Leverage eager tasks to optimize asyncio gather & TaskGroups further

[itamaro]

Feature or enhancement

gh-97696 introduced eager tasks factory, which speeds up some async-heavy workloads by up to 50% when opted in.

installing the eager tasks factory applies out-of-the-box when gathering tasks (asyncio.gather(...)), and when creating tasks as part of a TaskGroup, e.g.:

asyncio.get_event_loop().set_task_factory(asyncio.eager_task_factory)

await asyncio.gather(coro1, coro2, coro3)

async with asyncio.TaskGroup() as tg:
    tg.create_task(coro1)
    tg.create_task(coro2)
    tg.create_task(coro3)

in both examples, coro{1,2,3} will eagerly execute the first step, and potentially complete without scheduling to the event loop if the coros don't block.

the implementation of both gather and TaskGroup uses callbacks internally that end up getting scheduled to the event loop even if all the tasks were able to finish synchronously, and blocking the coroutine in which gather or TaskGroup() was awaited, preventing from the task to complete eagerly even if otherwise it could.

applications that use multiple levels of nested gathers / TaskGroups can benefit significantly from eagerly completing multiple levels without blocking.

this can be achieved by modifying the implementation of gather and TaskGroup to detect when it can complete without blocking, and skip scheduling wakeup & done callbacks.

Linked PRs

• gh-104144: Skip scheduling a done callback if a TaskGroup task completes eagerly #104140
• gh-104144: Optimize gather to finish eagerly when all futures complete eagerly #104138