plugin: fix bug that watch loop will refresh frequently when channel closed (#49275)

plugin/BUILD.bazel

@@ -47,6 +47,7 @@ go_test(
         "//testkit",
         "//testkit/testsetup",
         "@com_github_stretchr_testify//require",
+        "@io_etcd_go_etcd_client_v3//:client",
         "@org_uber_go_goleak//:goleak",
     ],
 )

plugin/plugin.go

@@ -20,6 +20,7 @@ import (
 	gplugin "plugin"
 	"strconv"
 	"sync/atomic"
+	"time"
 	"unsafe"
 
 	"github.com/pingcap/errors"
@@ -272,14 +273,32 @@ func (w *flushWatcher) refreshPluginState() error {
 	}
 	return nil
 }
-
 func (w *flushWatcher) watchLoop() {
-	watchChan := w.etcd.Watch(w.ctx, w.path)
+	const reWatchInterval = time.Second * 5
+	logutil.BgLogger().Info("plugin flushWatcher loop started", zap.String("plugin", w.manifest.Name))
+	for w.ctx.Err() == nil {
+		ch := w.etcd.Watch(w.ctx, w.path)
+		if exit := w.watchLoopWithChan(ch); exit {
+			break
+		}
+
+		logutil.BgLogger().Info(
+			"plugin flushWatcher old chan closed, restart loop later",
+			zap.String("plugin", w.manifest.Name),
+			zap.Duration("after", reWatchInterval))
+	}
+}
+
+func (w *flushWatcher) watchLoopWithChan(ch clientv3.WatchChan) (exit bool) {
 	for {
 		select {
 		case <-w.ctx.Done():
-			return
-		case <-watchChan:
+			return true
+		case _, ok := <-ch:
+			if !ok {
+				return false
+			}
+			logutil.BgLogger().Info("plugin flushWatcher detected event to reload plugin config", zap.String("plugin", w.manifest.Name))
 			_ = w.refreshPluginState()
 		}
 	}

plugin/plugin_test.go

@@ -18,10 +18,13 @@ import (
 	"context"
 	"io"
 	"strconv"
+	"sync/atomic"
 	"testing"
+	"time"
 
 	"github.com/pingcap/tidb/sessionctx/variable"
 	"github.com/stretchr/testify/require"
+	clientv3 "go.etcd.io/etcd/client/v3"
 )
 
 func TestLoadPluginSuccess(t *testing.T) {
@@ -242,3 +245,43 @@ func TestPluginsClone(t *testing.T) {
 	require.Equal(t, uint16(1), cps.versions["whitelist"])
 	require.Len(t, cps.dyingPlugins, 1)
 }
+
+func TestPluginWatcherLoop(t *testing.T) {
+	// exit when ctx done
+	ctx, cancel := context.WithCancel(context.Background())
+	watcher := &flushWatcher{
+		ctx: ctx,
+		manifest: &Manifest{
+			Name: "test",
+		},
+	}
+	ch := make(chan clientv3.WatchResponse)
+	var cancelled atomic.Bool
+	go func() {
+		time.Sleep(10 * time.Millisecond)
+		cancelled.Store(true)
+		cancel()
+	}()
+	exit := watcher.watchLoopWithChan(ch)
+	require.True(t, exit)
+	require.True(t, cancelled.Load())
+
+	// exit when ch closed
+	watcher = &flushWatcher{
+		ctx: context.Background(),
+		manifest: &Manifest{
+			Name: "test",
+		},
+	}
+
+	var closed atomic.Bool
+	ch = make(chan clientv3.WatchResponse)
+	go func() {
+		time.Sleep(10 * time.Millisecond)
+		closed.Store(true)
+		close(ch)
+	}()
+	exit = watcher.watchLoopWithChan(ch)
+	require.False(t, exit)
+	require.True(t, cancelled.Load())
+}