Add profiling_libunwind feature

[ndr-ds]

Configure jemalloc with --enable-prof-libunwind to use libunwind for heap profiling
backtraces. This avoids a known livelock in gcc's _Unwind_Backtrace in multi-threaded
programs (jemalloc/jemalloc#2282).

The feature implies profiling since jemalloc requires --enable-prof alongside
--enable-prof-libunwind. On Linux, the build emits cargo:rustc-link-lib=unwind to
link against libunwind. On Apple platforms no extra link is needed since unwind symbols
are part of libSystem.

As invited by @BusyJay in
#146 (comment).

Closes #146

Summary by CodeRabbit

• New Features
  • Added profiling_libunwind feature flag to enable libunwind-based backtracing for heap profiling, addressing a known livelock issue in multi-threaded environments. Requires libunwind-dev or libunwind-devel on Linux systems.

[ti-chi-bot]

Welcome @ndr-ds! It looks like this is your first PR to tikv/jemallocator 🎉

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: c06958ce-9f32-476f-83b5-e231311aeec2

📥 Commits

Reviewing files that changed from the base of the PR and between baae33d and 5cd1753.

📒 Files selected for processing (5)

• jemalloc-ctl/Cargo.toml
• jemalloc-sys/Cargo.toml
• jemalloc-sys/README.md
• jemalloc-sys/build.rs
• jemallocator/Cargo.toml

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📝 Walkthrough

Walkthrough

Adds a new profiling_libunwind Cargo feature across jemalloc-ctl, jemalloc-sys, and jemallocator crates. The feature configures jemalloc with --enable-prof-libunwind to use libunwind-based backtracing for heap profiling instead of default gcc-based unwinding, and automatically enables the profiling feature alongside optional library linking for supported platforms.

Changes

Cohort / File(s)	Summary
Cargo Feature Declarations jemalloc-ctl/Cargo.toml, jemalloc-sys/Cargo.toml, jemallocator/Cargo.toml	Added new profiling_libunwind feature flags across crates. In jemalloc-ctl and jemallocator, feature activates both tikv-jemalloc-sys/profiling_libunwind and profiling. In jemalloc-sys, feature serves as alias to profiling.
Documentation jemalloc-sys/README.md	Documented new profiling_libunwind feature flag explaining it configures jemalloc with --enable-prof-libunwind for libunwind-based backtracing, avoids _Unwind_Backtrace livelock bug in multi-threaded programs, and includes platform-specific dependency details (libunwind-dev on Linux; system-provided unwind symbols on macOS/iOS).
Build Configuration jemalloc-sys/build.rs	Added conditional build path gated on CARGO_FEATURE_PROFILING_LIBUNWIND. When enabled, passes --enable-prof-libunwind to configure and links unwind library on non-Apple and non-Windows targets.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 With libunwind in tow, profiling takes flight,
No more segfaults haunting the musl night,
Backtrace unwound by a gentler hand,
Across all the platforms, our heaps safely stand! 🧪✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding a new Cargo feature called profiling_libunwind across multiple crates.
Linked Issues check	✅ Passed	The PR implements profiling_libunwind to address the segfault in issue #146 by enabling libunwind-based backtracing instead of gcc's default, avoiding the known livelock bug.
Out of Scope Changes check	✅ Passed	All changes are focused on adding the profiling_libunwind feature across Cargo manifests, README, and build configuration—directly aligned with the issue objective.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[BusyJay]

Cool, but this requires libunwind installed on system, right? I remember rust itself also use libunwind, is it possible to reuse the package?

[ndr-ds]

Rust uses libgcc_s for unwinding on glibc Linux (source), not libunwind. And either way, jemalloc's configure needs libunwind.h at build time, so the dev package seems to be unavoidable for this opt-in feature, AFAICT.