pathlib: don't trust a zero Windows file ID in samefile_nofollow

On Windows st_ino holds the file ID, which file systems are free to not
support; WinFsp mounts such as sshfs-win report 0 for every file, because
SFTP has no inode field to fill it from. os.path.samestat() then reports
any two files on the volume as the same file, so the Windows short-path
fallback in Session.collect() matched every node and nothing got pruned:
passing a single file collected the whole suite.

Treat a zero file ID as unknown and fall back to path comparison, which
is what the other platforms do anyway.

Fixes #14864.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: RonnyPfannschmidt

changelog/14864.bugfix.rst

@@ -0,0 +1 @@
+Fixed collection on Windows collecting the whole suite instead of the given path, on file systems which do not support file IDs (``st_ino`` is ``0`` for every file, as seen for example on ``sshfs-win``/WinFsp mounts). The Windows-only short-path fallback used when matching collection arguments now ignores a zero file ID and compares paths instead.

src/_pytest/pathlib.py

@@ -1090,9 +1090,30 @@ def safe_exists(p: Path) -> bool:
         return False
 
 
-def samefile_nofollow(p1: Path, p2: Path) -> bool:
-    """Test whether two paths reference the same actual file or directory.
+if sys.platform == "win32":
+
+    def samefile_nofollow(p1: Path, p2: Path) -> bool:
+        """Test whether two paths reference the same actual file or directory.
+
+        Unlike Path.samefile(), does not resolve symlinks.
+
+        On Windows st_ino is the file ID, which file systems are free to not
+        support, in which case it is 0 for every file -- WinFsp mounts such as
+        sshfs-win are one example. os.path.samestat() would then consider any two
+        files on the volume to be the same (python/cpython#78116), so a zero file
+        ID is treated as "unknown", leaving the caller with plain path comparison
+        like on the other platforms (#14864).
+        """
+        s1, s2 = p1.lstat(), p2.lstat()
+        if not s1.st_ino or not s2.st_ino:
+            return False
+        return os.path.samestat(s1, s2)
 
-    Unlike Path.samefile(), does not resolve symlinks.
-    """
-    return os.path.samestat(p1.lstat(), p2.lstat())
+else:
+
+    def samefile_nofollow(p1: Path, p2: Path) -> bool:
+        """Test whether two paths reference the same actual file or directory.
+
+        Unlike Path.samefile(), does not resolve symlinks.
+        """
+        return os.path.samestat(p1.lstat(), p2.lstat())

testing/test_pathlib.py

@@ -38,6 +38,7 @@
 from _pytest.pathlib import resolve_package_path
 from _pytest.pathlib import resolve_pkg_root_and_module_name
 from _pytest.pathlib import safe_exists
+from _pytest.pathlib import samefile_nofollow
 from _pytest.pathlib import scandir
 from _pytest.pathlib import spec_matches_module_path
 from _pytest.pathlib import symlink_or_skip
@@ -570,6 +571,36 @@ def test_samefile_false_negatives(tmp_path: Path, monkeypatch: MonkeyPatch) -> N
     assert getattr(module, "foo")() == 42
 
 
+def test_samefile_nofollow(tmp_path: Path) -> None:
+    p1 = tmp_path / "test_one.py"
+    p2 = tmp_path / "test_two.py"
+    p1.touch()
+    p2.touch()
+
+    assert samefile_nofollow(p1, p1)
+    assert not samefile_nofollow(p1, p2)
+
+
+@pytest.mark.skipif(not sys.platform.startswith("win"), reason="Windows only")
+def test_samefile_nofollow_zero_file_id(
+    tmp_path: Path, monkeypatch: MonkeyPatch
+) -> None:
+    """On Windows file systems which do not support file IDs, st_ino is 0 for every
+    file, which must not make two distinct files compare equal (#14864)."""
+    p1 = tmp_path / "test_one.py"
+    p2 = tmp_path / "test_two.py"
+    p1.touch()
+    p2.touch()
+
+    # st_dev is a real value (the volume serial number), only the file ID is missing.
+    zero_file_id = os.stat_result((0o100644, 0, 3816903231, 1, 0, 0, 0, 0, 0, 0))
+    monkeypatch.setattr(Path, "lstat", lambda self: zero_file_id)
+
+    assert not samefile_nofollow(p1, p2)
+    # Also for the same file -- the caller compares paths first anyway.
+    assert not samefile_nofollow(p1, p1)
+
+
 def test_scandir_with_non_existent_directory() -> None:
     # Test with a directory that does not exist
     non_existent_dir = "path_to_non_existent_dir"