feat(messaging): migrate topic subscriptions to FCM v1 API

[lahirumaramba]

Migrate subscribeToTopic and unsubscribeFromTopic methods in admin.messaging.Messaging from legacy Instance ID (IID) endpoints (iid.googleapis.com/iid/v1) to the modernized single-token Firebase Cloud Messaging (FCM) v1 Topics Subscription REST API (fcm.googleapis.com/v1).

• Update subscribeToTopic (CreateTopicSubscription) and unsubscribeFromTopic (DeleteTopicSubscription) to use HTTP/2 transport by default (or HTTP/1.1 via enableLegacyHttpTransport()).
• Implement client-side concurrency pooling (runWithConcurrencyLimit) bounded to 100 concurrent requests per batch to avoid HTTP/2 stream exhaustion.
• Preserve idempotency by treating ALREADY_EXISTS (HTTP 409) as success for subscription requests, and NOT_FOUND (HTTP 404) as REGISTRATION_TOKEN_NOT_REGISTERED for unsubscription requests.
• Add legacy backup/escape-hatch methods subscribeToTopicLegacy and unsubscribeFromTopicLegacy annotated with @deprecated Use {@link Messaging.subscribeToTopic} instead. and @deprecated Use {@link Messaging.unsubscribeFromTopic} instead..

[wiz-9635d3485b]

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[gemini-code-assist]

Code Review

This pull request migrates the topic subscription and unsubscription management in the Firebase Messaging service to the FCM v1 API, while preserving the old behavior under deprecated legacy methods. It introduces concurrent request handling with a limit for both HTTP/1.1 and HTTP/2 transports. The review feedback highlights a potential runtime TypeError when parsing server error responses if the error field is null, and identifies an opportunity to reduce code duplication in the task mapping logic for HTTP/1.1 and HTTP/2 requests.

[gemini-code-assist]

If the server returns a JSON response where the error field is explicitly null (e.g., { "error": null }), evaluating (json as any).error.message will throw a TypeError: Cannot read properties of null (reading 'message') and crash the process. Adding a check to ensure json.error is a non-null object before accessing its properties prevents this potential runtime exception.

Suggested change

	const errorMessage = (validator.isNonNullObject(json) && 'error' in json && validator.isNonEmptyString((json as any).error.message))
	? (json as any).error.message
	: undefined;
	const errorMessage = (validator.isNonNullObject(json) && 'error' in json && validator.isNonNullObject((json as any).error) && validator.isNonEmptyString((json as any).error.message))
	? (json as any).error.message
	: undefined;

[gemini-code-assist]

The path construction and task mapping logic is identical between the HTTP/2 and HTTP/1.1 code paths. We can refactor this to construct the tasks array once and conditionally invoke the appropriate request handler inside the task function. This eliminates significant code duplication and improves maintainability.

        return this.getProjectId().then((projectId) => {
          const tasks = registrationTokensArray.map((token) => async () => {
            const path = methodName === 'subscribeToTopic'
              ? '/v1/projects/' + projectId + '/registrations/' + encodeURIComponent(token) + '/topicSubscriptions?topic_name=' + encodeURIComponent(topicName)
              : '/v1/projects/' + projectId + '/registrations/' + encodeURIComponent(token) + '/topicSubscriptions/' + encodeURIComponent(topicName) + '?allow_missing=true';
            const requestData = methodName === 'subscribeToTopic' ? {} : undefined;

            if (http2SessionHandler) {
              return this.messagingRequestHandler.invokeHttp2RequestHandlerForTopicSubscriptionResponse(
                FCM_SEND_HOST, path, methodName, requestData, http2SessionHandler,
              );
            } else {
              return this.messagingRequestHandler.invokeHttpRequestHandlerForTopicSubscriptionResponse(
                FCM_SEND_HOST, path, methodName, requestData,
              );
            }
          });
          return runWithConcurrencyLimit(tasks, 100);
        })