bpo-20369: concurrent.futures.wait() now deduplicates futures given a…

Doc/library/concurrent.futures.rst

@@ -444,7 +444,8 @@ Module Functions
 .. function:: wait(fs, timeout=None, return_when=ALL_COMPLETED)
 
    Wait for the :class:`Future` instances (possibly created by different
-   :class:`Executor` instances) given by *fs* to complete.  Returns a named
+   :class:`Executor` instances) given by *fs* to complete. Duplicate futures
+   given to *fs* are removed and will be returned only once. Returns a named
    2-tuple of sets.  The first set, named ``done``, contains the futures that
    completed (finished or cancelled futures) before the wait completed.  The
    second set, named ``not_done``, contains the futures that did not complete

Lib/concurrent/futures/_base.py

@@ -282,13 +282,14 @@ def wait(fs, timeout=None, return_when=ALL_COMPLETED):
         A named 2-tuple of sets. The first set, named 'done', contains the
         futures that completed (is finished or cancelled) before the wait
         completed. The second set, named 'not_done', contains uncompleted
-        futures.
+        futures. Duplicate futures given to *fs* are removed and will be 
+        returned only once.
     """
+    fs = set(fs)
     with _AcquireFutures(fs):
-        done = set(f for f in fs
-                   if f._state in [CANCELLED_AND_NOTIFIED, FINISHED])
-        not_done = set(fs) - done
-
+        done = {f for f in fs
+                   if f._state in [CANCELLED_AND_NOTIFIED, FINISHED]}
+        not_done = fs - done
         if (return_when == FIRST_COMPLETED) and done:
             return DoneAndNotDoneFutures(done, not_done)
         elif (return_when == FIRST_EXCEPTION) and done:
@@ -307,7 +308,7 @@ def wait(fs, timeout=None, return_when=ALL_COMPLETED):
             f._waiters.remove(waiter)
 
     done.update(waiter.finished_futures)
-    return DoneAndNotDoneFutures(done, set(fs) - done)
+    return DoneAndNotDoneFutures(done, fs - done)
 
 class Future(object):
     """Represents the result of an asynchronous computation."""

Lib/test/test_concurrent_futures.py

@@ -578,6 +578,14 @@ def test_shutdown_no_wait(self):
 
 
 class WaitTests:
+    def test_20369(self):
+        # See https://bugs.python.org/issue20369
+        future = self.executor.submit(time.sleep, 1.5)
+        done, not_done = futures.wait([future, future],
+                            return_when=futures.ALL_COMPLETED)
+        self.assertEqual({future}, done)
+        self.assertEqual(set(), not_done)
+
 
     def test_first_completed(self):
         future1 = self.executor.submit(mul, 21, 2)

Misc/NEWS.d/next/Library/2021-12-17-12-06-40.bpo-20369.zzLuBz.rst

@@ -0,0 +1 @@
+:func:`concurrent.futures.wait` no longer blocks forever when given duplicate Futures. Patch by Kumar Aditya.