Fix hearbeatWriter Close being stuck if waiting for a semi-sync ACK

go/mysql/fakesqldb/server.go

@@ -419,7 +419,11 @@ func (db *DB) HandleQuery(c *mysql.Conn, query string, callback func(*sqltypes.R
 		if pat.expr.MatchString(query) {
 			userCallback, ok := db.queryPatternUserCallback[pat.expr]
 			if ok {
+				// Since the user call back can be indefinitely stuck, we shouldn't hold the lock indefinitely.
+				// This is only test code, so no actual cause for concern.
+				db.mu.Unlock()
 				userCallback(query)
+				db.mu.Lock()
 			}
 			if pat.err != "" {
 				return fmt.Errorf(pat.err)

go/vt/vttablet/tabletserver/repltracker/writer.go

@@ -59,6 +59,7 @@ type heartbeatWriter struct {
 	appPool      *dbconnpool.ConnectionPool
 	allPrivsPool *dbconnpool.ConnectionPool
 	ticks        *timer.Timer
+	writeConnID  atomic.Int64
 
 	onDemandDuration            time.Duration
 	onDemandMu                  sync.Mutex
@@ -90,6 +91,7 @@ func newHeartbeatWriter(env tabletenv.Env, alias *topodatapb.TabletAlias) *heart
 		appPool:      dbconnpool.NewConnectionPool("HeartbeatWriteAppPool", env.Exporter(), 2, mysqlctl.DbaIdleTimeout, 0, mysqlctl.PoolDynamicHostnameResolution),
 		allPrivsPool: dbconnpool.NewConnectionPool("HeartbeatWriteAllPrivsPool", env.Exporter(), 2, mysqlctl.DbaIdleTimeout, 0, mysqlctl.PoolDynamicHostnameResolution),
 	}
+	w.writeConnID.Store(-1)
 	if w.onDemandDuration > 0 {
 		// see RequestHeartbeats() for use of onDemandRequestTicks
 		// it's basically a mechanism to rate limit operation RequestHeartbeats().
@@ -192,11 +194,6 @@ func (w *heartbeatWriter) write() error {
 	defer w.env.LogError()
 	ctx, cancel := context.WithDeadline(context.Background(), w.now().Add(w.interval))
 	defer cancel()
-	allPrivsConn, err := w.allPrivsPool.Get(ctx)
-	if err != nil {
-		return err
-	}
-	defer allPrivsConn.Recycle()
 
 	upsert, err := w.bindHeartbeatVars(sqlUpsertHeartbeat)
 	if err != nil {
@@ -207,6 +204,8 @@ func (w *heartbeatWriter) write() error {
 		return err
 	}
 	defer appConn.Recycle()
+	w.writeConnID.Store(appConn.Conn.ID())
+	defer w.writeConnID.Store(-1)
 	_, err = appConn.Conn.ExecuteFetch(upsert, 1, false)
 	if err != nil {
 		return err
@@ -215,6 +214,9 @@ func (w *heartbeatWriter) write() error {
 }
 
 func (w *heartbeatWriter) recordError(err error) {
+	if err == nil {
+		return
+	}
 	w.errorLog.Errorf("%v", err)
 	writeErrors.Add(1)
 }
@@ -238,14 +240,63 @@ func (w *heartbeatWriter) enableWrites(enable bool) {
 			w.ticks.Start(w.writeHeartbeat)
 		}()
 	case false:
-		w.ticks.Stop()
+		// We stop the ticks in a separate go routine because it can block if the write is stuck on semi-sync ACKs.
+		// At the same time we try and kill the write that is in progress. We use the context and its cancellation
+		// for coordination between the two go-routines. In the end we will have guaranteed that the ticks have stopped
+		// and no write is in progress.
+		ctx, cancel := context.WithCancel(context.Background())
+		go func() {
+			w.ticks.Stop()
+			cancel()
+		}()
+		w.killWritesUntilStopped(ctx)
+
 		if w.onDemandDuration > 0 {
 			// Let the next RequestHeartbeats() go through
 			w.allowNextHeartbeatRequest()
 		}
 	}
 }
 
