Use the replace fallback for AST cache writes

The AST extraction cache writes its own temp file and replace, a
separate code path from the shared atomic writer, and only caught
PermissionError. This was the exact write named in the report: worker
failed for a file with WinError 17 while replacing a cache entry
under the output directory's ast cache. Also covers the stat index
write, which had no Windows fallback at all.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017qfdzgbA5KedGEjD1AayNh

graphify/cache.py

@@ -17,6 +17,7 @@ from pathlib import Path
 # absolute path ("/shared/graphify-out"). Single source of truth in graphify.paths
 # (#1423); re-exported here as _GRAPHIFY_OUT for the existing call sites.
 from graphify.paths import GRAPHIFY_OUT as _GRAPHIFY_OUT
+from graphify.paths import os_replace_with_fallback as _os_replace_with_fallback
 
 # AST cache entries are the output of graphify's own extractor code, so they
 # are only valid for the version that wrote them: keying purely on file
@@ -399,7 +400,7 @@ def _flush_stat_index() -> None:
         try:
             os.write(fd, json.dumps(on_disk, separators=(",", ":")).encode())
             os.close(fd)
-            os.replace(tmp, p)
+            _os_replace_with_fallback(tmp, p)
         except Exception:
             try:
                 os.close(fd)
@@ -1126,14 +1127,7 @@ def save_cached(path: Path, result: dict, root: Path = Path("."), kind: str = "a
     try:
         os.write(fd, json.dumps(on_disk).encode())
         os.close(fd)
-        try:
-            os.replace(tmp_path, entry)
-        except PermissionError:
-            # Windows: os.replace can fail with WinError 5 if the target is
-            # briefly locked. Fall back to copy-then-delete.
-            import shutil
-            shutil.copy2(tmp_path, entry)
-            os.unlink(tmp_path)
+        _os_replace_with_fallback(tmp_path, entry)
     except Exception:
         try:
             os.close(fd)

tests/test_cache.py

@@ -221,6 +221,37 @@ def test_save_cached_relativizes_source_file(tmp_path):
     assert edge_sources == {"src/foo.py"}
 
 
+def test_save_cached_survives_a_winerror_17_replace(tmp_path, monkeypatch):
+    """#3508: on some Windows/filesystem combinations, `os.replace` raises
+    WinError 17 ("cannot move to a different disk drive") even for a temp file
+    and destination in the same directory on the same drive -- reported for
+    exactly this AST cache write path. WinError 17 is a plain OSError, not a
+    PermissionError, so it must still trigger the copy-then-delete fallback."""
+    import os
+    from graphify.cache import save_cached, file_hash, cache_dir
+
+    src = tmp_path / "foo.py"
+    src.write_text("def x(): pass\n")
+
+    real_replace = os.replace
+
+    def flaky_replace(a, b):
+        exc = OSError("cannot move to a different disk drive")
+        exc.winerror = 17
+        raise exc
+
+    monkeypatch.setattr(os, "replace", flaky_replace)
+    try:
+        save_cached(src, {"nodes": [], "edges": []}, root=tmp_path, kind="ast")
+    finally:
+        monkeypatch.setattr(os, "replace", real_replace)
+
+    h = file_hash(src, tmp_path)
+    entry = cache_dir(tmp_path, "ast") / f"{h}.json"
+    assert entry.exists()
+    assert not any(p.name.endswith(".tmp") for p in entry.parent.iterdir())
+
+
 def test_load_cached_absolutizes_source_file(tmp_path):
     """``load_cached`` returns the same absolute-path shape that a fresh
     extraction produces, so consumers don't need to special-case cache