tests: add test for ctypes.CDLL incorrectly picking up builtin extens…

…ion libraries

On linux and macOS, some of the built-ins are provided as extensions
(e.g., _sha1.cpython-39-x86_64-linux-gnu.so) that originally reside
in python3.X/lib-dynload directory. This directory is not in the
ctypes' library search path, therefore running
ctypes.CDLL('_sha1.cpython-39-x86_64-linux-gnu.so')
in python interpreter will come up empty.

In a frozen application, however, these extensions end up collected
directly in the _MEIPASS, which is searched by ctypes (because search
behavior is explicitly overriden by the hook). Therefore, trying to
load the extension's library file via ctypes.CDLL() will succeed,
resulting in inconsistent behavior between unfrozen and frozen
program.

On macOS, this causes issues with `pycryptodomex` (pyinstaller#5583), which,
due to its library search implementation, ends up with handle of
`_sha1` extension module instead of its private extension/library
 with the same name prefix.

tests/functional/test_import.py

@@ -11,6 +11,7 @@
 #-----------------------------------------------------------------------------
 
 import os
+import sys
 import glob
 import ctypes
 import ctypes.util
@@ -371,6 +372,31 @@ def g():
     pyi_builder.test_source(source % locals(), test_id=test_id)
 
 
+def test_ctypes_cdll_builtin_extension(pyi_builder):
+    # Take a built-in that is provided as an extension
+    builtin_ext = '_sha256'
+    if builtin_ext in sys.builtin_module_names:
+        # On Windows, built-ins do not seem to be extensions
+        pytest.skip(f"{builtin_ext} is a built-in module without extension.")
+
+    pyi_builder.test_source(
+        """
+        import ctypes
+        import importlib.machinery
+
+        # Try to load CDLL with all possible extension suffices; this
+        # should fail in all cases, as built-in extensions should not
+        # be in the ctypes' search path.
+        builtin_ext = '{0}'
+        for suffix in importlib.machinery.EXTENSION_SUFFIXES:
+            try:
+                lib = ctypes.CDLL(builtin_ext + suffix)
+            except OSError:
+                lib = None
+            assert lib is None, "Built-in extension picked up by ctypes.CDLL!"
+        """.format(builtin_ext))
+
+
 # TODO: Add test-cases for the prefabricated library loaders supporting
 # attribute accesses on windows. Example::
 #