fix: avoid deadlock in publisher and subscriber

db.go

@@ -2095,11 +2095,11 @@ func (db *DB) Subscribe(ctx context.Context, cb func(kv *KVList) error, matches
 	}
 
 	c := z.NewCloser(1)
-	recvCh, id := db.pub.newSubscriber(c, matches)
+	s := db.pub.newSubscriber(c, matches)
 	slurp := func(batch *pb.KVList) error {
 		for {
 			select {
-			case kvs := <-recvCh:
+			case kvs := <-s.sendCh:
 				batch.Kv = append(batch.Kv, kvs.Kv...)
 			default:
 				if len(batch.GetKv()) > 0 {
@@ -2109,6 +2109,16 @@ func (db *DB) Subscribe(ctx context.Context, cb func(kv *KVList) error, matches
 			}
 		}
 	}
+
+	drain := func() {
+		for {
+			select {
+			case <- s.sendCh:
+			default:
+				return
+			}
+		}
+	}
 	for {
 		select {
 		case <-c.HasBeenClosed():
@@ -2120,15 +2130,19 @@ func (db *DB) Subscribe(ctx context.Context, cb func(kv *KVList) error, matches
 			return err
 		case <-ctx.Done():
 			c.Done()
-			db.pub.deleteSubscriber(id)
+			atomic.StoreUint64(s.active, 0)
+			drain()
+			db.pub.deleteSubscriber(s.id)
 			// Delete the subscriber to avoid further updates.
 			return ctx.Err()
-		case batch := <-recvCh:
+		case batch := <-s.sendCh:
 			err := slurp(batch)
 			if err != nil {
 				c.Done()
+				atomic.StoreUint64(s.active, 0)
+				drain()
 				// Delete the subscriber if there is an error by the callback.
-				db.pub.deleteSubscriber(id)
+				db.pub.deleteSubscriber(s.id)
 				return err
 			}
 		}

publisher.go

@@ -18,6 +18,7 @@ package badger
 
 import (
 	"sync"
+	"sync/atomic"
 
 	"github.com/dgraph-io/badger/v3/pb"
 	"github.com/dgraph-io/badger/v3/trie"
@@ -26,9 +27,13 @@ import (
 )
 
 type subscriber struct {
+	id uint64
 	matches   []pb.Match
-	sendCh    chan<- *pb.KVList
+	sendCh    chan *pb.KVList
 	subCloser *z.Closer
+	// this will be atomic pointer which will be used to
+	// track whether the subscriber is active or not
+	active    *uint64
 }
 
 type publisher struct {
@@ -106,26 +111,32 @@ func (p *publisher) publishUpdates(reqs requests) {
 	}
 
 	for id, kvs := range batchedUpdates {
-		p.subscribers[id].sendCh <- kvs
+		if atomic.LoadUint64(p.subscribers[id].active) == 1 {
+			p.subscribers[id].sendCh <- kvs
+		}
 	}
 }
 
-func (p *publisher) newSubscriber(c *z.Closer, matches []pb.Match) (<-chan *pb.KVList, uint64) {
+func (p *publisher) newSubscriber(c *z.Closer, matches []pb.Match) subscriber {
 	p.Lock()
 	defer p.Unlock()
 	ch := make(chan *pb.KVList, 1000)
 	id := p.nextID
 	// Increment next ID.
 	p.nextID++
-	p.subscribers[id] = subscriber{
+	active := uint64(1)
+	s := subscriber{
+		active: &active,
+		id: id,
 		matches:   matches,
 		sendCh:    ch,
 		subCloser: c,
 	}
+	p.subscribers[id] = s
 	for _, m := range matches {
 		p.indexer.AddMatch(m, id)
 	}
-	return ch, id
+	return s
 }
 
 // cleanSubscribers stops all the subscribers. Ideally, It should be called while closing DB.

publisher_test.go

@@ -18,14 +18,69 @@ package badger
 import (
 	"context"
 	"fmt"
+	"github.com/pkg/errors"
 	"sync"
+	"sync/atomic"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/require"
 
 	"github.com/dgraph-io/badger/v3/pb"
 )
 
+// This test will result in deadlock for commits before this.
+// Exiting this test gracefully will be the proof that the
+// publisher is no longer stuck in deadlock.
+func TestPublisherDeadlock(t *testing.T) {
+	runBadgerTest(t, nil, func(t *testing.T, db *DB) {
+		var subWg sync.WaitGroup
+		subWg.Add(1)
+
+		var firstUpdate sync.WaitGroup
+		firstUpdate.Add(1)
+
+
+		var subDone sync.WaitGroup
+		subDone.Add(1)
+		go func() {
+			subWg.Done()
+			match := pb.Match{Prefix: []byte("ke"), IgnoreBytes: ""}
+			db.Subscribe(context.Background(), func(kvs *pb.KVList) error {
+				firstUpdate.Done()
+				time.Sleep(time.Second * 20)
+				return errors.New("error returned")
+			}, []pb.Match{match})
+			subDone.Done()
+		}()
+		subWg.Wait()
+		go db.Update(func(txn *Txn) error {
+			e := NewEntry([]byte(fmt.Sprintf("key%d", 0)), []byte(fmt.Sprintf("value%d", 0)))
+			return txn.SetEntry(e)
+		})
+
+		firstUpdate.Wait()
+		req := int64(0)
+		for i := 1; i < 1110; i++ {
+			time.Sleep(time.Millisecond * 10)
+			go func(i int) {
+				db.Update(func(txn *Txn) error {
+					e := NewEntry([]byte(fmt.Sprintf("key%d", i)), []byte(fmt.Sprintf("value%d", i)))
+					return txn.SetEntry(e)
+				})
+				atomic.AddInt64(&req, 1)
+			}(i)
+		}
+		for {
+			if atomic.LoadInt64(&req) == 1109 {
+				break
+			}
+			time.Sleep(time.Second)
+		}
+		subDone.Wait()
+	})
+}
+
 func TestPublisherOrdering(t *testing.T) {
 	runBadgerTest(t, nil, func(t *testing.T, db *DB) {
 		order := []string{}