fix(nightly): build the paired-base test's own history

quest/m0/nightly/README.md

@@ -1,4 +1,4 @@
-# Nightly passes on main
+# [S] Nightly passes on main
 
 ## Goal
 
@@ -16,7 +16,7 @@ Decided 2026-10-24 (maintainer), from a triage of run
 child per distinct cause, each landing on its own. The PR gate keeps
 skipping benches and examples; a green Nightly catches them within a day.
 
-This README's own work, after the children:
+This README's own work:
 
 - Nightly's multi-step jobs (Features first) keep going past a failed step
   and fail at the end, so one break never hides the steps after it.
@@ -30,7 +30,3 @@ This README's own work, after the children:
   [moq-dev/moq#6656](https://github.com/moq-dev/moq/pull/6656).
 
 Public API: none. Wire: none.
-
-## Required
-
-- [Nightly's base in a shallow checkout](/quest/m0/nightly/shallow-base.md) - `sh/gh/nightly.test.py` passes in Nightly's depth-1 checkout

sh/gh/nightly.test.py

@@ -16,7 +16,7 @@
 
 
 class NightlyTest(unittest.TestCase):
-    def run_group(self, group, fail="", **environment):
+    def run_group(self, group, fail="", cwd=ROOT, **environment):
         with tempfile.TemporaryDirectory() as directory:
             scratch = Path(directory)
             calls = scratch / "calls"
@@ -38,28 +38,36 @@ def run_group(self, group, fail="", **environment):
                        BENCH_RESULTS=str(scratch / "results"), NIGHTLY_CALLS=str(calls), NIGHTLY_FAIL=fail, BASH_ENV="/dev/null")
             env.pop("BENCH_BASE", None)
             env.update(environment)
-            result = subprocess.run(["bash", "sh/gh/nightly.sh", group], cwd=ROOT,
+            result = subprocess.run(["bash", ROOT / "sh/gh/nightly.sh", group], cwd=cwd,
                                     env=env, capture_output=True, text=True)
             self.bases = set(Path(f"{calls}.base").read_text().splitlines()) if calls.exists() else set()
             return result, calls.read_text().splitlines() if calls.exists() else []
 
     def test_paired_base_is_the_previous_scheduled_nightly(self):
-        head, parent = (subprocess.run(["git", "rev-parse", revision], cwd=ROOT, capture_output=True, text=True,
-                                       check=True).stdout.strip() for revision in ("HEAD", "HEAD~1"))
-        # This run is the newest scheduled one; the previous run's commit is the base.
-        runs = json.dumps([{"databaseId": 2, "headSha": parent}, {"databaseId": 1, "headSha": head}])
-        result, _ = self.run_group("rust-uring", BENCH_BASE="previous-nightly", GITHUB_RUN_ID="2", NIGHTLY_RUNS=runs)
-        self.assertEqual(result.returncode, 0, result.stderr)
-        self.assertEqual(self.bases, {head})
-        # A named revision reaches the runner as its commit.
-        result, _ = self.run_group("rust-uring", BENCH_BASE="HEAD~1")
-        self.assertEqual(result.returncode, 0, result.stderr)
-        self.assertEqual(self.bases, {parent})
-        result, calls = self.run_group("rust-uring", BENCH_BASE="previous-nightly", GITHUB_RUN_ID="1",
-                                       NIGHTLY_RUNS=json.dumps([{"databaseId": 1, "headSha": head}]))
-        self.assertEqual(result.returncode, 1)
-        self.assertIn("no previous scheduled Nightly", result.stderr)
-        self.assertEqual(calls, [])
+        # Nightly checks out shallowly, so the test builds the history it resolves against.
+        with tempfile.TemporaryDirectory() as repo:
+            git = ["git", "-C", repo, "-c", "user.name=nightly", "-c", "user.email=nightly@example.com",
+                   "-c", "commit.gpgsign=false"]
+            subprocess.run([*git, "init", "-q"], check=True)
+            for message in ("parent", "head"):
+                subprocess.run([*git, "commit", "-q", "--allow-empty", "-m", message], check=True)
+            head, parent = (subprocess.run([*git, "rev-parse", revision], capture_output=True, text=True,
+                                           check=True).stdout.strip() for revision in ("HEAD", "HEAD~1"))
+            # This run is the newest scheduled one; the previous run's commit is the base.
+            runs = json.dumps([{"databaseId": 2, "headSha": parent}, {"databaseId": 1, "headSha": head}])
+            result, _ = self.run_group("rust-uring", cwd=repo, BENCH_BASE="previous-nightly", GITHUB_RUN_ID="2",
+                                       NIGHTLY_RUNS=runs)
+            self.assertEqual(result.returncode, 0, result.stderr)
+            self.assertEqual(self.bases, {head})
+            # A named revision reaches the runner as its commit.
+            result, _ = self.run_group("rust-uring", cwd=repo, BENCH_BASE="HEAD~1")
+            self.assertEqual(result.returncode, 0, result.stderr)
+            self.assertEqual(self.bases, {parent})
+            result, calls = self.run_group("rust-uring", cwd=repo, BENCH_BASE="previous-nightly",
+                                           GITHUB_RUN_ID="1", NIGHTLY_RUNS=json.dumps([{"databaseId": 1, "headSha": head}]))
+            self.assertEqual(result.returncode, 1)
+            self.assertIn("no previous scheduled Nightly", result.stderr)
+            self.assertEqual(calls, [])
 
     def test_paired_base_applies_only_to_rust_bench_groups(self):
         for group in ("js", "rust-test", "bindings"):