From 013ea0ef7a402e9e718fd55e78771f89c1553d63 Mon Sep 17 00:00:00 2001
From: Chris Trzesniewski <k.trzesniewski@gmail.com>
Date: Thu, 26 Mar 2026 22:47:14 +0100
Subject: [PATCH] fix: use merge commit timestamp for PR benchmark entries
 [#327]

Replace pr.head.repo.updated_at (fork sync time) with the timestamp
from the local merge commit (HEAD), which is always accessible even
with shallow clones and is accurate to within seconds of the actual
commit time.
---
 src/extract.ts       | 12 ++++++++----
 test/extract.spec.ts | 10 ++++++----
 2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/src/extract.ts b/src/extract.ts
index 18cfaecb9..d035d0fe9 100644
--- a/src/extract.ts
+++ b/src/extract.ts
@@ -1,6 +1,7 @@
 /* eslint-disable @typescript-eslint/naming-convention */
 import { promises as fs } from 'fs';
 import * as github from '@actions/github';
+import { cmd } from './git';
 import { Config, ToolType } from './config';
 import { z } from 'zod';
 
@@ -239,21 +240,24 @@ function getHumanReadableUnitValue(seconds: number): [number, string] {
     }
 }
 
-function getCommitFromPullRequestPayload(pr: PullRequest): Commit {
-    // On pull_request hook, head_commit is not available
+async function getCommitFromPullRequestPayload(pr: PullRequest): Promise<Commit> {
+    // On pull_request hook, head_commit is not available.
+    // HEAD is the merge commit GitHub creates for the PR, always accessible even with shallow clones.
+    // Its timestamp is within seconds of the actual commit time.
     const id: string = pr.head.sha;
     const username: string = pr.head.user.login;
     const user = {
         name: username, // XXX: Fallback, not correct
         username,
     };
+    const timestamp = (await cmd([], 'log', '-1', '--format=%aI', 'HEAD')).trim();
 
     return {
         author: user,
         committer: user,
         id,
         message: pr.title,
-        timestamp: pr.head.repo.updated_at,
+        timestamp,
         url: `${pr.html_url}/commits/${id}`,
     };
 }
@@ -299,7 +303,7 @@ async function getCommit(githubToken?: string, ref?: string): Promise<Commit> {
     const pr = github.context.payload.pull_request;
 
     if (pr) {
-        return getCommitFromPullRequestPayload(pr);
+        return await getCommitFromPullRequestPayload(pr);
     }
 
     if (!githubToken) {
diff --git a/test/extract.spec.ts b/test/extract.spec.ts
index 631b64f43..9405b9f43 100644
--- a/test/extract.spec.ts
+++ b/test/extract.spec.ts
@@ -37,6 +37,10 @@ const dummyGitHubContext = {
     ref: 'abcd1234',
 };
 
+jest.mock('../src/git', () => ({
+    cmd: jest.fn().mockResolvedValue('2024-01-15T10:30:00+00:00\n'),
+}));
+
 jest.mock('@actions/github', () => ({
     get context() {
         return dummyGitHubContext;
@@ -218,9 +222,7 @@ describe('extractResult()', function () {
                     user: {
                         login: 'user',
                     },
-                    repo: {
-                        updated_at: 'repo updated at timestamp',
-                    },
+                    repo: {},
                 },
             },
         };
@@ -238,7 +240,7 @@ describe('extractResult()', function () {
         A.deepEqual(commit.committer, expectedUser);
         A.equal(commit.id, 'abcdef0123456789');
         A.equal(commit.message, 'this is title');
-        A.equal(commit.timestamp, 'repo updated at timestamp');
+        A.equal(commit.timestamp, '2024-01-15T10:30:00+00:00');
         A.equal(commit.url, 'https://github.com/dummy/repo/pull/1/commits/abcdef0123456789');
     });