init: import whichtests sources

.github/actions/setup-go/action.yaml

@@ -0,0 +1,24 @@
+name: "Setup Go"
+description: |
+  Sets up the Go environment for tests, builds, etc.
+inputs:
+  version:
+    description: "The Go version to use."
+    default: "1.26.2"
+  use-cache:
+    description: "Whether to use the cache."
+    default: "true"
+runs:
+  using: "composite"
+  steps:
+    - name: Setup Go
+      uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 # v5.5.0
+      with:
+        go-version: ${{ inputs.version }}
+        cache: ${{ inputs.use-cache }}
+
+    # It isn't necessary that we ever do this, but it helps separate the "setup"
+    # from the "run" times.
+    - name: go mod download
+      shell: bash
+      run: go mod download -x

.github/workflows/ci.yaml

@@ -0,0 +1,68 @@
+name: quality
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+# Cancel in-progress runs for pull requests when developers push additional
+# changes.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
+jobs:
+  fmt:
+    name: fmt
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          persist-credentials: false
+
+      - name: make fmt
+        run: make fmt
+
+      - name: Check unstaged
+        run: |
+          if [[ -n $(git ls-files --other --modified --exclude-standard) ]]; then
+            echo "Unexpected difference in directories after formatting. Run 'make fmt' and include the output in the commit."
+            exit 1
+          fi
+
+  lint:
+    name: lint
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          persist-credentials: false
+
+      - name: Setup Go
+        uses: ./.github/actions/setup-go
+
+      - name: make lint
+        run: make lint
+
+  test:
+    name: test
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - name: Checkout
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          persist-credentials: false
+
+      - name: Setup Go
+        uses: ./.github/actions/setup-go
+
+      - name: make test
+        run: make test

.gitignore

@@ -0,0 +1,34 @@
+# Binaries for programs and plugins
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+
+# Test binary, built with `go test -c`
+*.test
+
+# Code coverage profiles and other test artifacts
+*.out
+coverage.*
+*.coverprofile
+profile.cov
+
+# Go workspace file
+go.work
+go.work.sum
+
+# env file
+.env
+
+# Editor/IDE
+.idea/
+.vscode/
+
+# Key files
+*.key
+*.pub
+*.pem
+
+# Output directory
+build/

.golangci.yaml

@@ -0,0 +1,28 @@
+version: "2"
+
+linters:
+  exclusions:
+    rules:
+      - path: _test\.go
+        linters:
+          - gosec
+        text: "G304: Potential file inclusion via variable"
+  enable:
+    - goconst
+    - gocritic
+    - gosec
+    - misspell
+    - nakedret
+    - revive
+    - unconvert
+    - unparam
+  settings:
+    govet:
+      enable:
+        - shadow
+    misspell:
+      locale: US
+    revive:
+      rules:
+        - name: package-comments
+          disabled: true

.prettierrc

@@ -0,0 +1,14 @@
+{
+  "printWidth": 120,
+  "semi": false,
+  "trailingComma": "all",
+  "overrides": [
+    {
+      "files": ["./*.md", "./**/*.md"],
+      "options": {
+        "printWidth": 80,
+        "proseWrap": "always"
+      }
+    }
+  ]
+}

Makefile

@@ -0,0 +1,27 @@
+FIND_EXCLUSIONS= \
+	-not \( \( -path '*/.git/*' -o -path './build/*' -o -path './vendor/*' -o -path '*/.terraform/*' \) -prune \)
+GO_SRC_FILES := $(shell find . $(FIND_EXCLUSIONS) -type f -name '*.go' -not -name '*_test.go')
+GO_FMT_FILES := $(shell find . $(FIND_EXCLUSIONS) -type f -name '*.go' -print0 | xargs -0 grep -E --null -L '^// Code generated .* DO NOT EDIT\.$$' | tr '\0' ' ')
+
+default: build
+
+build/whichtests: $(GO_SRC_FILES) go.mod go.sum
+	mkdir -p ./build
+	go build -o ./build/whichtests .
+
+build: build/whichtests
+.PHONY: build
+
+fmt:
+	go mod tidy
+	go run golang.org/x/tools/cmd/goimports@v0.35.0 -w $(GO_FMT_FILES)
+	go run mvdan.cc/gofumpt@v0.8.0 -w -l $(GO_FMT_FILES)
+.PHONY: fmt
+
+lint:
+	go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.4.0 run ./...
+.PHONY: lint
+
+test:
+	go test -test.v -timeout 30s -cover ./...
+.PHONY: test

