server: return error on ping when in graceful shutdown (pingcap#58008)

ref pingcap#58007

errors.toml

@@ -2936,6 +2936,11 @@ error = '''
 Access denied for user '%-.48s'@'%-.255s' (using password: %s)
 '''
 
+["server:1053"]
+error = '''
+Server shutdown in progress
+'''
+
 ["server:1148"]
 error = '''
 The used command is not allowed with this MySQL version

pkg/server/conn.go

@@ -1390,7 +1390,10 @@ func (cc *clientConn) dispatch(ctx context.Context, data []byte) error {
 		return cc.writeStats(ctx)
 	// ComProcessInfo, ComConnect, ComProcessKill, ComDebug
 	case mysql.ComPing:
-		return cc.writeOK(ctx)
+		if cc.server.health.Load() {
+			return cc.writeOK(ctx)
+		}
+		return servererr.ErrServerShutdown
 	case mysql.ComChangeUser:
 		return cc.handleChangeUser(ctx, data)
 	// ComBinlogDump, ComTableDump, ComConnectOut, ComRegisterSlave

pkg/server/conn_test.go

@@ -613,6 +613,11 @@ func testDispatch(t *testing.T, inputs []dispatchInput, capability uint32) {
 	cfg.Status.ReportStatus = false
 	server, err := NewServer(cfg, tidbdrv)
 	require.NoError(t, err)
+
+	// Set healthy. This is used by graceful shutdown
+	// and is used in the response for ComPing and the
+	// /status HTTP endpoint
+	server.health.Store(true)
 	defer server.Close()
 
 	cc := &clientConn{

pkg/server/err/error.go

@@ -44,4 +44,6 @@ var (
 	ErrNetPacketTooLarge = dbterror.ClassServer.NewStd(errno.ErrNetPacketTooLarge)
 	// ErrMustChangePassword is returned when the user must change the password.
 	ErrMustChangePassword = dbterror.ClassServer.NewStd(errno.ErrMustChangePassword)
+	// ErrServerShutdown is returned when the server is shutting down.
+	ErrServerShutdown = dbterror.ClassServer.NewStd(errno.ErrServerShutdown)
 )