[3.6] bpo-29212: Fix the ugly repr() ThreadPoolExecutor thread name. (GH-2315) (#3276)

bpo-29212: Fix the ugly ThreadPoolExecutor thread name.

Fixes the newly introduced ugly default thread name for concurrent.futures
thread.ThreadPoolExecutor threads.  They'll now resemble the old <=3.5
threading default Thread-x names by being named ThreadPoolExecutor-y_n..
(cherry picked from commit a3d91b43c2)

Lib/concurrent/futures/thread.py

@@ -7,6 +7,7 @@ __author__ = 'Brian Quinlan (brian@sweetapp.com)'
 
 import atexit
 from concurrent.futures import _base
+import itertools
 import queue
 import threading
 import weakref
@@ -83,6 +84,10 @@ def _worker(executor_reference, work_queue):
         _base.LOGGER.critical('Exception in worker', exc_info=True)
 
 class ThreadPoolExecutor(_base.Executor):
+
+    # Used to assign unique thread names when thread_name_prefix is not supplied.
+    _counter = itertools.count().__next__
+
     def __init__(self, max_workers=None, thread_name_prefix=''):
         """Initializes a new ThreadPoolExecutor instance.
 
@@ -103,7 +108,8 @@ class ThreadPoolExecutor(_base.Executor):
         self._threads = set()
         self._shutdown = False
         self._shutdown_lock = threading.Lock()
-        self._thread_name_prefix = thread_name_prefix
+        self._thread_name_prefix = (thread_name_prefix or
+                                    ("ThreadPoolExecutor-%d" % self._counter()))
 
     def submit(self, fn, *args, **kwargs):
         with self._shutdown_lock: