A gc shared library registers the threads its host calls it on

[ASDAlexander77]

Problem

Boehm scans, and stops during a collection, only the threads registered with it. A tslang program registers its own threads: GC_init registers the one it starts on, and the async runtime registers its pool's. But a host that isn't a tslang program calls a -mm=gc shared library on threads nobody registered. That covers an Android app's JNI threads and a plain C/C++ program's threads. The first collection one of those threads started aborted with Collecting from unknown thread. Reproduced on Windows and on an Android API 29 emulator. Only the loading thread worked.

Fix

• __tslang_gc_enter (AsyncGCThreadsCommon.inc, in the async runtime's gc-only object):
  • checks the calling thread once, with a thread_local flag as the fast path;
  • registers the thread if the collector doesn't know it, and unregisters it at thread exit;
  • runs GC_enable_threads once. GC_register_my_thread refuses every thread until GC_allow_register_threads has run, and Boehm initializes itself on first use without it: a library with no top-level code had no GC_init injected at load. Without this, every foreign thread was silently refused (Threads explicit registering is not previously enabled), which was found on the emulator.
• GCPass injects the call first thing in every exported function (passthrough export/dllexport) of a --emit=dll gc build. The global constructors are excluded, since GC_init there registers the loading thread, and internal functions are left alone (checked in the disassembly).
• All OSes, not just Android: a Linux or Windows .so/.dll loaded by a non-tslang host had the same bug.

Tests

• New test-compile-foreign-thread-gc. foreign-thread-gc-host (C++, not tslang) loads a gc library and calls it 2,000 times on the loading thread, then 2,000 times on each of 8 threads of its own, checking every result. It does this with and without top-level code in the library. It crashed before this change (0x80000003, Boehm's abort) and passes 20/20 after.
  • The files are named foreign-thread-gc* because the repo's .gitignore ignores gc-*.
• Full Release suite on Windows: 3856/3856, including the 441 shared-library tests that now go through the entry hook.
• Android emulator (API 29, x86_64): libraries allocating nodes, number arrays, string arrays and class instances run correctly from 8 foreign threads (dlopen host).

Known issue: not fixed here, and not in tslang

On the Android emulator, multi-threaded allocation of growing pointer-free blocks can hang Boehm 8.2.12. That's the pattern the default library's join (result += v) produces: the collector is stuck in GC_start_world → resend_lost_signals while one registered thread never leaves GC_suspend_handler's sigsuspend. It reproduces in pure C with no tslang code: Boehm linked into an executable hangs 3 runs in 8, inside a .so 5 in 8. It isn't parallel marking: GC_MARKERS=1 hangs too. Being investigated against newer bdwgc and reported upstream. Until then, -mm=rc is the safe model for Android app libraries.

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