Filestore] add SIGUSR1 retries to FUSE loop thread shutdown to mitigate single lost SIGUSR1 wakeup

[neihar]

Problem

https://github.com/ydb-platform/nbs/actions/runs/27998974260/job/82867193543

Notes

TFuseLoopThread::Stop() sends one pthread_kill(SIGUSR1), then joins. The loop thread runs while(!fuse_session_exited(se)) read(/dev/fuse). A signal only breaks read() with EINTR if the thread is already sleeping inside it. There is a short window between the exit-flag check and the start of read(). If the signal arrives in that window, the no-op handler eats it. The thread then enters read() with nothing pending and blocks forever. No more FUSE requests come. No more signals are sent. The unmount that would break read() with ENODEV runs only after the join, and the join is already stuck on this thread. This window widens under host load. The bug predates the multi-queue refactor (#4045).

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🔴 linux-x86_64-release-tsan target: cloud/filestore/ (test time: 5275s): some tests FAILED for commit f3036c7.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
3500	3499	0	1	0	0	0

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🟢 linux-x86_64-relwithdebinfo target: cloud/filestore/ (test time: 6792s): all tests PASSED for commit f3036c7.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
4144	4144	0	0	0	0	0

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🟢 linux-x86_64-release-ubsan target: cloud/filestore/ (test time: 4561s): all tests PASSED for commit f3036c7.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
3560	3560	0	0	0	0	0

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🟢 linux-x86_64-release-ubsan target: cloud/filestore/ (test time: 4711s): all tests PASSED for commit 4063480.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
3560	3560	0	0	0	0	0

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🔴 linux-x86_64-release-tsan target: cloud/filestore/ (test time: 5129s): some tests FAILED for commit 4063480.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
3500	3498	0	2	0	0	0

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• ✅ filestore

🔴 linux-x86_64-relwithdebinfo target: cloud/filestore/ (test time: 6609s): some tests FAILED for commit 4063480.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
4144	4143	0	1	0	0	0

🟢 linux-x86_64-relwithdebinfo target: cloud/filestore/ (test time: 65s): all tests PASSED for commit 4063480.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
2	2	0	0	0	0	0