+// killWritesUntilStopped tries to kill the write in progress until the ticks have stopped.
+func (w *heartbeatWriter) killWritesUntilStopped(ctx context.Context) {
+	ticker := time.NewTicker(100 * time.Millisecond)
+	defer ticker.Stop()
+	for {
+		// Actually try to kill the query.
+		err := w.killWrite()
+		w.recordError(err)
+		select {
+		case <-ctx.Done():
+			// If the context has been cancelled, then we know that the ticks have stopped.
+			// This guarantees that there are no writes in progress, so there is nothing to kill.
+			return
+		case <-ticker.C:
+		}
+	}
+}
+
+// killWrite kills the write in progress (if any).
+func (w *heartbeatWriter) killWrite() error {
+	defer w.env.LogError()
+	writeId := w.writeConnID.Load()
+	if writeId == -1 {
+		return nil
+	}
+
+	ctx, cancel := context.WithDeadline(context.Background(), w.now().Add(w.interval))
+	defer cancel()
+	killConn, err := w.allPrivsPool.Get(ctx)
+	if err != nil {
+		log.Errorf("Kill conn didn't get connection :(")
+		return err
+	}
+	defer killConn.Recycle()
+
+	_, err = killConn.Conn.ExecuteFetch(fmt.Sprintf("kill %d", writeId), 1, false)
+	return err
+}
+
 // allowNextHeartbeatRequest ensures that the next call to RequestHeartbeats() passes through and
 // is not dropped.
 func (w *heartbeatWriter) allowNextHeartbeatRequest() {

go/vt/vttablet/tabletserver/repltracker/writer_test.go

@@ -17,7 +17,9 @@ limitations under the License.
 package repltracker
 
 import (
+	"context"
 	"fmt"
+	"sync"
 	"testing"
 	"time"
 
@@ -65,6 +67,48 @@ func TestWriteHeartbeatError(t *testing.T) {
 	assert.Equal(t, int64(1), writeErrors.Get())
 }
 
+// TestCloseWhileStuckWriting tests that Close shouldn't get stuck even if the heartbeat writer is stuck waiting for a semi-sync ACK.
+func TestCloseWhileStuckWriting(t *testing.T) {
+	db := fakesqldb.New(t)
+	tw := newTestWriter(db, nil)
+	tw.isOpen = true
+
+	killWg := sync.WaitGroup{}
+	killWg.Add(1)
+	startedWaitWg := sync.WaitGroup{}
+	startedWaitWg.Add(1)
+
+	// Insert a query pattern that causes the upsert to block indefinitely until it has been killed.
+	// This simulates a stuck primary write due to a semi-sync ACK requirement.
+	db.AddQueryPatternWithCallback(`INSERT INTO .*heartbeat \(ts, tabletUid, keyspaceShard\).*`, &sqltypes.Result{}, func(s string) {
+		startedWaitWg.Done()
+		killWg.Wait()
+	})
+
+	// When we receive a kill query, we want to finish running the wait group to unblock the upsert query.
+	db.AddQueryPatternWithCallback("kill.*", &sqltypes.Result{}, func(s string) {
+		killWg.Done()
+	})
+
+	// Now we enable writes, but the first write will get blocked.
+	tw.enableWrites(true)
+	// We wait until the write has blocked to ensure we only call Close after we are stuck writing.
+	startedWaitWg.Wait()
+	// Even if the write is blocked, we should be able to disable writes without waiting indefinitely.
+	// This is what we call, when we try to Close the heartbeat writer.
+	ctx, cancel := context.WithCancel(context.Background())
+	go func() {
+		tw.enableWrites(false)
+		cancel()
+	}()
+	select {
+	case <-ctx.Done():
+		db.Close()
+	case <-time.After(1000 * time.Second):
+		t.Fatalf("Timed out waiting for heartbeat writer to close")
+	}
+}
+
 func newTestWriter(db *fakesqldb.DB, frozenTime *time.Time) *heartbeatWriter {
 	config := tabletenv.NewDefaultConfig()
 	config.ReplicationTracker.Mode = tabletenv.Heartbeat