fix(resolve): attach SUBREQUEST_HTTP_ERROR on status-fallback path

[AvnerMaster]

Problem

With ApolloRouterCompatibilitySubrequestHTTPError enabled, every non-2XX subgraph response is expected to carry a SUBREQUEST_HTTP_ERROR-coded error (via addApolloRouterCompatibilityError).

However, that error is attached only on the errors-body path (mergeErrors, loader.go:741/788). When a subgraph returns a non-2XX with no GraphQL errors body — an empty/non-GraphQL body, or a null data._entities (typical of an LB/ingress 504 Gateway Timeout) — the loader takes renderErrorsStatusFallback (loader.go:502/565), which emits a bare {"message":"<status>: <statusText>"} with no extensions.code and never invokes addApolloRouterCompatibilityError.

Impact

The two non-2XX paths produce inconsistent error shapes: consumers relying on SUBREQUEST_HTTP_ERROR to normalize subgraph HTTP failures can normalize the errors-body case but not the status-only case (e.g. a bare 504 Gateway Timeout from an ingress/proxy in front of a subgraph).

Fix

In renderErrorsStatusFallback, when apolloRouterCompatibilitySubrequestHTTPError is enabled and statusCode >= 400, delegate to addApolloRouterCompatibilityError so status-only failures carry the same coded error as the errors-body path.

Test

Adds TestArenaGCSafety_ApolloRouterCompatError_StatusFallback (504 + flag on + empty body → coded SUBREQUEST_HTTP_ERROR). New test and the full pkg/engine/resolve suite pass, no regressions.

Notes

Reported by the monday.com platform API team while migrating from Apollo Router to Cosmo: an _entities fetch hitting a 504 Gateway Timeout reached clients as a raw, uncoded error instead of a normalized one, diverging from the Apollo Router path.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: cf7f8e53-34f7-4f86-ae5f-a3be45cf453c

📥 Commits

Reviewing files that changed from the base of the PR and between fb2d52a and 42d398b.

📒 Files selected for processing (2)

• v2/pkg/engine/resolve/loader.go
• v2/pkg/engine/resolve/resolve_arena_gc_test.go

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📝 Walkthrough

Walkthrough

The loader's renderErrorsStatusFallback function now checks for Apollo Router compatibility mode and, for HTTP status codes >= 400, emits a structured SUBREQUEST_HTTP_ERROR compatibility error instead of a generic status-message error. A new regression test validates this behavior under GC pressure with a 504 status response.

Changes

Status fallback compatibility fix

Layer / File(s)	Summary
Emit SUBREQUEST_HTTP_ERROR on status-only fallback v2/pkg/engine/resolve/loader.go, v2/pkg/engine/resolve/resolve_arena_gc_test.go	Adds an early return in renderErrorsStatusFallback that emits an Apollo Router compatibility SUBREQUEST_HTTP_ERROR when statusCode >= 400, replacing the generic status-message error path, and adds a test asserting this behavior for a 504 response with no GraphQL error/data body.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Subgraph
  participant Loader
  participant Resolvable

  Subgraph->>Loader: HTTP response, statusCode >= 400, no errors/data
  Loader->>Loader: check ApolloRouterCompatibilitySubrequestHTTPError
  Loader->>Resolvable: addApolloRouterCompatibilityError(res) with SUBREQUEST_HTTP_ERROR
  Loader-->>Loader: return, skip generic error message

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🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main fix: attaching SUBREQUEST_HTTP_ERROR on the status-fallback path.
Description check	✅ Passed	The description directly explains the same fix and test coverage for the status-only non-2XX path.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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