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

Lib/asyncio/tasks.py

@@ -813,6 +813,7 @@ def _done_callback(fut):
  children = []
  nfuts = 0
  nfinished = 0
+ done_futs = []
  loop = None
  outer = None # bpo-46672
  for arg in coros_or_futures:
@@ -829,7 +830,10 @@ def _done_callback(fut):
 
  nfuts += 1
  arg_to_fut[arg] = fut
- fut.add_done_callback(_done_callback)
+ if fut.done():
+ done_futs.append(fut)
+ else:
+ fut.add_done_callback(_done_callback)
 
  else:
  # There's a duplicate Future object in coros_or_futures.
@@ -838,6 +842,13 @@ def _done_callback(fut):
  children.append(fut)
 
  outer = _GatheringFuture(children, loop=loop)
+ # Run done callbacks after GatheringFuture created so any post-processing
+ # can be performed at this point
+ # optimization: in the special case that *all* futures finished eagerly,
+ # this will effectively complete the gather eagerly, with the last
+ # callback setting the result (or exception) on outer before returning it
+ for fut in done_futs:
+ _done_callback(fut)
  return outer
 
 

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

@@ -0,0 +1,3 @@
+Optimize :func:`asyncio.gather` when using :func:`asyncio.eager_task_factory`
+to complete eagerly if all fututres completed eagerly.
+Avoid scheduling done callbacks for fututres that complete eagerly.