I was hoping to trigger a Pub/Sub function (using functions.pubsub / onPublish) whenever a new Pub/Sub message is sent to a topic/subscription in a third-party project i.e. cross projects.
After some research and experimentation I found that TopicBuilder throws an error if the topic name contains a / and it defaults to "projects/" + process.env.GCLOUD_PROJECT + "/topics/" + topic (https://github.com/firebase/firebase-functions/blob/master/src/providers/pubsub.ts).
I also found a post in Stack Overflow that says that "Firebase provides a (relatively thin) wrapper around Google Cloud Functions" (What is the difference between Cloud Function and Firebase Functions?)
This led me to look into Google Cloud Functions. Whilst I was able to create a subscription in a project I own to a topic in a third-party project - after changing permissions in IAM - I could not find a way associate a function with the topic. Nor was I successful in associating a function with a topic and subscription in a third-party project. In the console I only see the topics in my project and I had no success using gcloud.
Has anyone had any success in using a function across projects and, if so, how did you achieve this and is there a documentation URL you could provide? If a function can't be triggered by a message to a topic and subscription in a third-party project can you think of a way that I could ingest third-party Pub/Sub data?
As Pub/Sub fees are billed to the project that contains the subscription I would prefer that the subscription resides in the third-party project with the topic.
Thank you
