whichtests

whichtests is the Go test-plan generator that drives the flake-go CI workflow in coder/coder. Given a base/head git revision pair (or a GitHub Actions event), it walks the diff, parses each changed test file, picks the smallest set of tests to rerun, and emits a workflow matrix plus a human-readable Markdown summary.

Building and running

go build ./
./whichtests --help

Typical invocation against the local working tree:

./whichtests \
  --repo-root . \
  --base-sha origin/main \
  --head-sha HEAD \
  --out-matrix ./flake-matrix.json \
  --out-summary -

In GitHub Actions:

go run ./ \
  --repo-root . \
  --github-actions \
  --out-matrix "$RUNNER_TEMP/flake-matrix.json"

For pull_request events, checkout must use the PR head SHA, for example github.event.pull_request.head.sha. The default synthetic merge ref is rejected because the checked-out HEAD must match pull_request.head.sha.

The matrix JSON contains include rows with package, run_regex, and test_count. By default, each row represents one safe Go package pattern and a precise regex for the directly changed runnable tests in that package. The generator does not emit whole-package fallback rows.

Coalescing into a single matrix row

By default whichtests emits one matrix row per selected package, so consumers can fan out across runners. Pass --coalesce to collapse the matrix into a single row whose package is the space-joined list of every selected package path and whose run_regex unions every selected test name across packages. This is useful when the consumer wants one go test invocation against all selected packages (for example, to amplify scheduling contention in a flake hunt) at the cost of giving up per-package precision in -run: a test name selected in one package will also match a same-named test in any other listed package.

File layout

The binary is a single package main, split into focused files:

File	Responsibility
cli.go	main, flag parsing, command orchestration (runCommand).
config.go	config / commandConfig types and defaults.
request.go	runRequest, diffRange, revision validation.
gitexec.go	gitRunner / gitFetcher types and the real exec.Command impl.
diff.go	Reading and parsing git diff, change kinds, hunks, line ranges.
snapshot.go	AST snapshot parsing, fileSnapshot, and top-level test ranges.
selection.go	Per-change direct test selection logic (selectChange).
inventory.go	inventoryCache for package/directory test discovery.
plan.go	Plan construction, matrix and summary rendering (buildExecutionPlan, selectTestPlan).
githubactions.go	GitHub Actions request builder and history preparation.
publish.go	Single sink for matrix and summary outputs.

Testing

go test ./...