README.md

@@ -0,0 +1,63 @@
+# 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
+
+```sh
+go build ./
+./whichtests --help
+```
+
+Typical invocation against the local working tree:
+
+```sh
+./whichtests \
+  --repo-root . \
+  --base-sha origin/main \
+  --head-sha HEAD \
+  --out-matrix ./flake-matrix.json \
+  --out-summary -
+```
+
+In GitHub Actions:
+
+```sh
+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`. `package` is normally one safe Go package pattern. If the matrix cap is hit, the final overflow row stores a space-separated list of safe package tokens in `package`, leaves `run_regex` empty, and sets `test_count` to `1`; this is the contract consumed by the current `flake-go` workflow.
+
+## 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 `sharedDecl`.             |
+| `broadening.go` | Per-kind broadening rules (`broadeningScope`).                      |
+| `selection.go`  | Per-change selection logic (`selectChange`, broaden vs narrow).     |
+| `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
+
+```sh
+go test ./...
+```

broadening.go

@@ -0,0 +1,60 @@
+package main
+
+type broadeningScope uint8
+
+const (
+	broadeningNone broadeningScope = iota
+	broadeningPackage
+	broadeningDirectory
+)
+
+func broadeningScopeForOldHunk(decls []sharedDecl, candidate lineRange) broadeningScope {
+	scope := broadeningNone
+	for _, decl := range decls {
+		if !decl.Range.overlaps(candidate) {
+			continue
+		}
+		scope = max(scope, decl.broadeningScope())
+	}
+	return scope
+}
+
+func broadeningScopeForNewHunk(decls []sharedDecl, oldSnapshot *fileSnapshot, candidate lineRange) broadeningScope {
+	scope := broadeningNone
+	for _, decl := range decls {
+		if !decl.Range.overlaps(candidate) {
+			continue
+		}
+		scope = max(scope, decl.broadeningScopeOnNewSide(oldSnapshot))
+	}
+	return scope
+}
+
+func (decl sharedDecl) broadeningScope() broadeningScope {
+	switch decl.Kind {
+	case sharedDeclInit, sharedDeclTestMain:
+		// Go builds package and package_test files into one test binary.
+		// Init and TestMain changes can affect every test in the directory.
+		return broadeningDirectory
+	case sharedDeclImport, sharedDeclVar, sharedDeclConst, sharedDeclType, sharedDeclHelper:
+		return broadeningPackage
+	}
+	return broadeningNone
+}
+
+func (decl sharedDecl) broadeningScopeOnNewSide(oldSnapshot *fileSnapshot) broadeningScope {
+	switch decl.Kind {
+	// TODO: Decide whether new imports should narrow to tests that still
+	// reference package-local declarations. Today any import edit broadens
+	// the package.
+	case sharedDeclImport:
+		return broadeningPackage
+	case sharedDeclInit, sharedDeclTestMain:
+		return broadeningDirectory
+	case sharedDeclVar, sharedDeclConst, sharedDeclType, sharedDeclHelper:
+		if oldSnapshot != nil && oldSnapshot.hasSharedKey(decl.Keys) {
+			return broadeningPackage
+		}
+	}
+	return broadeningNone
+}

broadening_test.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestBroadeningScopeForOldHunkChoosesMaxOverlappingScope(t *testing.T) {
+	t.Parallel()
+
+	data := []byte(`package sample
+
+import "testing"
+
+func init() {
+	register()
+}
+
+func TestAlpha(t *testing.T) {}
+`)
+	snapshot, err := parseFileSnapshot(data)
+	require.NoError(t, err)
+	candidate := rangeSpan(
+		singleLineRange(t, string(data), `import "testing"`),
+		singleLineRange(t, string(data), "register()"),
+	)
+	require.Equal(t, broadeningDirectory, broadeningScopeForOldHunk(snapshot.shared, candidate))
+}
+
+func TestBroadeningScopeForNewHunkChoosesMaxOverlappingScope(t *testing.T) {
+	t.Parallel()
+
+	data := []byte(`package sample
+
+import "testing"
+
+func TestMain(m *testing.M) {
+	m.Run()
+}
+
+func TestAlpha(t *testing.T) {}
+`)
+	snapshot, err := parseFileSnapshot(data)
+	require.NoError(t, err)
+	candidate := rangeSpan(
+		singleLineRange(t, string(data), `import "testing"`),
+		singleLineRange(t, string(data), "m.Run()"),
+	)
+	require.Equal(t, broadeningDirectory, broadeningScopeForNewHunk(snapshot.shared, nil, candidate))
+}