[dylink] Fix missing unlock in emscripten_dlopen()

[kleisauke]

The global write lock was acquired but never released if the
requested library was already loaded.

Noticed while investigating #26913.

[sbc100]

So this doesn't actually fix #26913?

[kleisauke]

So this doesn't actually fix #26913?

Sadly not. emscripten_dlopen() is a separate API, whereas #26913 relates to dlopen().

emscripten/system/include/emscripten/emscripten.h

Lines 179 to 182 in f46e0d8

	// Asynchronous version of dlopen. Since WebAssembly module loading in general
	// is asynchronous the normal dlopen function can't be used in all situations.
	typedef void (*em_dlopen_callback)(void* user_data, void* handle);
	void emscripten_dlopen(const char *filename, int flags, void* user_data, em_dlopen_callback onsuccess, em_arg_callback_func onerror);

(For full transparency, #26913 was originally identified in wasm-vips after the Emscripten 5.0.7 update.)

[kleisauke]

Having said that, I think #26913 also affects emscripten_dlopen(), since it also calls load_library_done() and subsequently _emscripten_dlsync_threads(), where this issue occurs.