[native] Remove the libc++ dependencies from the CoreCLR host's startup path

[simonrozsival]

PR relationship: Depends on #12552 (and transitively #12551). This is the top of stack #12654. The absolute symbol and size totals below were measured in the former serialized stack and include the parallel prerequisite work.

Part of #12533 (drop libc++ from the CoreCLR host).

This clears host.cc entirely. The CoreCLR host goes from 21 undefined libc++ references to 12, and every one that remains is now in a single file, assembly-store.cc.

The three uses in host.cc

1. The assembly store path

scan_filesystem_for_assemblies_and_libraries joined the native library directory and the store filename with a std::string. It now uses Util::join_paths, added earlier in this stack, which keeps the path in a stack buffer unless it does not fit and frees only when the returned pointer differs from that buffer.

2. APP_CONTEXT_BASE_DIRECTORY

static std::string app_context_base_directory;

The static is needed because the value has to outlive coreclr_initialize — CoreCLR keeps the pointer. But std::string has a non-trivial constructor, so a function-local static of that type forces a guard variable, and this was the only source of __cxa_guard_acquire/__cxa_guard_release in the file. A plain char* is constant-initialized, so the guard pair disappears along with the string. Runtime initialization is process-global, so replacing the stored value cannot invalidate a live CLR instance.

3. The FastDev TPA list

static std::string fastdev_tpa_list;
static std::vector<constchar*> fastdev_prop_names;
static std::vector<constchar*> fastdev_prop_values;

This block is inside if constexpr (Constants::is_debug_build), so it contributes nothing to the Release numbers below — but it has to go before libc++ can be dropped from Debug builds, which is why it is included here rather than left for later. The two property arrays are a fixed prop_count + 1 entries, so they become calloced arrays.

build_tpa_list now returns the string

It previously filled a std::string& out-parameter and returned bool; it now returns a malloced string the caller owns, or nullptr.

The TPA list is the one place in this PR with no useful upper bound — it holds one absolute path per assembly in the override directory — so a fixed buffer is not appropriate. It is accumulated through a small growable buffer that doubles on demand.

FastDev allocation failures are non-fatal: both list construction and extended runtime-property allocation log a warning, free partial state, reset TPA mode and fall back to the probe-only path. discard_tpa_list() owns that reset and has a no-op Release stub, avoiding references to Debug-only state from the discarded branch of the non-template if constexpr.

open_assembly

new uint8_t[]/delete[] become malloc/free. That buffer is handed to CoreCLR and never freed — see the existing TODO above it about ownership — so this is not a behavioral change.

It also gains a null check. That is a small bug fix: new uint8_t[] would have thrown std::bad_alloc and aborted, whereas the new path logs, closes the file descriptor and returns nullptr. The previous code would have leaked asm_fd had it ever returned rather than aborted.

Environment override buffer

The Debug environment override reader used std::make_unique<char[]>. It now uses malloc/free, preserves the previous non-fatal behavior for empty files without relying on malloc(0), and closes the override file descriptor after reading. Read errors retain their errno value across close() so diagnostics remain accurate.

Header cleanup

<string> and <vector> are no longer included by host.cc, and <string> is gone from fastdev-assemblies.hh — which is where host.cc was picking it up from in the first place.

Results

Undefined libc++ references in the CoreCLR host, Release:

object	before	after
host.cc.o	9	0
assembly-store.cc.o	12	12
total	21	12

The nine removed from host.cc.o: __cxa_guard_acquire, __cxa_guard_release, operator new, operator delete, std::string::append, std::string::push_back, std::string::__grow_by_and_replace, ~basic_string and __libcpp_verbose_abort.

	before	after	delta
libnet-android.release.so	523,224	520,112	−3,112 B

Verification

• CoreCLR, MonoVM and NativeAOT all build clean.
• The final review fixes were validated with dotnet build src/native/native-clr.csproj --no-restore.
• Debug was verified explicitly.fastdev-assemblies.cc is only added to the build under if(DEBUG_BUILD) (src/native/clr/host/CMakeLists.txt), so a Release build never compiles it and would not have caught a mistake in the FastDev work. There is no Debug CoreCLR build configured locally, so both host.cc and fastdev-assemblies.cc were compiled standalone with -DDEBUG -DDEBUG_BUILD using the exact flags from compile_commands.json. Both compile clean and llvm-nm --undefined-only reports 0 libc++ references for each.
• The Debug/Release API shape around discard_tpa_list() was compiled explicitly both with and without -DDEBUG.

Not covered

Only android-arm64 was built locally; the other ABIs rely on CI. The FastDev path is Debug-only and was not exercised on a device as part of this change.

[Copilot]

Copilot review overview

Review tier: Lite
Findings: 2

New issues introduced by this change (2)

Severity	Finding
	src/​native/​clr/​host/​fastdev-assemblies.cc — ERROR: FastDevAssemblies::tpa_in_use is set to true on success but never reset on failure. If…
	src/​native/​clr/​host/​host.cc — WARNING: This Debug-only FastDev path aborts the process if allocating the extended runtime…

What changed in this PR

This PR refactors the CoreCLR native host FastDev/initialization code to remove dependencies on libc++/STL types (e.g., std::string, std::vector) by switching to C allocation patterns and existing in-tree path helpers.

Changes:

• Change FastDev TPA list construction to return a malloc-owned char* instead of filling a std::string.
• Replace STL-based path/property list handling in host.cc with Util::join_paths, malloc/calloc, and explicit lifetime management.
• Update FastDev assembly loading buffer allocation to use malloc/free instead of new[]/delete[].

File	Description
src/​native/​clr/​include/​host/​fastdev-assemblies.hh	Updates the FastDev API to return an owned char* TPA list (no STL types).
src/​native/​clr/​host/​host.cc	Reworks CoreCLR init property handling and path building to avoid STL and manage memory explicitly.
src/​native/​clr/​host/​fastdev-assemblies.cc	Implements a growable C buffer for TPA list building and switches FastDev file buffer allocation to malloc.

[simonrozsival]

/review

[github-actions]

✅ Android PR Reviewer completed successfully!

Generated by Android PR Reviewer for #12570

[github-actions]

⚠️Needs Changes — 0 errors, 0 warnings, 1 suggestion. The raw-allocation ownership and cleanup paths are consistent, and the existing StackTraceContainsLineNumbers device test exercises the FastDev TPA behavior. The only feedback is a non-blocking utility-consolidation suggestion.

CI is not yet green: only license/cla has completed successfully, while no dotnet-android Azure DevOps checks are present for head 91690f72; the native ABI matrix therefore remains unvalidated.

Generated by Android PR Reviewer for #12570 · gpt56 · 99.4 AIC · ⌖ 9.54 AIC · ⊞ 25.7K
_{Comment /review to run again}

[simonrozsival]

@jonathanpeppers please re-approve