fix a bug in aio_cancel()

[esohns]

excerpt from the aio_cancel(3) manpage:
..."If aiocbp is not NULL, and fd differs from the file descriptor with which the asynchronous operation was initiated, unspecified results occur. ..."

I debugged this on Linux, and the call returns -1, setting errno to EINVAL.

This PR fixes that condition.

Summary by CodeRabbit

• Bug Fixes
  • Enhanced async I/O operation cancellation to target specific operations more precisely, improving reliability of asynchronous operations.

[coderabbitai]

Walkthrough

Modified POSIX_Proactor.cpp to add result pointer validation via assertion in cancel_aiocb and changed aio_cancel invocation to target a specific file descriptor from the result structure instead of canceling all operations globally.

Changes

Cohort / File(s)	Summary
Async I/O Cancellation Refinement ACE/ace/POSIX_Proactor.cpp	Added assertion to validate result pointer in cancel_aiocb; modified aio_cancel to use specific file descriptor (result->aio_fildes) instead of global cancellation (0)

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

• Single file with focused changes to a cancellation utility function
• Assertion addition is straightforward defensive programming
• File descriptor parameter change requires verification that targeting specific fd instead of global cancellation aligns with intended behavior and doesn't introduce unintended side effects in existing call sites

Poem

🐰 A cancel call, once broad and wide,
Now hops to targets, dignified.
With assertions firm, we guard the way—
Precision wins the proactor's day! ✨

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix a bug in aio_cancel()' directly corresponds to the main change in the pull request, which fixes a bug in the aio_cancel() function by using the correct file descriptor from the asynchronous I/O control block.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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📜 Recent review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 1a9f9ff and 4de982b.

📒 Files selected for processing (1)

• ACE/ace/POSIX_Proactor.cpp (1 hunks)

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🔇 Additional comments (2)

ACE/ace/POSIX_Proactor.cpp (2)

1505-1505: Good defensive programming with the assertion.

The assertion validates that the result pointer is not NULL before it's dereferenced on line 1509. While the call site at line 1484 already ensures this condition, the assertion provides an additional safety check and documents the precondition clearly.

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1509-1509: Correct fix per POSIX aio_cancel(3) specification.

The change from aio_cancel(0, result) to aio_cancel(result->aio_fildes, result) correctly addresses the bug. Per the POSIX specification, when the aiocbp parameter is not NULL, the fd parameter must match aiocbp->aio_fildes; otherwise, the behavior is unspecified. The old code violated this requirement, causing EINVAL errors on Linux. This fix ensures the file descriptor matches the one in the result structure.

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