feat: support Eventarc CE_PUBSUB_BINDING deliveries for CloudEvent functions

[dylayed]

Summary

Eventarc Firestore triggers push to https://<service>?__GCP_CloudEventsMode=CE_PUBSUB_BINDING. Normally the request is converted to HTTP binary content mode (ce-* headers) before it reaches the function. When an attribute can't be sent as an HTTP header, it isn't converted: the function receives the raw Pub/Sub push envelope, with the CloudEvent in binary content mode of the Google Cloud Pub/Sub protocol binding (ce--prefixed attributes, data in message.data).

The known trigger is a Firestore document ID with a trailing space, which puts a trailing space at the end of ce-subject and ce-document (firebase/firebase-functions#1922). FF handles the envelope as a plain Pub/Sub message and delivers a google.cloud.pubsub.topic.v1.messagePublished event with source: //pubsub.googleapis.com/. With firebase-functions 7.4.0 the handler now runs with params: {} and a non-existent snapshot, and the event is acked.

This adds a middleware, for the cloudevent signature type only, that decodes the envelope into the CloudEvent it carries and hands it to the existing structured-mode path. It's an alternative to #786, which spoofs HTTP headers and also matches unprefixed type/source attributes.

Scope

The middleware only applies when all of these hold. Everything else is left exactly as today:

• the query string has __GCP_CloudEventsMode=CE_PUBSUB_BINDING
• there are no ce-* HTTP headers (a binary-mode HTTP CloudEvent still takes precedence)
• the message has ce-specversion, ce-id, ce-source and ce-type attributes
• it is binary content mode (no content-type: application/cloudevents... attribute)

Data is decoded the way the HTTP body parsers in server.ts decode a binary-mode body: application/json is parsed (invalid JSON returns 400, like bodyParser.json), text/plain becomes a string, and anything else, including application/protobuf, stays a Buffer. datacontenttype comes from ce-datacontenttype, falling back to the content-type attribute.

Not included, on purpose: structured content mode, and the event signature type.

Backward compatibility

• Pub/Sub-source Eventarc triggers use a different mode (CUSTOM_PUBSUB_<topic>) and send their own ce-* headers, so user messages with ce-* attributes on those triggers are unaffected.
• User-configured push subscriptions don't carry the CE_PUBSUB_BINDING parameter, so they're unaffected.
• If the parameter ever changes, the middleware stops matching and behavior falls back to what it is today.
• The only behavior change: a cloudevent function that gets a CE_PUBSUB_BINDING envelope now receives the real event instead of a messagePublished wrapper.

Testing

• npm test: 120 passing (13 new in test/integration/cloud_event_pubsub_binding.ts); npm run check clean.
• With the middleware disabled, the 6 decoding tests fail and the 7 "left unchanged" tests still pass.
• End to end on GCP: a Firestore document.v1.written trigger → a firebase-functions 7.4.0 onDocumentWritten function, deployed with FUNCTION_SIGNATURE_TYPE=cloudevent (as the Firebase CLI does) and this branch packed in with npm pack. Seven document IDs were tested: normal, internal space, leading space, trailing space, %, ", é. All returned 200 and arrived as Firestore events. The trailing-space ID now gets params.docId = "trailing-space " and the correct snapshot, where it previously got empty params and a missing document. The other six are unchanged.

Unrelated issue found while testing: non-ASCII document IDs reach params garbled (é → é), because the header carries raw UTF-8 that Node reads as Latin-1. It's on the normal HTTP path and not touched here.

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