`concurrent.futures.Executor.map` temporarily exceeds its `buffersize` while collecting the next result

Bug report

Bug description:

Potential follow up after the merge of #125663 (introduce buffersize param in Executor.map).

"Bug": concurrent.futures.Executor.map temporarily exceeds its buffersize while collecting the next result:

Let's define:

results: Iterator = executor.map(fn, iterable, buffersize=buffersize)

What happens when calling next(results):

1. fetch the next arg from interable and put a task for fn(arg) in the buffer
2. wait for next result to be available
3. yield the collected result

So during step 2. the buffer's size is actually buffersize + 1.

If we want a strict guarantee that there is at most buffersize concurrent tasks even during a next, we have to swap step 1. and step 2.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-131467

Commencez par concurrent.futures.Executor.map et examinez le comportement décrit concernant la collecte du résultat suivant. Consultez la PR liée gh-131467 pour comprendre le travail déjà en cours. Le travail est terminé lorsqu’il est garanti que le tampon ne dépasse jamais temporairement la valeur de buffersize demandée pendant que next(results) attend un résultat.