internal/grpcsync: Provide an internal-only pub-sub type API

internal/grpcsync/pubsub.go

@@ -0,0 +1,136 @@
+/*
+ *
+ * Copyright 2023 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package grpcsync
+
+import (
+	"context"
+	"sync"
+)
+
+// Subscriber represents an entity that is subscribed to messages published on
+// a PubSub. It wraps the callback to be invoked by the PubSub when a new
+// message is published.
+type Subscriber interface {
+	// OnMessage is invoked when a new message is published. Implementations
+	// must not block in this method.
+	OnMessage(msg interface{})
+}
+
+// PubSub is a simple one-to-many publish-subscribe system that supports
+// messages of arbitrary type. It guarantees that messages are delivered in
+// the same order in which they were published.
+//
+// Publisher invokes the Publish() method to publish new messages, while
+// subscribers interested in receiving these messages register a callback
+// via the Subscribe() method.
+//
+// Once a PubSub is stopped, no more messages can be published, and
+// it is guaranteed that no more subscriber callback will be invoked.
+type PubSub struct {
+	cs     *CallbackSerializer
+	cancel context.CancelFunc
+
+	// Access to the below fields are guarded by this mutex.
+	mu          sync.Mutex
+	msg         interface{}
+	subscribers map[Subscriber]bool
+	stopped     bool
+}
+
+// NewPubSub returns a new PubSub instance.
+func NewPubSub() *PubSub {
+	ctx, cancel := context.WithCancel(context.Background())
+	return &PubSub{
+		cs:          NewCallbackSerializer(ctx),
+		cancel:      cancel,
+		subscribers: map[Subscriber]bool{},
+	}
+}
+
+// Subscribe registers the provided Subscriber to the PubSub.
+//
+// If the PubSub contains a previously published message, the Subscriber's
+// OnMessage() callback will be invoked asynchronously with the existing
+// message to begin with, and subsequently for every newly published message.
+//
+// The caller is responsible for invoking the returned cancel function to
+// unsubscribe itself from the PubSub.
+func (ps *PubSub) Subscribe(sub Subscriber) (cancel func()) {
+	ps.mu.Lock()
+	defer ps.mu.Unlock()
+
+	if ps.stopped {
+		return func() {}
+	}
+
+	ps.subscribers[sub] = true
+
+	if ps.msg != nil {
+		msg := ps.msg
+		ps.cs.Schedule(func(context.Context) {
+			ps.mu.Lock()
+			defer ps.mu.Unlock()
+			if !ps.subscribers[sub] {
+				return
+			}
+			sub.OnMessage(msg)
+		})
+	}
+
+	return func() {
+		ps.mu.Lock()
+		defer ps.mu.Unlock()
+		delete(ps.subscribers, sub)
+	}
+}
+
+// Publish publishes the provided message to the PubSub, and invokes
+// callbacks registered by subscribers asynchronously.
+func (ps *PubSub) Publish(msg interface{}) {
+	ps.mu.Lock()
+	defer ps.mu.Unlock()
+
+	if ps.stopped {
+		return
+	}
+
+	ps.msg = msg
+	for sub := range ps.subscribers {
+		s := sub
+		ps.cs.Schedule(func(context.Context) {
+			ps.mu.Lock()
+			defer ps.mu.Unlock()
+			if !ps.subscribers[s] {
+				return
+			}
+			s.OnMessage(msg)
+		})
+	}
+}
+
+// Stop shuts down the PubSub and releases any resources allocated by it.
+// It is guaranteed that no subscriber callbacks would be invoked once this
+// method returns.
+func (ps *PubSub) Stop() {
+	ps.mu.Lock()
+	defer ps.mu.Unlock()
+	ps.stopped = true
+
+	ps.cancel()
+}

