Bugfix: SignalTrace mode has memory and deadlock issues

[chenBright]

What problem does this PR solve?

Issue Number: resolve #3015

Problem Summary:

1. After the signal handler times out, the context may become invalid.
2. SignalSync destructor in signal handler may cause deadlock.

What is changed and the side effects?

Changed:

1. Use async-signal-safe absl instead of libunwind to trace the stack in the signal handler.
2. Hold additional ref of SignalSync to avoid SignalSync destructor in signal handler.

Side effects:

• Performance effects:

• Breaking backward compatibility:

---

Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

Pull Request Overview

This PR refactors the bthread stack tracing implementation to use Abseil’s async-signal-safe unwinder, removes the old enable_fast_unwind mode, simplifies Result, and updates build scripts and docs to pull in Abseil dependencies.

• Replace libunwind‐based fast/slow unwind toggling with a single Abseil unwinder (DefaultStackUnwinder)
• Simplify Result struct (single error buffer, remove semaphore/traditional pipe sync)
• Update CMake, Bazel, shell scripts, and CI workflows to add Abseil dependencies and flags

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/bthread_unittest.cpp	Remove fast/unwind flag tests, adjust loop break logic
src/bthread/task_tracer.h	Simplify Result, remove semaphore, add _inuse_signal_syncs
src/bthread/task_tracer.cpp	Remove old signal sync, implement Abseil unwinder, update APIs
docs/cn/bthread_tracer.md	Update installation steps to include Abseil
config_brpc.sh	Add Abseil header/lib discovery
MODULE.bazel	Normalize quotes, add Abseil deps
CMakeLists.txt	find_package(absl) and link Abseil targets
BUILD.bazel	Add Abseil debug libraries
.github/workflows/ci-linux.yml	Update CI flags for tracer
.github/actions/...	Install Abseil in custom actions

Comments suppressed due to low confidence (2)

src/bthread/task_tracer.cpp:62

• pipe_fds are never initialized via pipe(), so calling make_non_blocking on -1 will fail. You need to call pipe(pipe_fds) before setting them non-blocking.

    }

.github/workflows/ci-linux.yml:179

• You removed the --cc/--cxx=clang-12 flags from this step. If your build assumes clang-12, CI may now compile with the system default compiler, potentially causing failures. Re-add or confirm the intended compiler flags.

        options: --with-bthread-tracer

[wwbmmm]

LGTM

[chenBright]

1. FIx memory leak of SignalSync.
2. Fix wrong status of global priority bthread.

[chenBright]

@wwbmmm ping!

[wwbmmm]

LGTM