fix: properly rehydrate Eventarc CloudEvents from Raw Pub/Sub fallback

[ajperel]

Summary

When an HTTP trigger receives a CloudEvent, it strict-validates the incoming HTTP headers. However, Google Front End (GFE) aggressively rejects certain HTTP headers (e.g. ce-subject strings ending with a trailing space on the wire).

When Eventarc encounters these strict rejections during a push delivery, it falls back to Raw Pub/Sub message delivery, stuffing all of the critical ce- metadata into the Pub/Sub message attributes instead of the HTTP headers.

Previously, legacyPubSubEventMiddleware failed to recognize this fallback behavior. It assumed the body was a legacy Gen-1 Pub/Sub event wrapper if ce-type was missing from the HTTP headers, leading to payload shift and parsing failures in downstream SDKs like firebase-functions.

This addresses one of the primary root causes of firebase/firebase-functions#1922 (which contains a live reproduction case).

Reproduction

1. Deploy a Firestore CloudEvent trigger.
2. Create a Firestore document with a trailing space in its ID (e.g., Test Document2 ).
3. Eventarc falls back to Raw Pub/Sub because the HTTP specification forbids OWS (optional whitespace) at the end of header values (ce-subject: documents/... is illegal on the wire).
4. The Cloud Framework receives the raw Pub/Sub payload but fails to extract the CloudEvent wrapper, crashing the function execution.

Fix

Extracts a rehydrateCloudEvent helper inside pubsub_middleware.ts to cleanly catch and intercept Raw Pub/Sub fallbacks:

1. Header Promotion: Promotes all ce- prefixed (and unprefixed) Pub/Sub attributes into req.headers.
2. Payload Extraction: Base64 decodes message.data and attempts a JSON parse (unless skipped due to non-JSON ce-datacontenttype).
3. Safe Fallback: If JSON parsing fails (e.g. application/protobuf), it mirrors bodyParser.raw() by assigning the Buffer directly to req.body and passing it to the handler safely.

Because Express req.headers is just a JavaScript dictionary, assigning headers with trailing spaces internally perfectly bypasses the wire-level HTTP parser constraints that broke the initial Eventarc push.

Testing

• Manual Verification: We bundled this patched framework into a local firebase-functions dependency and successfully deployed it to a production playground executing firebase deploy --only functions. Successfully verified via Cloud Logging that documents with trailing spaces trigger the fallback and process correctly.
• Added unit test: Eventarc fallback CloudEvent (JSON payload) to verify successful promotion of attributes to headers and JSON deserialization.
• Added unit test: Eventarc fallback CloudEvent (Unprefixed attributes & Binary payload) to ensure CloudEvents with binary contents strictly bypass JSON parsing and arrive as Raw Buffers.
• Full suite green: 109 passing.