bpo-43517: Fix false positive in detection of circular imports

Lib/test/test_importlib/partial/cfimport.py

@@ -0,0 +1,38 @@
+import os
+import sys
+import threading
+import traceback
+
+
+NLOOPS = 50
+NTHREADS = 30
+
+
+def t1():
+    try:
+        from concurrent.futures import ThreadPoolExecutor
+    except Exception:
+        traceback.print_exc()
+        os._exit(1)
+
+def t2():
+    try:
+        from concurrent.futures.thread import ThreadPoolExecutor
+    except Exception:
+        traceback.print_exc()
+        os._exit(1)
+
+def main():
+    for j in range(NLOOPS):
+        threads = []
+        for i in range(NTHREADS):
+            threads.append(threading.Thread(target=t2 if i % 1 else t1))
+        for thread in threads:
+            thread.start()
+        for thread in threads:
+            thread.join()
+        sys.modules.pop('concurrent.futures', None)
+        sys.modules.pop('concurrent.futures.thread', None)
+
+if __name__ == "__main__":
+    main()

Lib/test/test_importlib/partial/pool_in_threads.py

@@ -0,0 +1,27 @@
+import multiprocessing
+import os
+import threading
+import traceback
+
+
+def t():
+    try:
+        with multiprocessing.Pool(1):
+            pass
+    except Exception:
+        traceback.print_exc()
+        os._exit(1)
+
+
+def main():
+    threads = []
+    for i in range(20):
+        threads.append(threading.Thread(target=t))
+    for thread in threads:
+        thread.start()
+    for thread in threads:
+        thread.join()
+
+
+if __name__ == "__main__":
+    main()

Lib/test/test_importlib/test_threaded_import.py

@@ -17,7 +17,7 @@
 from test.support import (verbose, run_unittest)
 from test.support.import_helper import forget
 from test.support.os_helper import (TESTFN, unlink, rmtree)
-from test.support import threading_helper
+from test.support import script_helper, threading_helper
 
 def task(N, done, done_tasks, errors):
     try:
@@ -245,6 +245,18 @@ def target():
         __import__(TESTFN)
         del sys.modules[TESTFN]
 
+    def test_concurrent_futures_circular_import(self):
+        # Regression test for bpo-43515
+        fn = os.path.join(os.path.dirname(__file__),
+                          'partial', 'cfimport.py')
+        script_helper.assert_python_ok(fn)
+
+    def test_multiprocessing_pool_circular_import(self):
+        # Regression test for bpo-41567
+        fn = os.path.join(os.path.dirname(__file__),
+                          'partial', 'pool_in_threads.py')
+        script_helper.assert_python_ok(fn)
+
 
 @threading_helper.reap_threads
 def test_main():

Misc/NEWS.d/next/Core and Builtins/2021-03-16-17-12-54.bpo-43517.zAo6Ws.rst

@@ -0,0 +1,2 @@
+Fix misdetection of circular imports when using ``from pkg.mod import
+attr``, which caused false positives in non-trivial multi-threaded code.

Python/import.c

@@ -1593,7 +1593,7 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
     }
 
     if (mod != NULL && mod != Py_None) {
-        if (import_ensure_initialized(tstate->interp, mod, name) < 0) {
+        if (import_ensure_initialized(tstate->interp, mod, abs_name) < 0) {
             goto error;
         }
     }