Add a typed exception for concurrent.futures executor shutdown

Proposal:

concurrent.futures.Executor.submit() implementations currently raise a generic RuntimeError when work is submitted after executor shutdown. For example, ThreadPoolExecutor.submit() raises one of these messages:

RuntimeError("cannot schedule new futures after shutdown")
RuntimeError("cannot schedule new futures after interpreter shutdown")

This makes it difficult for callers to distinguish expected executor lifecycle failures from unrelated RuntimeErrors without matching exception message strings:

try:
    future = executor.submit(fn)
except RuntimeError as exc:
    if str(exc) in {
        "cannot schedule new futures after shutdown",
        "cannot schedule new futures after interpreter shutdown",
    }:
        handle_executor_shutdown(exc)
    else:
        raise

It would be useful to expose a typed exception for this condition, for example:

class ExecutorShutdownError(RuntimeError):
    pass

Then executor implementations could raise ExecutorShutdownError for explicit shutdown and interpreter-shutdown submission failures. Because it would subclass RuntimeError, existing callers catching RuntimeError would remain compatible, while callers that need precise handling could avoid brittle string matching:

try:
    future = executor.submit(fn)
except concurrent.futures.ExecutorShutdownError:
    handle_executor_shutdown()

This would mirror the existing benefit of typed executor failures such as BrokenExecutor, where callers can distinguish executor state from arbitrary runtime failures without inspecting exception text.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

None. I searched existing CPython issues for ExecutorShutdownError, cannot schedule new futures after shutdown, cannot schedule new futures after interpreter shutdown, and related concurrent.futures RuntimeError shutdown exception type wording. I found issues about shutdown behavior itself, but not this typed-exception API request.

Linked PRs

• gh-153003

先閱讀 concurrent.futures.Executor.submit() 的實作以及現有的 BrokenExecutor API。檢視連結的 PR gh-153003 及其相關測試；完成標準是：關閉期間的提交會暴露專用例外，同時維持與 RuntimeError 的相容性，並涵蓋明確關閉和直譯器關閉的情況。