fix(internal): support for namespace packages [backport 2.5] (#8087)

Backport bed61ae from #8048 to 2.5.

We fix an issue with the import detection of namespace packages that
causes a warning to be emitted, notifying that the import machinery is
falling back to the legacy behaviour.

Fixes #8039.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)
- [x] If this PR touches code that signs or publishes builds or
packages, or handles credentials of any kind, I've requested a review
from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

Co-authored-by: Gabriele N. Tornetta <[email protected]>

Author: github-actions[bot]

ddtrace/appsec/_iast/_loader.py

@@ -25,5 +25,5 @@ def _exec_iast_patched_module(module_watchdog, module):
         # Patched source is executed instead of original module
         compiled_code = compile(patched_source, module_path, "exec")
         exec(compiled_code, module.__dict__)  # nosec B102
-    else:
+    elif module_watchdog.loader is not None:
         module_watchdog.loader.exec_module(module)

ddtrace/internal/module.py

@@ -144,9 +144,10 @@ def __init__(self, loader, spec=None):
 
         self.callbacks = {}  # type: Dict[Any, Callable[[ModuleType], None]]
 
-        if hasattr(loader, "create_module"):
+        # A missing loader is generally an indication of a namespace package.
+        if loader is None or hasattr(loader, "create_module"):
             self.create_module = self._create_module
-        if hasattr(loader, "exec_module"):
+        if loader is None or hasattr(loader, "exec_module"):
             self.exec_module = self._exec_module
 
     def __getattr__(self, name):

releasenotes/notes/fix-module-namespace-package-support-90b326382951be19.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fix for an import issue that caused the pytest plugin to fail to properly
+    initialize a test session and exit with an import exception.

tests/internal/test_module.py

@@ -438,3 +438,16 @@ def ns_hook(module):
     finally:
         sys.path.pop(0)
         ModuleWatchdog.uninstall()
+
+
+@pytest.mark.subprocess(
+    ddtrace_run=True,
+    env=dict(
+        PYTHONPATH=os.pathsep.join((str(Path(__file__).parent), os.environ.get("PYTHONPATH", ""))),
+        PYTHONDEVMODE="1",
+    ),
+)
+def test_module_watchdog_namespace_import_no_warnings():
+    # Test that the namespace import does not emit warnings (e.g. fallback to
+    # legacy import machinery).
+    import namespace_test.ns_module  # noqa:F401