gh-152915: Add ExecutorShutdownError to concurrent.futures

[zainnadeem786]

Summary

This PR introduces concurrent.futures.ExecutorShutdownError, a typed exception raised when work is submitted to an executor that can no longer accept new futures because it has been shut down.

The new exception subclasses RuntimeError, preserving backward compatibility while allowing callers to distinguish executor shutdown from unrelated runtime failures without relying on exception message matching.

Changes

• Add ExecutorShutdownError to concurrent.futures.
• Export the new public exception from concurrent.futures.
• Update ThreadPoolExecutor.submit() to raise ExecutorShutdownError after:
  • explicit executor shutdown
  • interpreter shutdown
• Update ProcessPoolExecutor.submit() to raise ExecutorShutdownError after:
  • explicit executor shutdown
  • interpreter shutdown
• Preserve the existing exception messages.
• Update the concurrent.futures test suite.
• Update the documentation.
• Add a Library NEWS entry.

InterpreterPoolExecutor inherits the updated thread executor behavior, so no additional implementation changes were required.

Backward Compatibility

ExecutorShutdownError subclasses RuntimeError, so existing code catching RuntimeError continues to work unchanged while enabling callers to distinguish executor shutdown from unrelated runtime failures.

Close gh-152915

[read-the-docs-community]

Documentation build overview

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2 files changed

± library/concurrent.futures.html
± whatsnew/changelog.html