[3.12] gh-111644: Fix support threading_cleanup() (GH-111714)

Lib/test/support/threading_helper.py

@@ -22,34 +22,37 @@
 
 
 def threading_setup():
-    return _thread._count(), threading._dangling.copy()
+    return _thread._count(), len(threading._dangling)
 
 
 def threading_cleanup(*original_values):
-    _MAX_COUNT = 100
-
-    for count in range(_MAX_COUNT):
-        values = _thread._count(), threading._dangling
-        if values == original_values:
-            break
-
-        if not count:
-            # Display a warning at the first iteration
-            support.environment_altered = True
-            dangling_threads = values[1]
-            support.print_warning(f"threading_cleanup() failed to cleanup "
-                                  f"{values[0] - original_values[0]} threads "
-                                  f"(count: {values[0]}, "
-                                  f"dangling: {len(dangling_threads)})")
-            for thread in dangling_threads:
-                support.print_warning(f"Dangling thread: {thread!r}")
-
-            # Don't hold references to threads
-            dangling_threads = None
-        values = None
-
-        time.sleep(0.01)
-        support.gc_collect()
+    orig_count, orig_ndangling = original_values
+
+    timeout = 1.0
+    for _ in support.sleeping_retry(timeout, error=False):
+        # Copy the thread list to get a consistent output. threading._dangling
+        # is a WeakSet, its value changes when it's read.
+        dangling_threads = list(threading._dangling)
+        count = _thread._count()
+
+        if count <= orig_count:
+            return
+
+    # Timeout!
+    support.environment_altered = True
+    support.print_warning(
+        f"threading_cleanup() failed to clean up threads "
+        f"in {timeout:.1f} seconds\n"
+        f"  before: thread count={orig_count}, dangling={orig_ndangling}\n"
+        f"  after: thread count={count}, dangling={len(dangling_threads)}")
+    for thread in dangling_threads:
+        support.print_warning(f"Dangling thread: {thread!r}")
+
+    # The warning happens when a test spawns threads and some of these threads
+    # are still running after the test completes. To fix this warning, join
+    # threads explicitly to wait until they complete.
+    #
+    # To make the warning more likely, reduce the timeout.
 
 
 def reap_threads(func):