feat(pubsublite): set finalizer for PublisherClient (googleapis#7109)

Mitigates `PublisherClient.Stop` not being called when the publisher is no longer used. Users must still call `Stop` to promptly shut down the publisher, as finalizers run after an arbitrary amount of time.

pubsublite/doc.go

@@ -187,7 +187,7 @@ client that may be able to handle messages.
 See https://cloud.google.com/pubsub/lite/docs/subscribing for more information
 about receiving messages.
 
-gRPC Connection Pools
+# gRPC Connection Pools
 
 Pub/Sub Lite utilizes gRPC streams extensively. gRPC allows a maximum of 100
 streams per connection. Internally, the library uses a default connection pool

pubsublite/pscompat/integration_test.go

@@ -17,6 +17,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"runtime"
 	"sort"
 	"strings"
 	"sync"
@@ -590,6 +591,32 @@ func TestIntegration_PublishSubscribeSinglePartition(t *testing.T) {
 		}
 	})
 
+	// Verifies that publisher clients are not stopped while still in use.
+	t.Run("Finalizer", func(t *testing.T) {
+		publisher := publisherClient(context.Background(), t, DefaultPublishSettings, topicPath)
+		runtime.GC() // Publisher should not be stopped
+
+		result := publisher.Publish(ctx, &pubsub.Message{Data: []byte("finalizer1")})
+		runtime.GC() // Publisher should not be stopped
+		if _, err := result.Get(ctx); err != nil {
+			t.Errorf("Publish() got err: %v", err)
+		}
+
+		result = publisher.Publish(ctx, &pubsub.Message{Data: []byte("finalizer2")})
+		// The finalizer runs during the next GC. Publish should still succeed
+		// because Stop flushes outstanding messages and waits for publish responses
+		// before closing connections.
+		runtime.GC()
+		if _, err := result.Get(ctx); err != nil {
+			t.Errorf("Publish() got err: %v", err)
+		}
+
+		// Explicitly clear the publisher reference, but the finalizer should have
+		// already been triggered.
+		publisher = nil
+		runtime.GC()
+	})
+
 	// Verifies that cancelling the context passed to NewSubscriberClient can shut
 	// down the subscriber.
 	t.Run("CancelSubscriberContext", func(t *testing.T) {

pubsublite/pscompat/publisher.go

@@ -15,6 +15,7 @@ package pscompat
 
 import (
 	"context"
+	"runtime"
 	"sync"
 
 	"cloud.google.com/go/pubsub"
@@ -73,6 +74,9 @@ type PublisherClient struct {
 // NewPublisherClient creates a new Pub/Sub Lite publisher client to publish
 // messages to a given topic, using DefaultPublishSettings. A valid topic path
 // has the format: "projects/PROJECT_ID/locations/LOCATION/topics/TOPIC_ID".
+//
+// Stop must be called to release resources when a PublisherClient is no longer
+// required.
 func NewPublisherClient(ctx context.Context, topic string, opts ...option.ClientOption) (*PublisherClient, error) {
 	return NewPublisherClientWithSettings(ctx, topic, DefaultPublishSettings, opts...)
 }
@@ -81,6 +85,9 @@ func NewPublisherClient(ctx context.Context, topic string, opts ...option.Client
 // publish messages to a given topic, using the specified PublishSettings. A
 // valid topic path has the format:
 // "projects/PROJECT_ID/locations/LOCATION/topics/TOPIC_ID".
+//
+// Stop must be called to release resources when a PublisherClient is no longer
+// required.
 func NewPublisherClientWithSettings(ctx context.Context, topic string, settings PublishSettings, opts ...option.ClientOption) (*PublisherClient, error) {
 	topicPath, err := wire.ParseTopicPath(topic)
 	if err != nil {
@@ -99,7 +106,16 @@ func NewPublisherClientWithSettings(ctx context.Context, topic string, settings
 	if err := wirePub.WaitStarted(); err != nil {
 		return nil, err
 	}
-	return &PublisherClient{settings: settings, wirePub: wirePub}, nil
+	publisher := &PublisherClient{settings: settings, wirePub: wirePub}
+
+	// Mitigation for Stop not being called when the publisher client is no longer
+	// used. Users must still call Stop to promptly shut down the publisher, as
+	// finalizers run after an arbitrary amount of time.
+	runtime.SetFinalizer(publisher, func(p *PublisherClient) {
+		// TODO: Log a warning.
+		go p.wirePub.Stop()
+	})
+	return publisher, nil
 }
 
 // Publish publishes `msg` to the topic asynchronously. Messages are batched and
@@ -136,13 +152,15 @@ func (p *PublisherClient) Publish(ctx context.Context, msg *pubsub.Message) *pub
 			ipubsub.SetPublishResult(result, "", err)
 		}
 	})
+	runtime.KeepAlive(p) // Delay finalizers up to this point
 	return result
 }
 
 // Stop sends all remaining published messages and closes publish streams.
 // Returns once all outstanding messages have been sent or have failed to be
 // sent. Stop should be called when the client is no longer required.
 func (p *PublisherClient) Stop() {
+	runtime.SetFinalizer(p, nil)
 	p.wirePub.Stop()
 	p.wirePub.WaitStopped()
 }