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fix(#2452): stop shallow-fetching the base ref in three-dot-diff CI gates (#2485)

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* fix(#2452): stop shallow-fetching the base ref in three-dot-diff CI gates

`mutation.yml` re-fetched the base branch with `--depth=1` after checking out
with `fetch-depth: 0`. The shallow re-fetch truncates the base ref's ancestry,
so `git diff --name-only origin/<base>...HEAD` in scripts/mutation-matrix.cjs
can no longer compute a merge base and aborts with
`fatal: origin/next...HEAD: no merge base` (exit 2).

The `detect` job then fails and the `mutate` shards never run — so the 80%
mutation-score threshold went UNVERIFIED rather than enforced. The failure is
branch-position dependent, which is why it went unnoticed: a branch already
level with the base incidentally passes (its merge base IS the single fetched
commit), while a branch that is BEHIND fails. Observed on PRs #2436 and #2005.

`changeset-required.yml` and `docs-required.yml` shallow-fetched the base ref
too (`--depth=50`), shrinking the same window further. All three now fetch the
BASE REF unshallowed.

Their shallow *checkout* depth is left at 50: that is a separate, deliberate
cost control with fail-closed semantics, owned by
tests/policy-lint-shallow-checkout.test.cjs. Only the base-ref fetch changes.

The two `${{ }}` interpolations in mutation.yml's run: blocks now pass the base
name through `env:`, matching the sibling workflows.

Regression coverage in tests/mutation-workflow-base-ref.test.cjs:
  - a per-workflow contract guard asserting the base fetch carries no --depth
    (RED on origin/next for all three files, GREEN here). The YAML step parser
    handles block scalars and skips commented-out steps, so a future refactor
    to a multi-line `run:` cannot silently degrade the guard.
  - a real-git mechanism proof with boundary coverage at the shallow edge:
    with the base advanced 60 commits past the branch point, --depth=1 and
    --depth=60 both fail with `no merge base`, --depth=61 (merge base exactly
    at the boundary) succeeds, and an unbounded fetch succeeds. Each variant
    uses an independent clone, because a plain fetch does not un-shallow a repo
    that already carries a .git/shallow boundary.

Also fixes a startup race in tests/run-with-timeout.test.cjs surfaced by this
branch's gsd-test run (C1, linux-node24). The heartbeat file only appeared
~100ms after the grandchild's runtime was up, but the window is 1s spanning two
cold node starts, so on a loaded runner the timeout fired before any heartbeat
existed and the precondition failed for reasons unrelated to reaping. The child
now writes its heartbeat once synchronously at startup; the frozen-vs-ticking
comparison that actually proves reaping is unchanged.

Closes #2452

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs(changeset): backfill PR number to 2485

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
Tom Boucher
2026-07-21 11:14:29 -04:00
committed by GitHub
parent c5e0371775
commit 3435218089
5 changed files with 297 additions and 4 deletions
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5
.changeset/witty-jaguars-gather.md Normal file
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@@ -0,0 +1,5 @@
---
type: Fixed
pr: 2485
---
**CI gates no longer fail with `no merge base` on branches behind the base.** The mutation, changeset-required, and docs-required workflows shallow-fetched the base *ref*, truncating the ancestry their three-dot `origin/<base>...HEAD` diffs depend on — so the mutation gate reported failure and silently skipped its Stryker shards, leaving the 80% threshold unverified on any PR not already level with `next`. (#2452)

11
.github/workflows/changeset-required.yml vendored
View File

@@ -18,9 +18,18 @@ jobs:
steps:
- uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5.0.1
with:
# Intentionally shallow — see tests/policy-lint-shallow-checkout.test.cjs.
# Depth 50 covers >99% of PRs; a deeper merge base fails CLOSED with a
# loud lint error, which is the accepted trade.
fetch-depth: 50
- name: Fetch base ref for diff
run: git fetch --depth=50 origin "${BASE_REF}:refs/remotes/origin/${BASE_REF}"
# The BASE REF itself must not be shallow (#2452): scripts/changeset/lint.cjs
# diffs with the three-dot form `origin/<base>...HEAD`, which needs a merge
# base. Truncating the base's ancestry makes git abort with
# `fatal: ...: no merge base` instead of reporting a real verdict.
# The shallow *checkout* above is the deliberate cost control; the shallow
# *base fetch* was not — it only shrank the window further.
run: git fetch origin "${BASE_REF}:refs/remotes/origin/${BASE_REF}"
env:
BASE_REF: ${{ github.event.pull_request.base.ref }}
- uses: actions/setup-node@a0853c24544627f65ddf259abe73b1d18a591444 # v5.0.0

