server: make clientConn() thread-safe

pkg/server/conn.go

@@ -169,8 +169,9 @@ type clientConn struct {
 	lastActive    time.Time             // last active time
 	authPlugin    string                // default authentication plugin
 	isUnixSocket  bool                  // connection is Unix Socket file
-	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.
+	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
 	// mu is used for cancelling the execution of current transaction.
 	mu struct {
@@ -349,28 +350,31 @@ func (cc *clientConn) Close() error {
 	return closeConn(cc, connections)
 }
 
-// closeConn should be idempotent.
+// 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 {
-	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

@@ -25,6 +25,7 @@ import (
 	"io"
 	"path/filepath"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -2052,3 +2053,42 @@ func TestStats(t *testing.T) {
 	require.Equal(t, "zstd", m["Compression_algorithm"])
 	require.Equal(t, 1, m["Compression_level"])
 }
+
+func TestCloseConn(t *testing.T) {
+	var outBuffer bytes.Buffer
+
+	store, _ := testkit.CreateMockStoreAndDomain(t)
+	cfg := serverutil.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:        internal.NewPacketIOForTest(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()
+}