Issue #19399: fix sporadic test_subprocess failure.

Change Thread.join() with a negative timeout to just return.  The
behavior isn't documented then, but this restores previous
behavior.

Lib/threading.py

@@ -1056,10 +1056,13 @@ class Thread:
             raise RuntimeError("cannot join thread before it is started")
         if self is current_thread():
             raise RuntimeError("cannot join current thread")
+
         if timeout is None:
             self._wait_for_tstate_lock()
-        else:
+        elif timeout >= 0:
             self._wait_for_tstate_lock(timeout=timeout)
+        # else it's a negative timeout - precise behavior isn't documented
+        # then, but historically .join() returned in this case
 
     def _wait_for_tstate_lock(self, block=True, timeout=-1):
         # Issue #18808: wait for the thread state to be gone.

Misc/NEWS

@@ -27,6 +27,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #19399: fix sporadic test_subprocess failure.
+
 - Issue #13234: Fix os.listdir to work with extended paths on Windows.
   Patch by Santoso Wijaya.