We are using a cloud function triggered by Pub/Sub to ensure delivery of an e-mail. Sometimes the e-mail service takes a long time to respond and our cloud function terminates before we get an error back. Since the message has already been acknowledged our e-mail gets lost.
The cloud function appears to be sending an ACK the Pub/Sub message automatically when we are called. Is there a way to delay the ACK until the successful completion of our code? Alternatively is there a way to catch timeouts and requeue the message for delivery? Something else we could try?
I heard from Google support that they do not currently provide the means to delay the ACK when a cloud function is invoked by Pub/Sub. If you want to use cloud functions with Pub/Sub you need to handle the error case yourself. For example you could have your cloud function requeue a message for the retry with a retry count.
This would seem to make it unnecessarily difficult to guarantee execution with Pub/Sub and cloud functions.
来源：https://stackoverflow.com/questions/43941315/google-cloud-functions-to-only-ack-pub-sub-on-success