gh-NNNN: Skip creating GatheringFuture if all futures finished eagerly

Author: itamaro

Lib/asyncio/tasks.py

@@ -815,6 +815,7 @@ def _done_callback(fut):
     nfinished = 0
     loop = None
     outer = None  # bpo-46672
+    all_finished = True
     for arg in coros_or_futures:
         if arg not in arg_to_fut:
             fut = ensure_future(arg, loop=loop)
@@ -829,15 +830,23 @@ def _done_callback(fut):
 
             nfuts += 1
             arg_to_fut[arg] = fut
-            fut.add_done_callback(_done_callback)
+            if not fut.done():
+                all_finished = False
+                fut.add_done_callback(_done_callback)
 
         else:
             # There's a duplicate Future object in coros_or_futures.
             fut = arg_to_fut[arg]
 
         children.append(fut)
 
-    outer = _GatheringFuture(children, loop=loop)
+    if all_finished:
+        # optimization: skip creating GatheringFuture if all children are done
+        # (e.g. when all coros are able to complete eagerly)
+        outer = futures.Future(loop=loop)
+        outer.set_result(c.result for c in children)
+    else:
+        outer = _GatheringFuture(children, loop=loop)
     return outer