drm_crtc_handle_vblank_get(crtc) returns the current per-crtc vblank
sequence number without incrementing, complementing the write-and-
increment behavior of drm_crtc_handle_vblank added in the prior commit.
Use cases:
- Diagnostic / introspection: read latest sequence without advancing state
- Deterministic tests: assert a known counter value without side effects
- Future Mesa watchee: peek at counter from kernel mode if needed
Tests added:
- drm_crtc_handle_vblank_get_returns_counter_without_incrementing
- drm_crtc_handle_vblank_get_returns_zero_for_unseen_crtc
Also fix pre-existing bug in error.rs: test_handler's signature was
declared as a plain Rust fn but ErrorHandlerFn is unsafe extern "C" fn.
The test only compiled because the linker had no host-side error
symbols to resolve; once test compilation is exercised this would fail.
Fix is one qualifiert (fn -> unsafe extern "C" fn).
cargo check --lib: clean. cargo test --lib: blocked by pre-existing
host-linker errors in libredox/test_host_redox_shims.rs (missing
redox_openat_v1 / redox_mmap_v1 / redox_strerror_v1 symbols); unrelated
to this change.
vasilito
86a162c803