Issue #22185: Fix an occasional RuntimeError in threading.Condition.wait() caused by mutation of the waiters queue without holding the lock.

Patch by Doug Zongker.

3 files changed, 11 insertions, 5 deletions

diff --git a/Lib/threading.py b/Lib/threading.py
index c7c4478..66620a9 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -290,6 +290,7 @@ class Condition:
         waiter.acquire()
         self._waiters.append(waiter)
         saved_state = self._release_save()
+        gotit = False
         try:    # restore state no matter what (e.g., KeyboardInterrupt)
             if timeout is None:
                 waiter.acquire()
@@ -299,14 +300,14 @@ class Condition:
                     gotit = waiter.acquire(True, timeout)
                 else:
                     gotit = waiter.acquire(False)
-                if not gotit:
-                    try:
-                        self._waiters.remove(waiter)
-                    except ValueError:
-                        pass
             return gotit
         finally:
             self._acquire_restore(saved_state)
+            if not gotit:
+                try:
+                    self._waiters.remove(waiter)
+                except ValueError:
+                    pass
 
     def wait_for(self, predicate, timeout=None):
         """Wait until a condition evaluates to True.
diff --git a/Misc/ACKS b/Misc/ACKS
index a8ea6d7..200d615 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1518,4 +1518,5 @@ Cheng Zhang
 Kai Zhu
 Tarek Ziadé
 Gennadiy Zlobin
+Doug Zongker
 Peter Åstrand
diff --git a/Misc/NEWS b/Misc/NEWS
index c9a40df..68c5d90 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -124,6 +124,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #22185: Fix an occasional RuntimeError in threading.Condition.wait()
+  caused by mutation of the waiters queue without holding the lock.  Patch
+  by Doug Zongker.
+
 - Issue #22287: On UNIX, _PyTime_gettimeofday() now uses
   clock_gettime(CLOCK_REALTIME) if available. As a side effect, Python now
   depends on the librt library on Solaris and on Linux (only with glibc older