`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

從 concurrent.futures.Executor.map 開始，檢視圍繞收集下一個結果所描述的行為。查看已連結的 PR gh-131467，以了解已在進行中的工作。完成的標準是確保在 next(results) 等待結果時，緩衝區絕不會暫時超過要求的 buffersize。