11
.github/workflows/docs-required.yml vendored
View File

@@ -18,9 +18,18 @@ jobs:
steps:
- uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5.0.1
with:
# Intentionally shallow — see tests/policy-lint-shallow-checkout.test.cjs.
# Depth 50 covers >99% of PRs; a deeper merge base fails CLOSED with a
# loud lint error, which is the accepted trade.
fetch-depth: 50
- name: Fetch base ref for diff
run: git fetch --depth=50 origin "${BASE_REF}:refs/remotes/origin/${BASE_REF}"
# The BASE REF itself must not be shallow (#2452): scripts/lint-docs-required.cjs
# diffs with the three-dot form `origin/<base>...HEAD`, which needs a merge
# base. Truncating the base's ancestry makes git abort with
# `fatal: ...: no merge base` instead of reporting a real verdict.
# The shallow *checkout* above is the deliberate cost control; the shallow
# *base fetch* was not — it only shrank the window further.
run: git fetch origin "${BASE_REF}:refs/remotes/origin/${BASE_REF}"
env:
BASE_REF: ${{ github.event.pull_request.base.ref }}
- uses: actions/setup-node@a0853c24544627f65ddf259abe73b1d18a591444 # v5.0.0

20
.github/workflows/mutation.yml vendored
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@@ -53,12 +53,28 @@ jobs:
# Ensure the base branch tip is available for the git diff below.
# fetch-depth: 0 above gets all history, but the remote ref name must exist.
# For workflow_dispatch (no base_ref) we fall back to `next`.
run: git fetch origin ${{ github.base_ref || 'next' }} --depth=1
#
# Do NOT pass --depth here (#2452): a shallow re-fetch truncates the
# freshly-fetched `next` history to a single commit, so the three-dot
# `origin/next...HEAD` diff in scripts/mutation-matrix.cjs can no longer
# compute a merge base and dies with `fatal: ... no merge base`. That
# only happens when the branch is BEHIND the base — an up-to-date branch
# incidentally passes because its merge base IS the fetched tip — which
# made the gate fail exactly on the PRs that most needed it, while the
# `mutate` shards never ran at all.
#
# BASE_NAME goes through `env:` rather than being interpolated straight
# into the shell, matching changeset-required.yml / docs-required.yml.
run: git fetch origin "${BASE_NAME}"
env:
BASE_NAME: ${{ github.base_ref || 'next' }}
- name: Compute mutation matrix
id: matrix
env:
BASE_NAME: ${{ github.base_ref || 'next' }}
run: |
BASE_REF="origin/${{ github.base_ref || 'next' }}"
BASE_REF="origin/${BASE_NAME}"
# Run the matrix script; capture the JSON output.
JSON=$(node scripts/mutation-matrix.cjs --base "${BASE_REF}")

