[native] Replace the AndroidSystem path statics with a POD buffer

[simonrozsival]

Part of #12533 (drop the libc++ dependency), stacked on #12551.

AndroidSystem kept five of its members in std::string / std::array<std::string, 1>: primary_override_dir, native_libraries_dir, app_code_cache_dir, single_app_lib_directory and override_dirs.

Because they are inline static with dynamic initialization, the compiler emits a guard variable and an atexit registration for them in every translation unit that includes android-system.hh — even in ones that never touch them. logger.cc, internal-pinvokes-clr.cc, internal-pinvokes-shared.cc and android-system-shared.cc each paid four libc++ references (~basic_string, operator delete, __cxa_guard_acquire, __cxa_guard_release) without using a single one of these directories:

$ llvm-nm --undefined-only logger.cc.o | llvm-cxxfilt
std::__ndk1::basic_string<...>::~basic_string()
operator delete(void*)
__cxa_guard_acquire
__cxa_guard_release

$ llvm-objdump -r logger.cc.o | grep _ZGV | llvm-cxxfilt
guard variable for xamarin::android::AndroidSystem::override_dirs
guard variable for xamarin::android::AndroidSystem::app_code_cache_dir
guard variable for xamarin::android::AndroidSystem::native_libraries_dir
guard variable for xamarin::android::AndroidSystem::primary_override_dir
guard variable for xamarin::android::AndroidSystem::single_app_lib_directory

What changed

All five become plain pointers. The three path members are const char* initialized to "" and assigned once, early during startup, with a copy made by a new Util::duplicate_string() helper that aborts if the allocation fails. Pointers to a string literal are constant-initialized, so neither a guard variable nor an atexit registration is emitted.

The two directory arrays become plain const char* arrays whose entries are malloced, which also drops an operator new[] from the non-split-APK path.

Since there is no longer a fixed-size buffer anywhere, there is also no hard limit on the path length and no abort when it is exceeded — which is what NativeAOT's char[SENSIBLE_PATH_MAX] primary_override_dir used to do. That lets primary_override_dir be shared by all three hosts, removing three #if defined (XA_HOST_NATIVEAOT) blocks and determine_primary_override_dir() entirely.

Results

Undefined libc++ references in the three CoreCLR archives — 58 → 31:

object	before	after
assembly-store.cc.o	13	11
host.cc.o	11	11
android-system.cc.o	11	5
timing-internal.cc.o	5	2
logger.cc.o	4	0
internal-pinvokes-shared.cc.o	4	0
internal-pinvokes-clr.cc.o	4	0
android-system-shared.cc.o	4	0
typemap.cc.o	2	2

Every __cxa_guard_* reference coming from this header is gone; the only ones left are host.cc's own function-local statics.

libnet-android.release.so: 539,464 → 536,368 bytes (−3,096).

The DEBUG-only code paths were compile-checked separately (there is no Debug ninja directory) and go from 12 to 7 references; llvm-nm confirms add_system_property, find_bundled_property and setup_environment_from_override_file are genuinely emitted rather than silently #if'd out.

CoreCLR, NativeAOT and MonoVM all build clean.

[Copilot]

Copilot review overview

Review tier: Lite
Findings: 1 · 2

New issues introduced by this change (3)

Severity	Finding
	src/​native/​clr/​runtime-base/​android-system.cc — ❌ error (security): dir_length + 1uz can overflow before malloc, which would lead to a…
	src/​native/​clr/​runtime-base/​android-system.cc — ❌ error: Allocating app_lib_directories_size * sizeof (const char*) should use overflow-checked…
	src/​native/​clr/​include/​runtime-base/​path-buffer.hh — ⚠️ warning: path_buffer is easy to default-initialize on the stack (e.g. path_buffer<N> p;),…

What changed in this PR

This PR reduces CoreCLR host libc++ dependencies and startup overhead by removing dynamically-initialized inline static std::string members from AndroidSystem, replacing them with a POD-style path buffer and const char* directory lists to avoid per-TU guard variables and atexit registrations.

Changes:

• Introduces path_buffer<N> for constant-initialized, mostly stack-buffer path storage with heap fallback.
• Converts AndroidSystem path members and directory lists away from std::string/std::array<std::string,...> to path_buffer / const char*, and adjusts DSO path formatting APIs to use std::string_view.
• Updates CoreCLR host components to consume the new AndroidSystem getters returning const char*.

File	Description
src/​native/​clr/​runtime-base/​android-system.cc	Switches app/override directory storage and DSO load iteration to const char* + malloced storage; updates DSO path formatting to string_view.
src/​native/​clr/​include/​runtime-base/​path-buffer.hh	Adds path_buffer<N> POD-style path storage abstraction with inline buffer + heap fallback.
src/​native/​clr/​include/​runtime-base/​android-system.hh	Reworks AndroidSystem statics/getters/setters to use path_buffer and const char* directory arrays/spans.
src/​native/​clr/​host/​host.cc	Updates native library directory usage to const char* from AndroidSystem.
src/​native/​clr/​host/​fastdev-assemblies.cc	Updates override directory usage/logging to const char* from AndroidSystem.
src/​native/​clr/​host/​assembly-store.cc	Updates code-cache-dir empty check for const char* API.