fix: several threading issues in discord_video_decoder #24
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1. Eliminated Main Thread Initialization Race Original: Had a two-step initialization where InitializeOnMainThread fetched GPU factories, then posted to media thread. Fix: Moved GPU factories fetching to a synchronous call before creating DiscordVideoDecoderMediaThread, eliminating the race between initialization and potential early deletion. 2. Introduced Proper Promise/Future Pattern Original: Used raw pointers with WaitableEvent that could be destroyed while still being accessed. Fix: Added WaitableSharedObject<T> using RefCountedThreadSafe to ensure the waiter object stays alive until completion. Note: Chromium does not use std::promise / std::future so these are work alikes. 3. Fixed DeleteSoon Race Original: Checked gpu_factories_ which was set asynchronously, creating a race where deletion could occur before initialization completed. Fix: Always has media_task_runner_ available from construction, ensuring proper thread-safe deletion. 4. Added GUARDED_BY Annotations Original: several members were properly mutex guarded but could easily be accidentally accessed in an unguarded context. Fix: GUARDED_BY annotations turn improper access into a compiler error.
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https://github.com/discord/discord/pull/251186