fix(python-capture): preserve __file__ on instrumented modules

Why: FlowtraceFinder rebuilt ModuleSpec from scratch, losing
spec.has_location=True (default False on bare ModuleSpec). CPython
only assigns module.__file__ when has_location is True, so user code
relying on Path(__file__).parent or pkg.__file__ broke under
instrumentation. Mutate the spec PathFinder returned instead — it
already has the correct flags. Adds regression test that imports an
instrumented module and asserts __file__ is set at top-level and
inside function scope.

Author: Rixmerz

capture/python/flowtrace_runtime/finder.py

@@ -78,12 +78,8 @@ def find_spec(
             return None
 
         loader = FlowtraceSourceLoader(name, origin)
-        new_spec = importlib.machinery.ModuleSpec(
-            name=name,
-            loader=loader,
-            origin=origin,
-            is_package=spec.submodule_search_locations is not None,
-        )
-        if spec.submodule_search_locations is not None:
-            new_spec.submodule_search_locations = list(spec.submodule_search_locations)
-        return new_spec
+        # Mutate the spec PathFinder gave us instead of rebuilding it.
+        # Rebuilding loses has_location=True (default False on bare ModuleSpec),
+        # which makes CPython skip setting __file__ on the module.
+        spec.loader = loader
+        return spec

capture/python/tests/test_integration.py

@@ -105,3 +105,61 @@ def test_source_not_modified():
     h1 = _original_sha256()
     h2 = _original_sha256()
     assert h1 == h2
+
+
+def test_instrumented_module_preserves_dunder_file():
+    """Regression: FlowtraceFinder must preserve spec.has_location so __file__ is set.
+
+    Reason: a previous implementation rebuilt ModuleSpec without copying
+    has_location=True, which made CPython skip assigning module.__file__.
+    User code relying on Path(__file__).parent or pkg.__file__ then broke.
+    """
+    with tempfile.TemporaryDirectory() as tmpdir:
+        pkg_dir = Path(tmpdir) / "ft_pkg_dunder"
+        pkg_dir.mkdir()
+        (pkg_dir / "__init__.py").write_text("", encoding="utf-8")
+        mod_path = pkg_dir / "leaf.py"
+        mod_path.write_text(
+            "from pathlib import Path\n"
+            "MY_FILE = __file__\n"
+            "MY_PARENT = str(Path(__file__).parent)\n"
+            "def hello():\n"
+            "    return MY_FILE\n",
+            encoding="utf-8",
+        )
+
+        probe = (
+            "import sys, json\n"
+            f"sys.path.insert(0, {str(tmpdir)!r})\n"
+            "import ft_pkg_dunder.leaf as m\n"
+            "print(json.dumps({'file': m.__file__, 'my_file': m.MY_FILE, 'parent': m.MY_PARENT, 'hello': m.hello()}))\n"
+        )
+
+        existing_pythonpath = os.environ.get("PYTHONPATH", "")
+        pythonpath = os.pathsep.join(
+            [str(STUB_DIR), str(CAPTURE_PKG)] + ([existing_pythonpath] if existing_pythonpath else [])
+        )
+
+        env = {
+            **os.environ,
+            "PYTHONPATH": pythonpath,
+            "FLOWTRACE_ENABLE": "1",
+            "FLOWTRACE_PACKAGE_PREFIX": "ft_pkg_dunder",
+        }
+        env.pop("FLOWTRACE_OUTPUT", None)
+
+        result = subprocess.run(
+            [sys.executable, "-c", probe],
+            env=env,
+            capture_output=True,
+            text=True,
+            timeout=15,
+        )
+        assert result.returncode == 0, f"probe failed: {result.stderr}"
+        payload = json.loads(result.stdout.strip().splitlines()[-1])
+
+        expected_file = str(mod_path)
+        assert payload["file"] == expected_file, f"module.__file__ mismatch: {payload}"
+        assert payload["my_file"] == expected_file, f"top-level __file__ broken: {payload}"
+        assert payload["parent"] == str(pkg_dir), f"Path(__file__).parent broken: {payload}"
+        assert payload["hello"] == expected_file, f"function-scope __file__ broken: {payload}"