testing/cross_backend/differential.ts

The cross-backend differential: each impl is the oracle for the other.

A conformance case pins its status and reason on one backend at a time, and an output schema is often loose (z.looseObject, or Zod's default strip), so an extra field from one spine — possibly a leak — passes, as does any body difference on a case pinned by status alone. describe_differential_conformance_tests sends every case to two backends, normalizes both responses (normalize_differential_response), and asserts they are equal: status, header names, and the whole body.

Each case runs on a fresh fixture per backend — each backend's own setup_test, so separate databases and separate keepers, which is why the normalization is positional. The principal resolves and the request is sent through the table runner's own send_conformance_case, so a case means the same thing here as in describe_conformance_table_tests. The per-case expectations (expect.status, error_reason, …) are not asserted here — each backend's conformance leg already holds them; the differential holds the two answers to each other.

Every HTTP request asks for an uncompressed response (DIFFERENTIAL_REQUEST_HEADERS): content negotiation is the transport's, not the app's — Deno's Deno.serve compresses a response whose request accepts gzip, as Node's fetch sends by default, while other servers send it as is — so letting a server choose would read as a content-encoding and vary difference on every case.

A difference that is tracked and accepted goes in the known-divergence ledger (KnownDivergence), matched by case names or by methods, naming the paths that differ (what_differs), each optionally with the value each side must show there. A case may match several entries and differ at any of their paths, with the declared values, and nowhere else: any other difference fails like any other. The ledger cleans itself, with the xfail_until semantics — red when a divergence closes, naming the entry to delete: a case an entry names by case_name must still show it, checked in that case's own test, and an entry matched by method must still show it on at least one of its cases, checked by a closing test once they have all run. The closing test also fails an entry that matches no case, or that names a case that doesn't exist, and leaves an entry whose cases didn't all run (a -t filter, a send that threw) unjudged rather than calling it closed.

Use the backends' cross-process fixtures — the differential needs two live backends, so it runs in a dual-spawn project. fuz_app's own wiring is the cross_backend_parity project's differential.cross.test.ts.