254
tests/mutation-workflow-base-ref.test.cjs Normal file
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@@ -0,0 +1,254 @@
'use strict';
/**
* tests/mutation-workflow-base-ref.test.cjs
*
* Regression tests for the mutation-gate base-ref fetch (issue #2452).
*
* Background: `.github/workflows/mutation.yml` checks out with `fetch-depth: 0`
* (full history) and then re-fetched the base branch with `--depth=1`. That
* shallow re-fetch truncates the base ref's ancestry, so the three-dot diff in
* scripts/mutation-matrix.cjs (`git diff --name-only origin/<base>...HEAD`)
* can no longer compute a merge base and aborts with
* `fatal: origin/next...HEAD: no merge base`, exit 2. The `detect` job then
* fails and the `mutate` shards never run — the 80% mutation-score threshold
* goes UNVERIFIED rather than enforced.
*
* The failure is branch-position dependent, which is why it went unnoticed: a
* branch already level with the base incidentally passes (its merge base IS
* the single fetched commit), while a branch that is BEHIND the base fails.
*
* Test 1 is the contract guard (RED on origin/next, GREEN after the fix).
* Test 2 is a real-git mechanism proof: it reconstructs the runner's ref
* topology in a temp repo and demonstrates that the shallow fetch breaks the
* three-dot diff while a full fetch resolves it — proving the fix is both
* necessary and sufficient rather than asserting on YAML text alone.
*/
const { test, describe } = require('node:test');
const assert = require('node:assert/strict');
const fs = require('fs');
const os = require('os');
const path = require('path');
const { execFileSync } = require('child_process');
const helpers = require('./helpers.cjs');
const WORKFLOWS_DIR = path.resolve(__dirname, '..', '.github', 'workflows');
/**
* Every workflow whose lint step diffs against the base with the three-dot
* form (`origin/<base>...HEAD`). All of them need the BASE REF's ancestry, so
* none may shallow-fetch it. mutation.yml used --depth=1 and failed outright;
* the other two used --depth=50, shrinking the window further still.
*
* This guard covers the base-ref FETCH only. The shallow *checkout* depth on
* changeset-required.yml / docs-required.yml is a separate, deliberate cost
* control with fail-closed semantics, owned by
* tests/policy-lint-shallow-checkout.test.cjs — do not conflate the two.
*/
const THREE_DOT_WORKFLOWS = [
{ file: 'mutation.yml', consumer: 'scripts/mutation-matrix.cjs' },
{ file: 'changeset-required.yml', consumer: 'scripts/changeset/lint.cjs' },
{ file: 'docs-required.yml', consumer: 'scripts/lint-docs-required.cjs' },
];
// Bounded: git subprocesses in tests must never hang a CI lane.
const GIT_TIMEOUT_MS = 30_000;
function git(cwd, args) {
return execFileSync('git', args, {
cwd,
encoding: 'utf8',
timeout: GIT_TIMEOUT_MS,
stdio: ['ignore', 'pipe', 'pipe'],
});
}
/**
* Extract the `run:` body of the named step from the workflow YAML.
* Deliberately a small hand parser rather than a YAML dep: this asserts on the
* literal command line the runner executes, which is the contract at issue.
*
* Handles both inline (`run: git fetch ...`) and block-scalar (`run: |`) forms;
* a block scalar returns its dedented body so a future refactor to multi-line
* `run:` cannot silently degrade the guard into asserting on the literal "|".
* Comment lines are skipped so a commented-out step of the same name cannot
* shadow the real one.
*/
function runBodyForStep(yaml, stepName) {
const lines = yaml.split(/\r?\n/);
const nameIdx = lines.findIndex(
(l) => !/^\s*#/.test(l) && l.includes(`- name: ${stepName}`),
);
if (nameIdx === -1) return null;
for (let i = nameIdx + 1; i < lines.length; i++) {
const line = lines[i];
// Next step begins -> the step had no run: body.
if (/^\s*- name:/.test(line) && !/^\s*#/.test(line)) return null;
const m = line.match(/^\s*run:\s*(.*)$/);
if (!m) continue;
const inline = m[1].trim();
if (!/^[|>][-+]?$/.test(inline)) return inline;
// Block scalar: collect the indented body until the indentation drops.
const runIndent = line.match(/^(\s*)/)[1].length;
const body = [];
for (let j = i + 1; j < lines.length; j++) {
const bodyLine = lines[j];
if (bodyLine.trim() === '') {
body.push('');
continue;
}
const indent = bodyLine.match(/^(\s*)/)[1].length;
if (indent <= runIndent) break;
body.push(bodyLine.trim());
}
return body.join('\n').trim();
}
return null;
}
describe('#2452 CI gates: base-ref fetch must preserve ancestry', () => {
for (const { file, consumer } of THREE_DOT_WORKFLOWS) {
test(`${file}: "Fetch base ref for diff" does not shallow-fetch the base`, () => {
const yaml = fs.readFileSync(path.join(WORKFLOWS_DIR, file), 'utf8');
const runBody = runBodyForStep(yaml, 'Fetch base ref for diff');
assert.ok(
runBody,
`Expected a "Fetch base ref for diff" step with a run: body in ${file}. ` +
'If the step was renamed, update this test to match — do not delete the guard.',
);
assert.ok(
runBody.startsWith('git fetch origin'),
`Expected the step to fetch the base ref, got: ${runBody}`,
