ func TestLDAPAuthSwitch(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, []byte(mysql.AuthMySQLClearPassword), respAuthSwitch)
 }
+
+func TestCloseConn(t *testing.T) {
+	var outBuffer bytes.Buffer
+
+	store, _ := testkit.CreateMockStoreAndDomain(t)
+	cfg := newTestConfig()
+	cfg.Port = 0
+	cfg.Status.StatusPort = 0
+	drv := NewTiDBDriver(store)
+	server, err := NewServer(cfg, drv)
+	require.NoError(t, err)
+
+	cc := &clientConn{
+		connectionID: 0,
+		salt: []byte{
+			0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A,
+			0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0x10, 0x11, 0x12, 0x13, 0x14,
+		},
+		server: server,
+		pkt: &packetIO{
+			bufWriter: bufio.NewWriter(&outBuffer),
+		},
+		collation:  mysql.DefaultCollationID,
+		peerHost:   "localhost",
+		alloc:      arena.NewAllocator(512),
+		chunkAlloc: chunk.NewAllocator(),
+		capability: mysql.ClientProtocol41,
+	}
+
+	var wg sync.WaitGroup
+	const numGoroutines = 10
+	wg.Add(numGoroutines)
+	for i := 0; i < numGoroutines; i++ {
+		go func() {
+			defer wg.Done()
+			err := closeConn(cc, 1)
+			require.NoError(t, err)
+		}()
+	}
+	wg.Wait()
+}