gh-104144: Optimize gather to finish eagerly when all futures complete eagerly

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 +830,26 @@ def _done_callback(fut):
 
             nfuts += 1
             arg_to_fut[arg] = fut
-            fut.add_done_callback(_done_callback)
+            if fut.done():
+                # call the callback immediately instead of scheduling it
+                _done_callback(fut)
+            else:
+                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 completed
+        # (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
 
 

Misc/NEWS.d/next/Library/2023-05-03-16-50-24.gh-issue-104144.yNkjL8.rst

@@ -0,0 +1,3 @@
+Optimize asyncio.gather when using eager tasks factory Skip creating
+gathering future and scheduling done callbacks when all futures finish
+without blocking - for up to 3x speedup