Issue #22853: Fixed a deadlock when use multiprocessing.Queue at import time.

Patch by Florian Finkernagel and Davin Potts.

Lib/multiprocessing/queues.py

@@ -44,10 +44,10 @@ import weakref
 
 from Queue import Empty, Full
 import _multiprocessing
-from multiprocessing import Pipe
-from multiprocessing.synchronize import Lock, BoundedSemaphore, Semaphore, Condition
-from multiprocessing.util import debug, info, Finalize, register_after_fork
-from multiprocessing.forking import assert_spawning
+from . import Pipe
+from .synchronize import Lock, BoundedSemaphore, Semaphore, Condition
+from .util import debug, info, Finalize, register_after_fork, is_exiting
+from .forking import assert_spawning
 
 #
 # Queue type using a pipe, buffer and thread
@@ -229,8 +229,6 @@ class Queue(object):
     @staticmethod
     def _feed(buffer, notempty, send, writelock, close):
         debug('starting thread to feed data to pipe')
-        from .util import is_exiting
-
         nacquire = notempty.acquire
         nrelease = notempty.release
         nwait = notempty.wait

Lib/test/test_multiprocessing.py

@@ -620,6 +620,26 @@ class _TestQueue(BaseTestCase):
         for p in workers:
             p.join()
 
+    def test_no_import_lock_contention(self):
+        with test_support.temp_cwd():
+            module_name = 'imported_by_an_imported_module'
+            with open(module_name + '.py', 'w') as f:
+                f.write("""if 1:
+                    import multiprocessing
+
+                    q = multiprocessing.Queue()
+                    q.put('knock knock')
+                    q.get(timeout=3)
+                    q.close()
+                """)
+
+            with test_support.DirsOnSysPath(os.getcwd()):
+                try:
+                    __import__(module_name)
+                except Queue.Empty:
+                    self.fail("Probable regression on import lock contention;"
+                              " see Issue #22853")
+
 #
 #
 #

Misc/ACKS

@@ -411,6 +411,7 @@ Vincent Fiack
 Anastasia Filatova
 Tomer Filiba
 Jeffrey Finkelstein
+Florian Finkernagel
 Russell Finn
 Dan Finnie
 Nils Fischbeck
@@ -1073,6 +1074,7 @@ Martin Pool
 Iustin Pop
 Claudiu Popa
 John Popplewell
+Davin Potts
 Guillaume Pratte
 Amrit Prem
 Paul Prescod

Misc/NEWS

@@ -18,6 +18,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #22853: Fixed a deadlock when use multiprocessing.Queue at import time.
+  Patch by Florian Finkernagel and Davin Potts.
+
 - Issue #23476: In the ssl module, enable OpenSSL's X509_V_FLAG_TRUSTED_FIRST
   flag on certificate stores when it is available.