Shutdown regressions in signal handler (from #848)

[j-rivero]

🦟 Bug fix

Summary

While reviewing #848 one of my testing agents running TSAN was complaining about different problems with races. See full report in: https://gist.github.com/j-rivero/2132044f9d4d0083f1e116729b23c0ed. Summarized of what I understand this are real problems:

• src/WaitHelpers.cc:61,74,91 - g_shutdownPipe is shared unsafely between first-call initialization and the signal handler; TSan reported a race between pipe() setup and handler write().

• src/WaitHelpers.cc:104 - one signal writes one byte, so only one blocked waiter wakes. A 2-waiter probe left one thread blocked, regressing the old notify_all semantics.

• src/WaitHelpers.cc:70-82 - the signal handler should save and restore errno, otherwise it can clobber interrupted code's error state.

I've asked the agent to create tests that demonstrate the problem failing hard.

• Add POSIX-only waitForShutdown regression tests that fail in a normal build when one signal only wakes a single waiter,
• Repeated signals block on the shutdown pipe
• The signal handler clobbers errno.

In draft by now until we confirm that these are real scenarios that can happen and prepare a fix in this PR for them.

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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Generated-by: Claude Sonnet 4.6 + GPT-5.4 (Copilot)

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