Never Trust a Failed git ls-files Call's stdout, Even Non-Empty

.github/actions/repo-gate/repo_gate.py

@@ -127,10 +127,12 @@ def tracked(root: Path, exclude: list[str] | None = None) -> list[str]:
     if exclude:
         args += ["--", *(f":!{pattern}" for pattern in exclude)]
     result = subprocess.run(args, capture_output=True, text=True, check=False)
-    # A failed command and a valid empty result would otherwise look the same to the caller.
-    # The command's own stderr (a bad pathspec, an unreadable root) is surfaced rather than dropped.
-    if result.returncode != 0 and result.stderr.strip():
-        print(f"git ls-files failed: {result.stderr.strip()}", file=sys.stderr)
+    if result.returncode != 0:
+        # A failed command's stdout is never trusted, even where it is non-empty.
+        # A partial listing read as complete is a scan that missed files and said nothing.
+        reason = result.stderr.strip() or f"exit {result.returncode}, no stderr"
+        print(f"git ls-files failed: {reason}", file=sys.stderr)
+        return []
     return [l for l in result.stdout.split("\n") if l]
 
 

scripts/tests/test_repo_gate.py

@@ -617,6 +617,28 @@ def test_a_failed_ls_files_call_prints_gits_own_error(self) -> None:
         self.assertIn("git ls-files failed", err.getvalue())
         self.assertIn("Invalid pathspec magic", err.getvalue())
 
+    def test_a_failed_call_is_never_trusted_even_with_nonempty_stdout(self) -> None:
+        """A partial listing read as a complete one is a scan that missed files silently."""
+        proc = subprocess.CompletedProcess([], 128, "kept.py\n", "fatal: something went wrong\n")
+        with (
+            mock.patch.object(repo_gate.subprocess, "run", return_value=proc),
+            contextlib.redirect_stderr(io.StringIO()),
+        ):
+            files = repo_gate.tracked(self.tmp, ["vendor/**"])
+        self.assertEqual([], files)
+
+    def test_a_failure_with_no_stderr_still_prints_a_reason(self) -> None:
+        """Empty stderr on a nonzero exit must not read as a silent, unexplained empty scan."""
+        proc = subprocess.CompletedProcess([], 1, "", "")
+        with (
+            mock.patch.object(repo_gate.subprocess, "run", return_value=proc),
+            contextlib.redirect_stderr(io.StringIO()) as err,
+        ):
+            files = repo_gate.tracked(self.tmp, ["vendor/**"])
+        self.assertEqual([], files)
+        self.assertIn("git ls-files failed", err.getvalue())
+        self.assertIn("exit 1", err.getvalue())
+
 
 class TestHarness(unittest.TestCase):
     def test_this_module_collects_a_plausible_number_of_cases(self) -> None: