Issue #20778: Fix modulefinder to work with bytecode-only modules.

Bug filed and initial attempt at a patch by Bohuslav Kabrda.

Lib/modulefinder.py

@@ -287,7 +287,7 @@ class ModuleFinder:
             if fp.read(4) != imp.get_magic():
                 self.msgout(2, "raise ImportError: Bad magic number", pathname)
                 raise ImportError("Bad magic number in %s" % pathname)
-            fp.read(4)
+            fp.read(8)  # Skip mtime and size.
             co = marshal.load(fp)
         else:
             co = None

Lib/test/test_modulefinder.py

 import os
 import errno
+import importlib.machinery
+import py_compile
 import shutil
 import unittest
 import tempfile
@@ -208,6 +210,14 @@ a/module.py
                                 from . import *
 """]
 
+bytecode_test = [
+    "a",
+    ["a"],
+    [],
+    [],
+    ""
+]
+
 
 def open_file(path):
     dirname = os.path.dirname(path)
@@ -288,6 +298,16 @@ class ModuleFinderTest(unittest.TestCase):
     def test_relative_imports_4(self):
         self._do_test(relative_import_test_4)
 
+    def test_bytecode(self):
+        base_path = os.path.join(TEST_DIR, 'a')
+        source_path = base_path + importlib.machinery.SOURCE_SUFFIXES[0]
+        bytecode_path = base_path + importlib.machinery.BYTECODE_SUFFIXES[0]
+        with open_file(source_path) as file:
+            file.write('testing_modulefinder = True\n')
+        py_compile.compile(source_path, cfile=bytecode_path)
+        os.remove(source_path)
+        self._do_test(bytecode_test)
+
 
 def test_main():
     support.run_unittest(ModuleFinderTest)

Misc/NEWS

@@ -29,6 +29,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #20778: Fix modulefinder to work with bytecode-only modules.
+
 - Issue #20791: copy.copy() now doesn't make a copy when the input is
   a bytes object.  Initial patch by Peter Otten.