internal/grpcsync/pubsub_test.go

@@ -0,0 +1,211 @@
+/*
+ *
+ * Copyright 2023 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package grpcsync
+
+import (
+	"fmt"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+type testSubscriber struct {
+	mu      sync.Mutex
+	msgs    []int
+	onMsgCh chan struct{}
+}
+
+func newTestSubscriber(chSize int) *testSubscriber {
+	return &testSubscriber{onMsgCh: make(chan struct{}, chSize)}
+}
+
+func (ts *testSubscriber) OnMessage(msg interface{}) {
+	ts.mu.Lock()
+	defer ts.mu.Unlock()
+	ts.msgs = append(ts.msgs, msg.(int))
+	select {
+	case ts.onMsgCh <- struct{}{}:
+	default:
+	}
+}
+
+func (ts *testSubscriber) receivedMsgs() []int {
+	ts.mu.Lock()
+	defer ts.mu.Unlock()
+
+	msgs := make([]int, len(ts.msgs))
+	copy(msgs, ts.msgs)
+
+	return msgs
+}
+
+func (s) TestPubSub_PublishNoMsg(t *testing.T) {
+	pubsub := NewPubSub()
+	defer pubsub.Stop()
+
+	ts := newTestSubscriber(1)
+	pubsub.Subscribe(ts)
+
+	select {
+	case <-ts.onMsgCh:
+		t.Fatalf("Subscriber callback invoked when no message was published")
+	case <-time.After(defaultTestShortTimeout):
+	}
+}
+
+func (s) TestPubSub_PublishMsgs_RegisterSubs_And_Stop(t *testing.T) {
+	pubsub := NewPubSub()
+
+	const numPublished = 10
+
+	ts1 := newTestSubscriber(numPublished)
+	pubsub.Subscribe(ts1)
+	wantMsgs1 := []int{}
+
+	var wg sync.WaitGroup
+	wg.Add(2)
+	// Publish ten messages on the pubsub and ensure that they are received in order by the subscriber.
+	go func() {
+		for i := 0; i < numPublished; i++ {
+			pubsub.Publish(i)
+			wantMsgs1 = append(wantMsgs1, i)
+		}
+		wg.Done()
+	}()
+
+	isTimeout := false
+	go func() {
+		for i := 0; i < numPublished; i++ {
+			select {
+			case <-ts1.onMsgCh:
+			case <-time.After(defaultTestTimeout):
+				isTimeout = true
+			}
+		}
+		wg.Done()
+	}()
+
+	wg.Wait()
+	if isTimeout {
+		t.Fatalf("Timeout when expecting the onMessage() callback to be invoked")
+	}
+	if gotMsgs1 := ts1.receivedMsgs(); !cmp.Equal(gotMsgs1, wantMsgs1) {
+		t.Fatalf("Received messages is %v, want %v", gotMsgs1, wantMsgs1)
+	}
+
+	// Register another subscriber and ensure that it receives the last published message.
+	ts2 := newTestSubscriber(numPublished)
+	pubsub.Subscribe(ts2)
+	wantMsgs2 := wantMsgs1[len(wantMsgs1)-1:]
+
+	select {
+	case <-ts2.onMsgCh:
+	case <-time.After(defaultTestShortTimeout):
+		t.Fatalf("Timeout when expecting the onMessage() callback to be invoked")
+	}
+	if gotMsgs2 := ts2.receivedMsgs(); !cmp.Equal(gotMsgs2, wantMsgs2) {
+		t.Fatalf("Received messages is %v, want %v", gotMsgs2, wantMsgs2)
+	}
+
+	wg.Add(3)
+	// Publish ten messages on the pubsub and ensure that they are received in order by the subscribers.
+	go func() {
+		for i := 0; i < numPublished; i++ {
+			pubsub.Publish(i)
+			wantMsgs1 = append(wantMsgs1, i)
+			wantMsgs2 = append(wantMsgs2, i)
+		}
+		wg.Done()
+	}()
+	errCh := make(chan error, 1)
+	go func() {
+		for i := 0; i < numPublished; i++ {
+			select {
+			case <-ts1.onMsgCh:
+			case <-time.After(defaultTestTimeout):
+				errCh <- fmt.Errorf("")
+			}
+		}
+		wg.Done()
+	}()
+	go func() {
+		for i := 0; i < numPublished; i++ {
+			select {
+			case <-ts2.onMsgCh:
+			case <-time.After(defaultTestTimeout):
+				errCh <- fmt.Errorf("")
+			}
+		}
+		wg.Done()
+	}()
+	wg.Wait()
+	select {
+	case <-errCh:
+		t.Fatalf("Timeout when expecting the onMessage() callback to be invoked")
+	default:
+	}
+	if gotMsgs1 := ts1.receivedMsgs(); !cmp.Equal(gotMsgs1, wantMsgs1) {
+		t.Fatalf("Received messages is %v, want %v", gotMsgs1, wantMsgs1)
+	}
+	if gotMsgs2 := ts2.receivedMsgs(); !cmp.Equal(gotMsgs2, wantMsgs2) {
+		t.Fatalf("Received messages is %v, want %v", gotMsgs2, wantMsgs2)
+	}
+
+	pubsub.Stop()
+
+	go func() {
+		pubsub.Publish(99)
+	}()
+	// Ensure that the subscriber callback is not invoked as instantiated
+	// pubsub has already closed.
+	select {
+	case <-ts1.onMsgCh:
+		t.Fatalf("The callback was invoked after pubsub being stopped")
+	case <-ts2.onMsgCh:
+		t.Fatalf("The callback was invoked after pubsub being stopped")
+	case <-time.After(defaultTestShortTimeout):
+	}
+}
+
+func (s) TestPubSub_PublishMsgs_BeforeRegisterSub(t *testing.T) {
+	pubsub := NewPubSub()
+	defer pubsub.Stop()
+
+	const numPublished = 3
+	for i := 0; i < numPublished; i++ {
+		pubsub.Publish(i)
+	}
+
+	ts := newTestSubscriber(numPublished)
+	pubsub.Subscribe(ts)
+
+	wantMsgs := []int{numPublished - 1}
+	// Ensure that the subscriber callback is invoked with a previously
+	// published message.
+	select {
+	case <-ts.onMsgCh:
+		if gotMsgs := ts.receivedMsgs(); !cmp.Equal(gotMsgs, wantMsgs) {
+			t.Fatalf("Received messages is %v, want %v", gotMsgs, wantMsgs)
+		}
+	case <-time.After(defaultTestShortTimeout):
+		t.Fatalf("Timeout when expecting the onMessage() callback to be invoked")
+	}
+}