bpo-27144: concurrent.futures as_complete and map iterators do not keep reference to returned object (#1560)

* bpo-27144: concurrent.futures as_complie and map iterators do not keep
reference to returned object

* Some nits.  Improve wordings in docstrings and comments, and avoid relying on
sys.getrefcount() in tests.

1 files changed, 13 insertions, 1 deletions

diff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
index 8f1d714193a..03b28ab5d68 100644
--- a/Lib/concurrent/futures/process.py
+++ b/Lib/concurrent/futures/process.py
@@ -357,6 +357,18 @@ def _check_system_limits():
     raise NotImplementedError(_system_limited)
 
 
+def _chain_from_iterable_of_lists(iterable):
+    """
+    Specialized implementation of itertools.chain.from_iterable.
+    Each item in *iterable* should be a list.  This function is
+    careful not to keep references to yielded objects.
+    """
+    for element in iterable:
+        element.reverse()
+        while element:
+            yield element.pop()
+
+
 class BrokenProcessPool(RuntimeError):
     """
     Raised when a process in a ProcessPoolExecutor terminated abruptly
@@ -482,7 +494,7 @@ class ProcessPoolExecutor(_base.Executor):
         results = super().map(partial(_process_chunk, fn),
                               _get_chunks(*iterables, chunksize=chunksize),
                               timeout=timeout)
-        return itertools.chain.from_iterable(results)
+        return _chain_from_iterable_of_lists(results)
 
     def shutdown(self, wait=True):
         with self._shutdown_lock: