Description

Make child task submission idempotent and thread-safe in FuturesDependencyExecutor. This specifically targets _Task._submit which used to assert it is only ever called once. It can now be called multiple times, but the submission will only ever be performed by the first thread.

I thought about using locks more liberally in other places to maintain the internal assertion that _submit() is only called once, but ultimately I think this solution is easier to reason about.

Related to #145, which addressed some other race conditions.

Fixes #156

Type of Change

• Bug (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds or improves functionality)
• Internal (refactoring, performance, and testing)
• Breaking (fix or feature that would cause existing functionality to not work as expected)
• Documentation (no changes to the code)
• Development (CI workflows, packages, templates, etc.)
• Package (package dependencies and versions)

Testing

Not really... I couldn't reliably reproduce the error.

Pull Request Checklist

Please confirm the PR meets the following requirements.

• Relevant tags are added (breaking, bug, documentation, enhancement, package, etc.).
• Code changes pass pre-commit (e.g., ruff, mypy, etc.).
• Tests have been added to show the fix is effective or that the new feature works.
• New and existing unit tests pass locally with the changes.
• Docs have been updated and reviewed if relevant.