fix(storage): Make Writer thread-safe.

storage.Writer took an assumption that CloseWithError() could be called
more than once, and was thread-safe with respect to concurrent Write(),
Flush(), and Close() calls. This was not honored in the refactor in
#12422.

Modify Writer so that it is thread-safe to provide these behaviors, and
support repeated Close() and CloseWithError() calls.

To address this, we start the sender goroutine earlier, and gather the
first buffer in that goroutine. It's possible that some workloads which
gather less than one buffer worth of data with a sequence of small
writes will observe a performance hit here, since those writes used to
be direct copies but will now be a channel ping-pong. If that's an
issue, it could be improved by wrapping the buffer in a mutex and doing
more explicit concurrency control.

Author: cjc25

storage/client_test.go

@@ -1490,6 +1490,115 @@ func TestWriterSmallFlushEmulated(t *testing.T) {
 	})
 }
 
+func TestWriterAsyncCancelEmulated(t *testing.T) {
+	transportClientTest(context.Background(), t, func(t *testing.T, ctx context.Context, project, bucket string, client storageClient) {
+		// Create test bucket.
+		_, err := client.CreateBucket(ctx, project, bucket, &BucketAttrs{
+			Name: bucket,
+		}, nil)
+		if err != nil {
+			t.Fatalf("client.CreateBucket: %v", err)
+		}
+		objName := fmt.Sprintf("object-%d", time.Now().Nanosecond())
+
+		vc := &Client{tc: client}
+
+		ctx, cancel := context.WithCancel(ctx)
+		defer cancel()
+
+		w := vc.Bucket(bucket).Object(objName).NewWriter(ctx)
+		defer w.Close()
+
+		if _, err := w.Write(randomBytes3MiB); err != nil {
+			t.Fatalf("first w.Write(): %v", err)
+		}
+		// Cancel concurrently with an additional write
+		go cancel()
+		// We don't actually care if this succeeds or fails - it will blow up under
+		// the race detector if writes and cancels are not thread-safe.
+		w.Write(randomBytes3MiB)
+		// Make sure the cancel got processed.
+		<-ctx.Done()
+
+		// The writer close should observe the cancelled error.
+		if err := w.Close(); err != context.Canceled {
+			t.Errorf("w.Close(): got %v, want %v", err, context.Canceled)
+		}
+	})
+}
+
+func TestWriterCloseTwiceEmulated(t *testing.T) {
+	transportClientTest(context.Background(), t, func(t *testing.T, ctx context.Context, project, bucket string, client storageClient) {
+		// Create test bucket.
+		_, err := client.CreateBucket(ctx, project, bucket, &BucketAttrs{
+			Name: bucket,
+		}, nil)
+		if err != nil {
+			t.Fatalf("client.CreateBucket: %v", err)
+		}
+		objName := fmt.Sprintf("object-%d", time.Now().Nanosecond())
+
+		vc := &Client{tc: client}
+		obj := vc.Bucket(bucket).Object(objName)
+		w := obj.NewWriter(ctx)
+		if err := w.Close(); err != nil {
+			t.Fatalf("closing writer: %v", err)
+		}
+		// Closing a writer twice is allowed!
+		if err := w.Close(); err != nil {
+			t.Fatalf("closing writer: %v", err)
+		}
+
+		// The object is present with 0 contents.
+		attrs, err := obj.Attrs(ctx)
+		if err != nil {
+			t.Fatalf("obj.Attrs: %v", err)
+		}
+		if attrs.Size != 0 {
+			t.Errorf("incorrect object size; got %v, want 0", attrs.Size)
+		}
+	})
+}
+
+func TestWriterCloseWithErrorTwiceEmulated(t *testing.T) {
+	transportClientTest(context.Background(), t, func(t *testing.T, ctx context.Context, project, bucket string, client storageClient) {
+		// Create test bucket.
+		_, err := client.CreateBucket(ctx, project, bucket, &BucketAttrs{
+			Name: bucket,
+		}, nil)
+		if err != nil {
+			t.Fatalf("client.CreateBucket: %v", err)
+		}
+		objName := fmt.Sprintf("object-%d", time.Now().Nanosecond())
+
+		vc := &Client{tc: client}
+		obj := vc.Bucket(bucket).Object(objName)
+		w := obj.NewWriter(ctx)
+		if _, err := w.Write(randomBytes3MiB); err != nil {
+			t.Errorf("w.Write(): %v", err)
+		}
+
+		errOne := errors.New("the first error")
+		if err := w.CloseWithError(errOne); err != nil {
+			// CloseWithError always returns nil
+			t.Fatalf("w.CloseWithError(errOne): %v", err)
+		}
+		if err := w.Close(); err != errOne {
+			t.Errorf("first w.Close(); got %v, want %v", err, errOne)
+		}
+
+		errTwo := errors.New("the second error")
+		if err := w.CloseWithError(errTwo); err != nil {
+			// CloseWithError always returns nil
+			t.Fatalf("w.CloseWithError(errOne): %v", err)
+		}
+		// The error is _not_ replaced by a subsequent call.
+		if err := w.Close(); err != errOne {
+			t.Errorf("second w.Close(); got %v, want %v", err, errTwo)
+		}
+	})
+}
+
 // customObjSizeReadStream intercepts BidiReadObjectResponse messages and
 // changes the object size in the BidiReadObjectResponse.Metadata to
 // customRecvSize.