Fix threaded video shader-backend detection (black screen / crash on glcore)

[bulzipke]

Description

With video_threaded = true, the glcore driver fails to create its slang filter chain, resulting in a black screen (audio only). On some platforms (e.g. H700) this crashes with a SIGSEGV in the video thread:

#0  0x0000000000ba1d20 in std::vector<std::unique_ptr<gl3_shader::Pass, std::default_delete<gl3_shader::Pass> >, std::allocator<std::unique_ptr<gl3_shader::Pass, std::default_delete<gl3_shader::Pass> > > >::size (this=0x0)
    at /data/rocknix/build.ROCKNIX-H700.aarch64/toolchain/aarch64-rocknix-linux-gnu/include/c++/15.2.0/bits/stl_vector.h:1119
#1  0x0000000000b997f0 in gl3_filter_chain::set_frame_count (this=0x0, count=0) at gfx/drivers_shader/shader_gl3.cpp:2645
#2  0x0000000000b9b50c in gl3_filter_chain_set_frame_count (chain=0x0, count=0) at gfx/drivers_shader/shader_gl3.cpp:3235
#3  0x0000000000b8f6cc in gl3_frame (data=0xffffec000b70, frame=0xfffff250f010, frame_width=256, frame_height=224, frame_count=0, pitch=1024, msg=0x0, video_info=0xfffff250d1d0) at gfx/drivers/gl3.c:4283
#4  0x0000000000b4f94c in video_thread_loop (data=0x1d29a00) at gfx/video_thread_wrapper.c:476
#5  0x0000000000b4deec in thread_wrap (data_=0x25cbe90) at libretro-common/rthreads/rthreads.c:150
#6  0x0000fffff4f987c8 in ?? () from /usr/lib/libc.so.6
#7  0x0000fffff501664c in ?? () from /usr/lib/libc.so.6

gl3_get_fallback_shader_type() queries the context driver's advertised shader flags, which (e.g. in gfx_ctx_wl_get_flags) gate on video_driver_get_ident() matching glcore.

In video_init_thread(), current_video is repointed to the thread wrapper before the underlying driver's init() runs, but VIDEO_FLAG_THREAD_WRAPPER_ACTIVE is only set after init() completes. During that window the underlying gl3_init() runs on the worker thread and calls video_driver_get_ident(), which — with the flag still unset — resolves to the wrapper's ident Thread wrapper instead of the wrapped driver glcore.

As a result the slang backend is not detected, gl3_get_fallback_shader_type() returns RARCH_SHADER_NONE, and no filter chain is ever created (filter_chain and filter_chain_default both NULL), which gl3_frame() then dereferences. In non-threaded mode current_video is the real driver at init time, so it works.

Related Pull Requests

The change was introduced by #18151, which made glcore shader-backend selection depend on the context-driver ident.