Issue #18351: Fix various issues with

importlib._bootstrap._get_sourcefile().

Thanks to its only use by the C API, it was never properly tested
until now.

Thanks to Neal Norwitz for discovering the bug and Madison May for the patch.

Lib/test/test_import.py

@@ -1,5 +1,6 @@
 # We import importlib *ASAP* in order to test #15386
 import importlib
+from importlib._bootstrap import _get_sourcefile
 import builtins
 import imp
 from test.test_importlib.import_ import util as importlib_util
@@ -11,6 +12,7 @@ import random
 import stat
 import sys
 import unittest
+import unittest.mock as mock
 import textwrap
 import errno
 import shutil
@@ -864,6 +866,40 @@ class ImportlibBootstrapTests(unittest.TestCase):
         self.assertIs(imp.new_module, mod.new_module)
 
 
+@cpython_only
+class GetSourcefileTests(unittest.TestCase):
+
+    """Test importlib._bootstrap._get_sourcefile() as used by the C API.
+
+    Because of the peculiarities of the need of this function, the tests are
+    knowingly whitebox tests.
+
+    """
+
+    def test_get_sourcefile(self):
+        # Given a valid bytecode path, return the path to the corresponding
+        # source file if it exists.
+        with mock.patch('importlib._bootstrap._path_isfile') as _path_isfile:
+            _path_isfile.return_value = True;
+            path = TESTFN + '.pyc'
+            expect = TESTFN + '.py'
+            self.assertEqual(_get_sourcefile(path), expect)
+
+    def test_get_sourcefile_no_source(self):
+        # Given a valid bytecode path without a corresponding source path,
+        # return the original bytecode path.
+        with mock.patch('importlib._bootstrap._path_isfile') as _path_isfile:
+            _path_isfile.return_value = False;
+            path = TESTFN + '.pyc'
+            self.assertEqual(_get_sourcefile(path), path)
+
+    def test_get_sourcefile_bad_ext(self):
+        # Given a path with an invalid bytecode extension, return the
+        # bytecode path passed as the argument.
+        path = TESTFN + '.bad_ext'
+        self.assertEqual(_get_sourcefile(path), path)
+
+
 class ImportTracebackTests(unittest.TestCase):
 
     def setUp(self):
@@ -1028,7 +1064,7 @@ def test_main(verbose=None):
     run_unittest(ImportTests, PycacheTests, FilePermissionTests,
                  PycRewritingTests, PathsTests, RelativeImportTests,
                  OverridingImportBuiltinTests,
-                 ImportlibBootstrapTests,
+                 ImportlibBootstrapTests, GetSourcefileTests,
                  TestSymbolicallyLinkedPackage,
                  ImportTracebackTests)