gitlab-mcp-server: Instructions file for GitHub Copilot

Assertions off the test goroutine

testing.T.FailNow — and therefore t.Fatal/t.Fatalf — must be called from the goroutine running the test. Anywhere else (an http.HandlerFunc literal, a go statement, an errgroup.Group.Go task, an MCP tool handler registered on an in-memory server) it only terminates that goroutine: the HTTP response is truncated or never written, the client observes a transport error, and the test continues against the wreckage. go vet's testinggoroutine analyzer does not catch handler literals; cmd/audit_test_goroutines does (make audit-test-goroutines / make check-test-goroutines).

The six rules

Every assertion inside a function literal that crosses a goroutine boundary MUST follow all of these:

1. Never t.Fatal/t.Fatalf/t.FailNow inside the literal. Use t.Errorf (or record and assert later — see rule 5).
2. Always return immediately after the t.Errorf, even when it is the last statement. t.Fatal provided that exit for free; a later edit appending code below a bare t.Errorf silently reintroduces the bug (this exact mistake produced a nil-dereference panic in PR #270).
3. Write a deterministic response before returning. For HTTP handlers, http.Error(w, "<what failed>", http.StatusInternalServerError); for MCP tool handlers, an error result. Without it the client silently receives a 200 with an empty body — a worse signal than the EOF it replaces.
4. Nothing the failed check would have validated may be used afterwards. If the check guarded a nil (r.MultipartForm, a decoded struct), the return must come before any use of it.
5. Prefer recording over asserting. Store observed values in locals (or a struct) inside the handler and assert on the test goroutine after the client call returns. This removes the problem instead of mitigating it.
6. "Must not be called" guards become a recorded flag — var called atomic.Bool (or atomic.Int64 for counts) written in the handler and asserted afterwards. The atomic is mandatory: an unsynchronised variable written from the handler goroutine is a data race.