From 1d98d1a64770827ce0bc862b298482641fdfb567 Mon Sep 17 00:00:00 2001
From: fufesou <linlong1266@gmail.com>
Date: Sat, 12 Sep 2026 16:56:48 +0800
Subject: [PATCH] fix(cursor): keep DRM density lookup off the polling path

Read optional Wayland metadata from the cache without starting discovery or waiting on a refresh. Keep delivering cursor shapes with unknown density when metadata is unavailable, and cover lock contention and repeated empty-cache reads.
---
 libs/scrap/src/wayland/display.rs | 33 +++++++++++++++++++++++++++++++
 src/server/drm_capturer.rs        |  3 ++-
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/libs/scrap/src/wayland/display.rs b/libs/scrap/src/wayland/display.rs
index 6ff4177cc..1e19bb415 100644
--- a/libs/scrap/src/wayland/display.rs
+++ b/libs/scrap/src/wayland/display.rs
@@ -248,6 +248,19 @@ pub fn wayland_failure_stamped() -> bool {
     LAST_FAILED_LOOKUP.lock().unwrap().is_some()
 }
 
+/// Cursor polls must neither start discovery nor wait for an in-progress display refresh.
+#[cfg(feature = "drm")]
+pub fn get_cached_displays() -> Option<Arc<Displays>> {
+    match DISPLAYS.try_lock() {
+        Ok(cache) => cache.clone(),
+        Err(std::sync::TryLockError::WouldBlock) => None,
+        Err(err) => {
+            warn!("Failed to read cached Wayland displays: {}", err);
+            None
+        }
+    }
+}
+
 pub fn get_displays() -> Arc<Displays> {
     let mut lock = DISPLAYS.lock().unwrap();
     match lock.as_ref() {
@@ -516,6 +529,26 @@ fn map_axis(v: i32, base_origin: i32, base_extent: i32, live_origin: i32, live_e
 mod tests {
     use super::*;
 
+    #[cfg(feature = "drm")]
+    #[test]
+    fn cursor_metadata_does_not_wait_for_display_discovery() {
+        let cache = DISPLAYS.lock().unwrap();
+        let (tx, rx) = std::sync::mpsc::channel();
+        let reader = std::thread::spawn(move || {
+            tx.send(get_cached_displays().is_none()).unwrap();
+        });
+        // Discovery owns this lock until it completes or times out.
+        let result = rx.recv_timeout(Duration::from_secs(1));
+        drop(cache);
+        reader.join().unwrap();
+        assert!(result.unwrap());
+
+        clear_wayland_displays_cache();
+        for _ in 0..100 {
+            assert!(get_cached_displays().is_none());
+        }
+    }
+
     #[test]
     fn test_lookup_backoff_boundaries() {
         // Future `now`s sidestep Instant subtraction, which can panic near boot.
diff --git a/src/server/drm_capturer.rs b/src/server/drm_capturer.rs
index 76c2a3d09..ed1826ce9 100644
--- a/src/server/drm_capturer.rs
+++ b/src/server/drm_capturer.rs
@@ -995,7 +995,8 @@ pub fn drm_cursor_snapshot<T>(
         None
     } else {
         display_info_of(display).and_then(|display| {
-            let wayland = scrap::wayland::display::get_displays();
+            // Display discovery runs outside the cursor service; missing metadata means unknown DPI.
+            let wayland = scrap::wayland::display::get_cached_displays()?;
             let index = identity_matches(&[display], &wayland.displays)
                 .into_iter()
                 .next()