bpo-33540: Add block_on_close attr to socketserver (GH-6911)

Add a new block_on_close class attribute to ForkingMixIn and
ThreadingMixIn classes of socketserver to opt-in for pre-3.7 behaviour.

Doc/library/socketserver.rst

@@ -116,10 +116,13 @@ server classes.
    only available on POSIX platforms that support :func:`~os.fork`.
 
    :meth:`socketserver.ForkingMixIn.server_close` waits until all child
-   processes complete.
+   processes complete, except if
+   :attr:`socketserver.ForkingMixIn.block_on_close` attribute is false.
 
    :meth:`socketserver.ThreadingMixIn.server_close` waits until all non-daemon
-   threads complete. Use daemonic threads by setting
+   threads complete, except if
+   :attr:`socketserver.ThreadingMixIn.block_on_close` attribute is false. Use
+   daemonic threads by setting
    :data:`ThreadingMixIn.daemon_threads` to ``True`` to not wait until threads
    complete.
 
@@ -128,6 +131,8 @@ server classes.
       :meth:`socketserver.ForkingMixIn.server_close` and
       :meth:`socketserver.ThreadingMixIn.server_close` now waits until all
       child processes and non-daemonic threads complete.
+      Add a new :attr:`socketserver.ForkingMixIn.block_on_close` class
+      attribute to opt-in for the pre-3.7 behaviour.
 
 
 .. class:: ForkingTCPServer

Doc/whatsnew/3.7.rst

@@ -1216,6 +1216,18 @@ by default.
 (Contributed by Christian Heimes in :issue:`28134`.)
 
 
+socketserver
+------------
+
+:meth:`socketserver.ThreadingMixIn.server_close` now waits until all non-daemon
+threads complete. :meth:`socketserver.ForkingMixIn.server_close` now waits
+until all child processes complete.
+
+Add a new :attr:`socketserver.ForkingMixIn.block_on_close` class attribute to
+:class:`socketserver.ForkingMixIn` and :class:`socketserver.ThreadingMixIn`
+classes. Set the class attribute to ``False`` to get the pre-3.7 behaviour.
+
+
 sqlite3
 -------
 
@@ -2156,10 +2168,17 @@ Changes in the Python API
   and module are affected by this change. (Contributed by INADA Naoki and
   Eugene Toder in :issue:`29463`.)
 
-* :meth:`~socketserver.BaseServer.server_close` in
+* :meth:`socketserver.ThreadingMixIn.server_close` now waits until all
-  :class:`socketserver.ThreadingMixIn` and :class:`socketserver.ForkingMixIn`
+  non-daemon threads complete.  Set the new
-  now waits until all non-daemon threads complete.
+  :attr:`socketserver.ThreadingMixIn.block_on_close` class attribute to
-  (Contributed by Victor Stinner in :issue:`31233` and :issue:`31151`.)
+  ``False`` to get the pre-3.7 behaviour.
+  (Contributed by Victor Stinner in :issue:`31233` and :issue:`33540`.)
+
+* :meth:`socketserver.ForkingMixIn.server_close` now waits until all
+  child processes complete. Set the new
+  :attr:`socketserver.ForkingMixIn.block_on_close` class attribute to ``False``
+  to get the pre-3.7 behaviour.
+  (Contributed by Victor Stinner in :issue:`31151` and :issue:`33540`.)
 
 * The :func:`locale.localeconv` function now temporarily sets the ``LC_CTYPE``
   locale to the value of ``LC_NUMERIC`` in some cases.

Lib/socketserver.py

@@ -543,6 +543,8 @@ if hasattr(os, "fork"):
         timeout = 300
         active_children = None
         max_children = 40
+        # If true, server_close() waits until all child processes complete.
+        block_on_close = True
 
         def collect_children(self, *, blocking=False):
             """Internal routine to wait for children that have exited."""
@@ -620,7 +622,7 @@ if hasattr(os, "fork"):
 
         def server_close(self):
             super().server_close()
-            self.collect_children(blocking=True)
+            self.collect_children(blocking=self.block_on_close)
 
 
 class ThreadingMixIn:
@@ -629,6 +631,8 @@ class ThreadingMixIn:
     # Decides how threads will act upon termination of the
     # main process
     daemon_threads = False
+    # If true, server_close() waits until all non-daemonic threads terminate.
+    block_on_close = True
     # For non-daemonic threads, list of threading.Threading objects
     # used by server_close() to wait for all threads completion.
     _threads = None
@@ -659,11 +663,12 @@ class ThreadingMixIn:
 
     def server_close(self):
         super().server_close()
-        threads = self._threads
+        if self.block_on_close:
-        self._threads = None
+            threads = self._threads
-        if threads:
+            self._threads = None
-            for thread in threads:
+            if threads:
-                thread.join()
+                for thread in threads:
+                    thread.join()
 
 
 if hasattr(os, "fork"):

Misc/NEWS.d/next/Library/2018-05-16-18-10-38.bpo-33540.wy9LRV.rst

@@ -0,0 +1,2 @@
+Add a new ``block_on_close`` class attribute to ``ForkingMixIn`` and
+``ThreadingMixIn`` classes of :mod:`socketserver`.