Compare also paths on Windows when considering ImportPathMismatchError

On Windows, os.path.samefile returns false for paths mounted in UNC paths which
point to the same location.

I couldn't reproduce the actual case reported, but looking at the code it seems
this commit should fix the issue.

Fix #7678
Fix #8076

changelog/7678.bugfix.rst

@@ -0,0 +1,2 @@
+Fixed bug where ``ImportPathMismatchError`` would be raised for files compiled in
+the host and loaded later from an UNC mounted path (Windows).

src/_pytest/pathlib.py

@@ -543,7 +543,7 @@ def import_path(
             module_file = module_file[: -(len(os.path.sep + "__init__.py"))]
 
         try:
-            is_same = os.path.samefile(str(path), module_file)
+            is_same = _is_same(str(path), module_file)
         except FileNotFoundError:
             is_same = False
 
@@ -553,6 +553,20 @@ def import_path(
     return mod
 
 
+# Implement a special _is_same function on Windows which returns True if the two filenames
+# compare equal, to circumvent os.path.samefile returning False for mounts in UNC (#7678).
+if sys.platform.startswith("win"):
+
+    def _is_same(f1: str, f2: str) -> bool:
+        return Path(f1) == Path(f2) or os.path.samefile(f1, f2)
+
+
+else:
+
+    def _is_same(f1: str, f2: str) -> bool:
+        return os.path.samefile(f1, f2)
+
+
 def resolve_package_path(path: Path) -> Optional[Path]:
     """Return the Python package path by looking for the last
     directory upwards which still contains an __init__.py.

testing/test_pathlib.py

@@ -7,6 +7,7 @@ from textwrap import dedent
 import py
 
 import pytest
+from _pytest.monkeypatch import MonkeyPatch
 from _pytest.pathlib import bestrelpath
 from _pytest.pathlib import commonpath
 from _pytest.pathlib import ensure_deletable
@@ -414,3 +415,23 @@ def test_visit_ignores_errors(tmpdir) -> None:
         "bar",
         "foo",
     ]
+
+
+@pytest.mark.skipif(not sys.platform.startswith("win"), reason="Windows only")
+def test_samefile_false_negatives(tmp_path: Path, monkeypatch: MonkeyPatch) -> None:
+    """
+    import_file() should not raise ImportPathMismatchError if the paths are exactly
+    equal on Windows. It seems directories mounted as UNC paths make os.path.samefile
+    return False, even when they are clearly equal.
+    """
+    module_path = tmp_path.joinpath("my_module.py")
+    module_path.write_text("def foo(): return 42")
+    monkeypatch.syspath_prepend(tmp_path)
+
+    with monkeypatch.context() as mp:
+        # Forcibly make os.path.samefile() return False here to ensure we are comparing
+        # the paths too. Using a context to narrow the patch as much as possible given
+        # this is an important system function.
+        mp.setattr(os.path, "samefile", lambda x, y: False)
+        module = import_path(module_path)
+    assert getattr(module, "foo")() == 42