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 a new feature flag, profiling_libunwind, to enable libunwind-based backtracing for heap profiling; it also enables profiling automatically.
• Bug Fixes
  • Avoids a known livelock issue with unwind-based backtraces in multi-threaded programs.
• Documentation
  • Updated docs to describe the feature and platform requirements (libunwind packages on Linux; system unwind symbols on macOS/iOS).

[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: 6c23083e-9ce4-4a67-8905-3c866f951b3d

📥 Commits

Reviewing files that changed from the base of the PR and between 5cd1753 and cf42d64.

📒 Files selected for processing (2)

• jemalloc-sys/Cargo.toml
• jemalloc-sys/README.md

✅ Files skipped from review due to trivial changes (1)

• jemalloc-sys/README.md

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📝 Walkthrough

Walkthrough

Adds a new profiling_libunwind Cargo feature across crates to enable jemalloc heap-profiling with libunwind (configures with --enable-prof-libunwind), and documents platform linking and dependency notes.

Changes

Profiling Libunwind Feature

Layer / File(s)	Summary
Cargo Feature Declarations jemalloc-ctl/Cargo.toml, jemalloc-sys/Cargo.toml, jemallocator/Cargo.toml	Added profiling_libunwind feature flags across crates. In jemalloc-ctl and jemallocator, the feature enables tikv-jemalloc-sys/profiling_libunwind and profiling. In jemalloc-sys, the feature aliases to profiling.
Build Configuration jemalloc-sys/build.rs	Build script now respects CARGO_FEATURE_PROFILING_LIBUNWIND, passing --enable-prof-libunwind to configure and linking unwind on non-Apple/non-Windows targets.
Documentation jemalloc-sys/README.md	Documented profiling_libunwind: uses --enable-prof-libunwind to force libunwind-based backtraces for profiling, notes a livelock workaround for _Unwind_Backtrace, and lists platform dependency guidance (Linux: libunwind-dev/devel; macOS/iOS: system unwind symbols).

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested reviewers

• BusyJay

Poem

🐰 I hopped through Cargo.toml with glee,
Enabled libunwind for your heap's decree,
Build flags set and docs now sing,
Profiling backtraces take to wing,
A rabbit cheers—no more deadlocking spree! 🥕

🚥 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 and concisely describes the main change: adding a new Cargo feature called profiling_libunwind across multiple crates.
Linked Issues check	✅ Passed	The PR adds the profiling_libunwind feature to address the musl + profiling segfault by using libunwind-based backtracing, directly implementing the solution approach described in issue #146.
Out of Scope Changes check	✅ Passed	All changes are scoped to adding the profiling_libunwind feature: updated Cargo.toml files, README documentation, and build.rs logic, with no unrelated modifications.
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.

[ndr-ds]

@BusyJay Happy to adjust the approach if you have any concerns!

[BusyJay]

Can you verify whether it's linked dynamically or statically?