Replace custom timer with bthread timer and improve lifecycle safety

[zchuango]

What problem does this PR solve?

Issue Number: #3463（Phase2）

Problem Summary:

This PR addresses the Phase 2 timer and lifecycle stability improvements for UBShmTransport/UBRing discussed in #3463.

The existing UBRing timer and teardown implementation has several lifecycle and scalability issues, including:

• a custom timerfd/epoll-based timer subsystem with per-fd state;
• stale fd / fd reuse risks and races between timer callbacks and deletion;
• blocking cleanup work running on the timer thread;
• cleanup races with UbrTrx release and slot reuse;
• SHM teardown ordering issues.

What is changed and the side effects?

Changed:

• Replace the custom timerfd + epoll/kqueue implementation with bthread_timer_add / bthread_timer_del.
• Introduce explicit timer task lifetime management and UbrTimerDelAndWait for teardown paths that need to wait for in-flight callbacks.
• Introduce UbrCleanupCtl to coordinate delayed cleanup and force-close ownership independently of pooled UbrTrx objects.
• Add generation checks to prevent stale cleanup paths from affecting reused trx slots.
• Remove the blocking ub_flying_io_timeout_s sleep from the timer callback and use delayed cleanup instead.
• Move SHM cleanup SDK calls outside shm_lock and stop the cleanup timer before ubsmem_finalize.
• Add exponential backoff for idle close-check polling.
• Make concurrently accessed UBRing I/O sequence counters atomic.
• Update the related UBRing documentation and clean up obsolete timer code.

Side effects:

• Performance effects:

  • Removes the per-fd timer context table and dedicated timer thread.
  • Reduces close-check polling while links are idle.
  • Avoids blocking the timer thread during delayed cleanup.

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
  • CI passes, including Linux Build and Test, macOS Build, and License Check.
• When providing us with a new feature, it is best to add related tests.
  • brpc_ubring_unittest passes.
  • Client/master communication was verified successfully in a real UB
    environment.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

Pull request overview

This PR refactors the UBRing/UBShmTransport timer and teardown machinery to use bthread timers, with additional lifecycle-safety mechanisms (generation checks + anchored cleanup control objects) to prevent stale timer/callback paths from touching reused trx slots and to avoid blocking work on the timer thread.

Changes:

• Replace the custom timerfd/epoll timer subsystem with a bthread-timer-based facade (UbrTimerStart / UbrTimerDel / UbrTimerDelAndWait) and update call sites accordingly.
• Introduce explicit delayed-cleanup ownership tracking via UbrCleanupCtl anchored in UBRingManager, plus generation checks to avoid slot-reuse races.
• Reduce idle close-check polling via exponential backoff, and improve teardown ordering (e.g., stop UBS SHM cleanup timer before SDK finalize).

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/brpc_ubring_unittest.cpp	Extends configuration tests to cover the new close-check backoff cap flag default.
src/brpc/ubshm/ubr_trx.h	Replaces fd-based timers with bthread timer handles; adds cleanup control state and makes hot counters atomic.
src/brpc/ubshm/ub_ring.h	Updates UBRing APIs to remove timer-fd parameters and align cleanup entry points with new lifecycle model.
src/brpc/ubshm/ub_ring.cpp	Implements bthread-timer scheduling, delayed cleanup ownership, close-check backoff, and atomic I/O sequencing.
src/brpc/ubshm/ub_ring_manager.h	Adds per-slot generation + cleanup-ctl anchoring APIs to coordinate delayed cleanup across slot reuse.
src/brpc/ubshm/ub_ring_manager.cpp	Implements generation-safe release and teardown-time cancellation/waiting for in-flight cleanups.
src/brpc/ubshm/ub_helper.cpp	Removes eager timer module init (timers become lazy via bthread facade).
src/brpc/ubshm/ub_endpoint.cpp	Fixes poller SID updates by replacing existing set entries on ADD/MOD.
src/brpc/ubshm/timer/timer_mgr.h	Defines the bthread-timer facade API and documents callback/teardown expectations.
src/brpc/ubshm/timer/timer_mgr.cpp	Replaces timerfd/epoll/kqueue implementation with bthread timer task management and delete/wait semantics.
src/brpc/ubshm/shm/shm_ubs.cpp	Moves UBS cleanup work outside locks, switches cleanup timer to bthread, and fixes teardown ordering vs SDK finalize.
src/brpc/ubshm/common/common.h	Adds SEC_TO_USEC for consistent time conversions used by the new timer paths.
docs/en/ubring.md	Updates timer documentation to reflect bthread timer usage and backoff policy.
docs/cn/ubring.md	Same as English docs: updates timer documentation to reflect new bthread timer approach.

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[zchuango]

I've updated the timer deletion contract to clarify the one-shot and periodic timer semantics, and addressed the related comments.

@wwbmmm could you please take another look when you have time?