[sbc100]

When a side module built with --embed-file is dlopened from a pthread, its static constructors run on that worker thread (firstLoad is true there). Without __proxy: 'sync',
_emscripten_fs_load_embedded_files populated the worker's local FS (or un-flushed WasmFS preloaded arrays) instead of the main thread's filesystem.

Also, add some assertions to make this kind of thing easier to detect in the future.

Fixes: #28256

[gemini-code-assist]

Code Review

This pull request introduces thread-safety assertions for the File System (FS) on pthreads, proxies embedded file loading to the main thread, and adds a new test for loading embedded files via dlopen on a pthread. The review feedback highlights two main issues: first, in the new C test, returning from the thread main function during an asynchronous dlopen can prematurely terminate the thread runtime, so emscripten_exit_with_live_runtime() should be used instead; second, the Python test violates the newly added documentation guidelines by mutating self.cflags directly rather than passing compiler flags inline.

[gemini-code-assist]

When using asynchronous emscripten_dlopen on a pthread, returning NULL from thread_main immediately exits the thread and cleans up its resources. This can prevent the asynchronous on_dl_success callback from executing, or cause it to run on a defunct thread context, leading to undefined behavior or test flakiness. To keep the thread's runtime alive so it can process the async callback, use emscripten_exit_with_live_runtime() instead of returning.

#ifdef USE_ASYNC_DLOPEN
  emscripten_dlopen("libside.so", RTLD_NOW | RTLD_GLOBAL, NULL, on_dl_success, on_dl_fail);
  emscripten_exit_with_live_runtime();
#else
  void* handle = dlopen("libside.so", RTLD_NOW | RTLD_GLOBAL);
  assert(handle);
  func_t read_embedded_file = (func_t)dlsym(handle, "read_embedded_file");
  assert(read_embedded_file);
  read_embedded_file();
  check_embedded_file("pthread");
  dlclose(handle);
  thread_done = true;
  return NULL;
#endif

[gemini-code-assist]

This test mutates self.cflags directly, which violates the newly added guideline in docs/process.md stating that compiler/linker flags should be passed via inline cflags= arguments to test methods rather than mutating self.cflags. We can pass the flags directly to build_dlfcn_lib and do_runf instead.

    self.build_dlfcn_lib(test_file('core/pthread/test_pthread_dlopen_embed_side.c'),
                         cflags=['--embed-file', 'embedded.txt', '-Wno-experimental', '-pthread'])

    self.prep_dlfcn_main()
    self.do_runf('core/pthread/test_pthread_dlopen_embed.c', 'done\n',
                 cflags=args + ['-sEXIT_RUNTIME', '-Wno-experimental', '-pthread'])