[native] Replace std::mutex with pthread_mutex_t in the CoreCLR host

[simonrozsival]

PR relationship: Prerequisite: #12534 (merged). This is the bottom of stack #12653.

Part of #12533 (drop libc++ from the CoreCLR host). Stacked on top of #12534.

std::mutex is a thin wrapper over pthread_mutex_t, but using it pulls <mutex> and out-of-line libc++ symbols into the native host. This PR stores pthread_mutex_t directly and provides a small CoreCLR RAII guard so locked scopes retain automatic unlock behavior without libc++.

What

• Replace CoreCLR's std::mutex instances with pthread_mutex_t, initialized using PTHREAD_MUTEX_INITIALIZER.
• Add xamarin::android::lock_guard under the CoreCLR include tree. It takes pthread_mutex_t& and calls pthread_mutex_lock / pthread_mutex_unlock.
• Update CoreCLR's StartupAwareLock to take pthread_mutex_t& directly.
• Convert shared Timing to direct pthread calls and keep it explicitly non-copyable and non-movable.
• Leave src/native/mono unchanged; MonoVM retains its existing mutex and templated lock_guard implementation in mono/shared/cppcompat.hh.

PTHREAD_MUTEX_INITIALIZER keeps static instances constant-initialized, so this adds no thread-safe initialization guards. The CoreCLR guard is header-only and compiles down to direct pthread calls.

Effect

This removes the last #include <mutex> in the repository:

	files including <mutex>	std::mutex / std::lock_guard uses
before	6	13
after	0	0

It also drops eight undefined libc++ references:

symbol	before	after
std::__ndk1::mutex::lock()	3	0
std::__ndk1::mutex::unlock()	3	0
std::__ndk1::mutex::~mutex()	2	0

	refs	__cxa_guard_*
#12534 (base)	55	10
this PR	47	10

The link-time libc++ requirement only disappears when every cause reaches zero, so this is one of several prerequisites rather than a self-sufficient win. The remaining causes (operator new/delete[], __cxa_guard_*, std::string, __libcpp_verbose_abort) are tracked in #12533.

Verification

• CoreCLR, MonoVM and NativeAOT build clean.
• fastdev-assemblies.cc is #if defined(DEBUG) and was additionally compiled with -DDEBUG.
• The CoreCLR guard compiled with the runtime's no-C++-exceptions settings references only pthread_mutex_lock and pthread_mutex_unlock; it introduces no C++ exception-runtime or initialization-guard symbols.
• Real libc++ references: CoreCLR 55 → 47, NativeAOT 0. The eight removed references are exactly the mutex members listed above.
• __cxa_guard_* undefined references remain at 10, confirming that the static mutexes remain constant-initialized.

[Copilot]

Pull request overview

This PR reduces the CoreCLR host’s dependence on libc++ by replacing std::mutex/std::lock_guard usage with a minimal pthread-backed Mutex/MutexGuard wrapper in the shared native runtime-base headers.

Changes:

• Add Mutex and MutexGuard wrappers around pthread_mutex_t in runtime-base/mutex.hh.
• Update CoreCLR host code paths (assembly store, FastDev assemblies, DSO loader, startup-aware lock) to use the new wrapper instead of <mutex>.
• Update shared timing code to use the new wrapper (affecting other runtime lanes via common sources).

Show a summary per file

File	Description
src/native/common/include/runtime-base/timing.hh	Switch timing sequence locking to Mutex/MutexGuard.
src/native/common/include/runtime-base/mutex.hh	Introduce pthread-backed Mutex and MutexGuard.
src/native/clr/include/runtime-base/startup-aware-lock.hh	Replace std::mutex reference with Mutex.
src/native/clr/include/runtime-base/monodroid-dl.hh	Replace static std::mutex with Mutex for DSO handle write lock.
src/native/clr/include/host/fastdev-assemblies.hh	Replace override directory lock from std::mutex to Mutex.
src/native/clr/include/host/assembly-store.hh	Replace assembly decompress lock from std::mutex to Mutex.
src/native/clr/host/fastdev-assemblies.cc	Replace std::lock_guard usage with MutexGuard.
src/native/clr/host/assembly-store.cc	Replace internal state_lock and std::lock_guard usage with Mutex/MutexGuard.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 2
• Review effort level: Lite

[simonrozsival]

/review

[github-actions]

✅ Android PR Reviewer completed successfully!

Generated by Android PR Reviewer for #12541

[github-actions]

⚠️ Needs Changes

Findings: 0 errors, 1 warning, 0 suggestions.

The direct pthread conversion is otherwise mechanically consistent, keeps the static mutexes constant-initialized, and preserves the startup-aware locking behavior. The remaining issue is that Timing no longer inherits std::mutex’s non-copyable semantics; its copy/move operations should be explicitly deleted before merging.

CI is still in progress: 41 of 44 checks had passed, with two running and one queued at review time; no failures were reported.

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