Reject Symlinks From Shebang Discovery, Propagate read Failures

.github/workflows/validate-task.yml

@@ -115,10 +115,10 @@ jobs:
           mapfile -d '' -t candidates < <(git ls-files -z)
           for file in "${candidates[@]}"; do
             base="${file##*/}"
-            if [[ "$base" != *.* ]] && [ -f "$file" ]; then
-              # `read` fails at EOF even when it fills first_line, so the check reads the content regardless.
-              first_line=""
-              IFS= read -r first_line < "$file" || true
+            # Never follows a tracked symlink: -h short-circuits before -f's own dereferencing stat.
+            if [[ "$base" != *.* ]] && [ ! -h "$file" ] && [ -f "$file" ]; then
+              # `head` reads a no-trailing-newline file cleanly and still fails on a genuine read error.
+              first_line="$(head -n 1 -- "$file")"
               if is_shell_shebang "$first_line"; then
                 scripts+=("$file")
               fi

scripts/docker_lint.py

@@ -194,9 +194,15 @@ def shell_shebang_interpreter(line: str) -> str | None:
 
 
 def has_shell_shebang(root: Path, relative_path: str) -> bool:
-    """Report whether a tracked file's shebang directly names bash or sh."""
+    """Report whether a tracked file's shebang directly names bash or sh.
+
+    Never follows a tracked symlink: `is_symlink()` uses `lstat`, keeping the target unreached.
+    """
+    path = root / relative_path
     try:
-        with (root / relative_path).open("rb") as handle:
+        if path.is_symlink():
+            return False
+        with path.open("rb") as handle:
             first_line = handle.readline(256)
     except OSError:
         return False

scripts/tests/test_docker_lint.py

@@ -44,13 +44,21 @@ class DockerLintCase(unittest.TestCase):
     def setUp(self) -> None:
         self.root = Path(self.enterContext(tempfile.TemporaryDirectory()))
         subprocess.run(["git", "init", "-q", str(self.root)], check=True)
+        # A separate temp dir, not self.root's own parent, which is the shared system temp root.
+        self.outside = Path(self.enterContext(tempfile.TemporaryDirectory()))
 
     def track(self, name: str, body: str = "content\n") -> None:
         path = self.root / name
         path.parent.mkdir(parents=True, exist_ok=True)
         path.write_text(body, encoding="utf-8")
         subprocess.run(["git", "-C", str(self.root), "add", "--", name], check=True)
 
+    def track_symlink(self, name: str, target: Path) -> None:
+        path = self.root / name
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.symlink_to(target)
+        subprocess.run(["git", "-C", str(self.root), "add", "--", name], check=True)
+
     def invoke(self, selected: set[str], runner: FakeRunner) -> tuple[int, str]:
         output = io.StringIO()
         with contextlib.redirect_stdout(output):
@@ -157,6 +165,21 @@ def test_extensionless_shebang_script_merges_with_glob_matched_scripts(self) ->
             docker_lint.tracked_files(self.root, linter),
         )
 
+    def test_extensionless_symlink_is_never_followed(self) -> None:
+        secret = self.outside / "secret"
+        secret.write_text("#!/usr/bin/env bash\n", encoding="utf-8")
+        self.track_symlink("ops/evil-symlink", secret)
+        linter = next(linter for linter in docker_lint.LINTERS if linter.name == "shellcheck")
+        with mock.patch.object(Path, "open", side_effect=AssertionError("symlink target opened")):
+            self.assertEqual([], docker_lint.tracked_files(self.root, linter))
+
+    def test_has_shell_shebang_reports_false_for_a_symlink_without_reading_it(self) -> None:
+        secret = self.outside / "secret"
+        secret.write_text("#!/usr/bin/env bash\n", encoding="utf-8")
+        self.track_symlink("ops/evil-symlink", secret)
+        with mock.patch.object(Path, "open", side_effect=AssertionError("symlink target opened")):
+            self.assertFalse(docker_lint.has_shell_shebang(self.root, "ops/evil-symlink"))
+
     def test_extensionless_untracked_shebang_script_is_not_picked_up(self) -> None:
         path = self.root / "ops" / "vps-backup-pull"
         path.parent.mkdir(parents=True, exist_ok=True)