bpo-24882: Let ThreadPoolExecutor reuse idle threads before creating new thread (#6375)

* Fixes issue 24882

* Add news file entry for change.

* Change test_concurrent_futures.ThreadPoolShutdownTest

Adjust the shutdown test so that, after submitting three jobs
to the executor, the test checks for less than three threads,
instead of looking for exactly three threads.

If idle threads are being recycled properly, then we should have
less than three threads.

* Switched idle count to semaphor, Updated tests

As suggested by reviewer tomMoral, swapped lock-protected counter
with a semaphore to track the number of unused threads.

Adjusted test_threads_terminate to wait for completiton of the
previous future before submitting a new one (and checking the
number of threads used).

Also added a new test to confirm the thread pool can be saturated.

* Updates tests as requested by pitrou.

* Correct minor whitespace error.

* Make test_saturation faster

1 files changed, 13 insertions, 2 deletions

diff --git a/Lib/concurrent/futures/thread.py b/Lib/concurrent/futures/thread.py
index 2af31a1..ad6b4c2 100644
--- a/Lib/concurrent/futures/thread.py
+++ b/Lib/concurrent/futures/thread.py
@@ -80,7 +80,14 @@ def _worker(executor_reference, work_queue, initializer, initargs):
                 work_item.run()
                 # Delete references to object. See issue16284
                 del work_item
+
+                # attempt to increment idle count
+                executor = executor_reference()
+                if executor is not None:
+                    executor._idle_semaphore.release()
+                del executor
                 continue
+
             executor = executor_reference()
             # Exit if:
             #   - The interpreter is shutting down OR
@@ -133,6 +140,7 @@ class ThreadPoolExecutor(_base.Executor):
 
         self._max_workers = max_workers
         self._work_queue = queue.SimpleQueue()
+        self._idle_semaphore = threading.Semaphore(0)
         self._threads = set()
         self._broken = False
         self._shutdown = False
@@ -178,12 +186,15 @@ class ThreadPoolExecutor(_base.Executor):
     submit.__doc__ = _base.Executor.submit.__doc__
 
     def _adjust_thread_count(self):
+        # if idle threads are available, don't spin new threads
+        if self._idle_semaphore.acquire(timeout=0):
+            return
+
         # When the executor gets lost, the weakref callback will wake up
         # the worker threads.
         def weakref_cb(_, q=self._work_queue):
             q.put(None)
-        # TODO(bquinlan): Should avoid creating new threads if there are more
-        # idle threads than items in the work queue.
+
         num_threads = len(self._threads)
         if num_threads < self._max_workers:
             thread_name = '%s_%d' % (self._thread_name_prefix or self,