# HG changeset patch
# User Joshua Harlow <harlowja@gmail.com>
# Date 1441135537 25200
#      Tue Sep 01 12:25:37 2015 -0700
# Node ID 21c7f1439ee3cb4bab5af0bdcc8f1e35af3031f2
# Parent  0073f17ed1f8aa28aa98682c878e2d66af8719fd
Add a 'on_new_thread' callback to concurrent thread executor

This addition allows for users to provide a callback that
will be called once per spawned thread. It can be used to setup
thread local state (for example).

diff -r 0073f17ed1f8 -r 21c7f1439ee3 Lib/concurrent/futures/thread.py
--- a/Lib/concurrent/futures/thread.py	Tue Sep 01 02:33:20 2015 -0700
+++ b/Lib/concurrent/futures/thread.py	Tue Sep 01 12:25:37 2015 -0700
@@ -58,8 +58,13 @@
         else:
             self.future.set_result(result)
 
-def _worker(executor_reference, work_queue):
+def _worker(executor_reference, work_queue, on_new_thread):
     try:
+        if on_new_thread is not None:
+            try:
+                on_new_thread()
+            finally:
+                del on_new_thread
         while True:
             work_item = work_queue.get(block=True)
             if work_item is not None:
@@ -81,12 +86,15 @@
         _base.LOGGER.critical('Exception in worker', exc_info=True)
 
 class ThreadPoolExecutor(_base.Executor):
-    def __init__(self, max_workers=None):
+    def __init__(self, max_workers=None, on_new_thread=None):
         """Initializes a new ThreadPoolExecutor instance.
 
         Args:
             max_workers: The maximum number of threads that can be used to
                 execute the given calls.
+            on_new_thread: Callback that will be called by each newly spawned
+                thread; typically useful for setting up any thread local
+                state (or other thread-specific data).
         """
         if max_workers is None:
             # Use this number because ThreadPoolExecutor is often
@@ -95,6 +103,7 @@
         if max_workers <= 0:
             raise ValueError("max_workers must be greater than 0")
 
+        self._on_new_thread = on_new_thread
         self._max_workers = max_workers
         self._work_queue = queue.Queue()
         self._threads = set()
@@ -124,7 +133,8 @@
         if len(self._threads) < self._max_workers:
             t = threading.Thread(target=_worker,
                                  args=(weakref.ref(self, weakref_cb),
-                                       self._work_queue))
+                                       self._work_queue,
+                                       self._on_new_thread))
             t.daemon = True
             t.start()
             self._threads.add(t)