Mesa 19.3 Now Exposes Its Own GL_MESA_EGL_sync OpenGL Extension
GL_MESA_EGL_sync is basically about bringing EGL_KHR_fence_sync support to desktop OpenGL. This OpenGL synchronization support was being pursued to complement the existing GL_ARB_sync synchronization extension in order to match the semantics of EGL_ANDROID_native_fence_sync support and other similar use-cases with some platform extensions being layered atop EGL_KHR_fence_sync's functionality rather than ARB_sync.
While the extension is still considered a work-in-progress and hasn't been merged into the OpenGL Registry, as of this weekend in Mesa 19.3 Git the extension is in place. The patches merged this morning by Heinrich Fink are now in there for next quarter's Mesa release with this synchronization extension.