fix(link-checker): distinguish empty diffs from unavailable bases

tools/check_all_links.py

@@ -308,9 +308,9 @@ def get_changed_files_from_git(repo_root: pathlib.Path) -> List[str]:
             cmd = ["git", "diff", "--name-only", f"{base}...HEAD"]
             out = subprocess.check_output(cmd, cwd=repo_root, text=True, stderr=subprocess.DEVNULL)
             lines = [line.strip() for line in out.splitlines() if line.strip().endswith((".md", ".ipynb"))]
-            if lines:
-                logger.info("Detected %d changed file(s) against %s", len(lines), base)
-                return lines
+            # A successful empty diff still establishes the comparison base.
+            logger.info("Detected %d changed file(s) against %s", len(lines), base)
+            return lines
         except Exception:
             continue
     logger.warning("Could not determine base branch for --changed. Falling back to all files.")

tools/test_check_all_links.py

@@ -15,16 +15,23 @@
 
 """Unit tests for tools/check_all_links.py link verification logic."""
 
+import contextlib
+import io
 import json
 import pathlib
+import subprocess
+import sys
 import tempfile
 import unittest
+from unittest import mock
 
 from tools.check_all_links import (
     LinkCheckerConfig,
     extract_anchors_from_content,
     extract_links_from_file,
+    get_changed_files_from_git,
     get_file_anchors,
+    main,
     normalize_cell_source,
     run_link_audit,
     slugify_heading,
@@ -289,6 +296,56 @@ def test_github_blob_url_with_query_params_and_casing(self) -> None:
         self.assertIn("nonexistent.md", err)
         self.assertNotIn("?raw=true", err)
 
+    def test_changed_files_stops_at_available_base_without_documents(self) -> None:
+        """An empty document diff must not select changes from an older base."""
+        for diff in ("", "tools/check_all_links.py\n"):
+            with self.subTest(diff=diff):
+                with mock.patch(
+                    "tools.check_all_links.subprocess.check_output",
+                    side_effect=[diff, "quickstarts/guide.md\n"],
+                ):
+                    self.assertEqual(get_changed_files_from_git(self.repo_root), [])
+
+    def test_changed_files_falls_back_when_base_is_unavailable(self) -> None:
+        """An unavailable upstream ref still falls back to origin and filters paths."""
+        with mock.patch(
+            "tools.check_all_links.subprocess.check_output",
+            side_effect=[
+                subprocess.CalledProcessError(128, "git diff"),
+                "README.md\ntools/check_all_links.py\nquickstarts/sample.ipynb\n",
+            ],
+        ):
+            self.assertEqual(
+                get_changed_files_from_git(self.repo_root),
+                ["README.md", "quickstarts/sample.ipynb"],
+            )
+
+    def test_changed_cli_skips_unrelated_broken_links(self) -> None:
+        """No document changes against origin must not audit an older broken guide."""
+        for diff in ("", "tools/check_all_links.py\n"):
+            with self.subTest(diff=diff):
+                output = io.StringIO()
+                with (
+                    mock.patch(
+                        "tools.check_all_links.subprocess.check_output",
+                        side_effect=[
+                            subprocess.CalledProcessError(128, "git diff"),
+                            diff,
+                            "quickstarts/guide.md\n",
+                        ],
+                    ),
+                    mock.patch.object(
+                        sys,
+                        "argv",
+                        ["check_all_links.py", "--repo-dir", str(self.repo_root), "--changed"],
+                    ),
+                    contextlib.redirect_stdout(output),
+                    self.assertRaises(SystemExit) as exit_context,
+                ):
+                    main()
+                self.assertEqual(exit_context.exception.code, 0, output.getvalue())
+                self.assertIn("No modified .md or .ipynb files detected", output.getvalue())
+
 
 if __name__ == "__main__":
     unittest.main()