gh-145587: fix busy loop in multiprocessing.connection.wait on Windows (GH-145597)

Ensure wait() blocks for the specified timeout when object_list is empty, preventing 100% CPU usage. This aligns the Windows behavior with the Unix implementation.

Co-authored-by: AN Long <aisk@users.noreply.github.com>

diff --git a/Lib/multiprocessing/connection.py b/Lib/multiprocessing/connection.py
index 41b36066c62fcbf1d112fc3e30a0816ff92b0a96..9ce996c9ccd2116f03f7061cfd9a985adbe19e87 100644 (file)
--- a/Lib/multiprocessing/connection.py
+++ b/Lib/multiprocessing/connection.py
@@ -1085,14 +1085,22 @@ if sys.platform == 'win32':
 
         Returns list of those objects in object_list which are ready/readable.
         '''
+        object_list = list(object_list)
+
+        if not object_list:
+            if timeout is None:
+                while True:
+                    time.sleep(1e6)
+            elif timeout > 0:
+                time.sleep(timeout)
+            return []
+
         if timeout is None:
             timeout = INFINITE
         elif timeout < 0:
             timeout = 0
         else:
             timeout = int(timeout * 1000 + 0.5)
-
-        object_list = list(object_list)
         waithandle_to_obj = {}
         ov_list = []
         ready_objects = set()

diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index cc07062eee6f9806c9116f360905d20ba69bcfef..d67fd13fa33beda27684a42361b24df570eea6ea 100644 (file)
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -3890,6 +3890,19 @@ class _TestConnection(BaseTestCase):
             self.assertRaises(OSError, a.recv)
             self.assertRaises(OSError, b.recv)
 
+    @warnings_helper.ignore_fork_in_thread_deprecation_warnings()
+    def test_wait_empty(self):
+        if self.TYPE != 'processes':
+            self.skipTest('test not appropriate for {}'.format(self.TYPE))
+        # gh-145587: wait() with empty list should respect timeout
+        timeout = 0.5
+        start = time.monotonic()
+        res = self.connection.wait([], timeout=timeout)
+        duration = time.monotonic() - start
+
+        self.assertEqual(res, [])
+        self.assertGreaterEqual(duration, timeout - 0.1)
+
 class _TestListener(BaseTestCase):
 
     ALLOWED_TYPES = ('processes',)

diff --git a/Misc/NEWS.d/next/Library/2026-03-07-14-34-39.gh-issue-145587.flFQ5-.rst b/Misc/NEWS.d/next/Library/2026-03-07-14-34-39.gh-issue-145587.flFQ5-.rst
new file mode 100644 (file)
index 0000000..c17d01f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-03-07-14-34-39.gh-issue-145587.flFQ5-.rst
@@ -0,0 +1 @@
+Resolved a performance regression in ``multiprocessing.connection.wait`` on Windows that caused infinite busy loops when called with no objects. The function now properly yields control to the OS to conserve CPU resources. Patch By Shrey Naithani