);
assert.ok(
!/--depth[=\s]/.test(runBody),
`${file}: the base-ref fetch must NOT be shallow. A --depth fetch truncates ` +
"the base branch's ancestry, so the three-dot diff in " +
`${consumer} cannot compute a merge base and the job dies with ` +
'`fatal: ...: no merge base` (#2452). Offending command: ' +
runBody,
);
});
}
// How far the base branch advances past the branch point. Chosen so the
// merge base sits OUTSIDE the old --depth=50 window, making the boundary
// between "cushion masks the bug" and "cushion exhausted" directly testable.
const BASE_ADVANCE = 60;
// Base chain is: tip … BASE_ADVANCE commits … branch point. So the branch
// point is the (BASE_ADVANCE + 1)-th commit from the tip — the exact depth
// at which a shallow base fetch first contains a usable merge base.
const MERGE_BASE_DEPTH = BASE_ADVANCE + 1;
test('base-ref fetch depth determines whether the three-dot diff resolves', () => {
const tmp = fs.mkdtempSync(path.join(os.tmpdir(), 'gsd-2452-'));
try {
// ---- origin: a base branch that advances past a feature branch --------
const origin = path.join(tmp, 'origin');
fs.mkdirSync(origin);
git(origin, ['init', '--quiet', '--initial-branch=base']);
git(origin, ['config', 'user.email', 'test@example.com']);
git(origin, ['config', 'user.name', 'Test']);
// Ambient commit.gpgsign=true would otherwise break these commits in CI.
git(origin, ['config', 'commit.gpgsign', 'false']);
fs.writeFileSync(path.join(origin, 'seed.txt'), 'seed\n');
git(origin, ['add', '.']);
git(origin, ['commit', '--quiet', '-m', 'seed']);
// Feature branch diverges here — this commit is the merge base.
git(origin, ['checkout', '--quiet', '-b', 'feature']);
fs.writeFileSync(path.join(origin, 'covered.cts'), 'export const x = 1;\n');
git(origin, ['add', '.']);
git(origin, ['commit', '--quiet', '-m', 'feature change']);
// Base then advances, leaving `feature` BEHIND — the failing condition.
git(origin, ['checkout', '--quiet', 'base']);
for (let n = 1; n <= BASE_ADVANCE; n++) {
fs.writeFileSync(path.join(origin, `base-${n}.txt`), `${n}\n`);
git(origin, ['add', '.']);
git(origin, ['commit', '--quiet', '-m', `base advance ${n}`]);
}
// ---- runner: has the PR head, must fetch the base ref separately ------
// Each variant gets its OWN clone, modelling independent workflow runs.
// They must not share a repo: once a shallow fetch writes a .git/shallow
// boundary, a later plain `git fetch` does NOT un-shallow it (that needs
// --unshallow), so repairing in place would test a scenario the workflow
// never encounters.
function runnerDiff(name, baseFetchArgs) {
const dir = path.join(tmp, name);
fs.mkdirSync(dir);
git(dir, ['init', '--quiet']);
git(dir, ['config', 'user.email', 'test@example.com']);
git(dir, ['config', 'user.name', 'Test']);
git(dir, ['config', 'commit.gpgsign', 'false']);
git(dir, ['remote', 'add', 'origin', origin]);
git(dir, ['fetch', '--quiet', 'origin', 'feature']);
git(dir, ['checkout', '--quiet', 'FETCH_HEAD']);
git(dir, ['fetch', '--quiet', 'origin', 'base', ...baseFetchArgs]);
try {
return { ok: true, out: git(dir, ['diff', '--name-only', 'origin/base...HEAD']).trim() };
} catch (err) {
return { ok: false, err: String(err.stderr || err.message) };
}
}
// (a) --depth=1 — mutation.yml's pre-fix command. Always broken.
const depth1 = runnerDiff('runner-depth-1', ['--depth=1']);
assert.equal(
depth1.ok,
false,
'Expected the three-dot diff to FAIL after a --depth=1 base fetch. ' +
'If this stops holding, the #2452 mechanism no longer reproduces and ' +
'this guard needs revisiting.',
);
assert.match(
depth1.err,
/no merge base/i,
'Expected git to report "no merge base" for a depth-1 base ref',
);
// (b) BOUNDARY, just below: the merge base is one commit out of reach.
// This is the changeset-required.yml / docs-required.yml --depth=50 case
// generalized — the cushion only ever postponed the same failure.
const below = runnerDiff('runner-depth-below', [`--depth=${MERGE_BASE_DEPTH - 1}`]);
assert.equal(
below.ok,
false,
`Expected FAIL at --depth=${MERGE_BASE_DEPTH - 1} (merge base one commit ` +
'beyond the shallow boundary) — this is why a bounded cushion is not a fix',
);
assert.match(below.err, /no merge base/i);
// (c) BOUNDARY, exactly deep enough: the merge base is the last commit in.
const atDepth = runnerDiff('runner-depth-at', [`--depth=${MERGE_BASE_DEPTH}`]);
assert.equal(
atDepth.ok,
true,
`Expected SUCCESS at --depth=${MERGE_BASE_DEPTH} (merge base exactly at the ` +
`shallow boundary), got: ${atDepth.err}`,
);
assert.equal(atDepth.out, 'covered.cts');
// (d) Unbounded — the shipped fix. Correct regardless of how far behind.
const full = runnerDiff('runner-full', []);
assert.equal(full.ok, true, `Expected the full-fetch diff to succeed, got: ${full.err}`);
assert.equal(
full.out,
'covered.cts',
'After a full base fetch the three-dot diff must resolve and report ' +
"exactly the feature branch's changed files",
);
} finally {
helpers.cleanup(tmp);
}
});
});