egl: Handle buffer age query errors

src/backend/egl/surface.rs

@@ -81,18 +81,28 @@ impl EGLSurface {
     }
 
     /// Returns the buffer age of the underlying back buffer
-    pub fn buffer_age(&self) -> i32 {
+    pub fn buffer_age(&self) -> Option<i32> {
         let surface = self.surface.load(Ordering::SeqCst);
         let mut age = 0;
-        unsafe {
+        let ret = unsafe {
             ffi::egl::QuerySurface(
                 **self.display,
                 surface as *const _,
                 ffi::egl::BUFFER_AGE_EXT as i32,
                 &mut age as *mut _,
+            )
+        };
+        if ret == ffi::egl::FALSE {
+            slog::debug!(
+                self.logger,
+                "Failed to query buffer age value for surface {:?}: {}",
+                self,
+                EGLError::from_last_call().unwrap_err()
             );
+            None
+        } else {
+            Some(age)
         }
-        age
     }
 
     /// Swaps buffers at the end of a frame.

src/backend/winit/mod.rs

@@ -306,12 +306,15 @@ impl WinitGraphicsBackend {
     ///
     /// This will only return a meaningful value, if this `WinitGraphicsBackend`
     /// is currently bound (by previously calling [`WinitGraphicsBackend::bind`]).
-    /// Otherwise the contents of the return value are undefined.
-    pub fn buffer_age(&self) -> usize {
+    ///
+    /// Otherwise and on error this function returns `None`.
+    /// If you are using this value actively e.g. for damage-tracking you should
+    /// likely interpret an error just as if "0" was returned.
+    pub fn buffer_age(&self) -> Option<usize> {
         if self.damage_tracking {
-            self.egl.buffer_age() as usize
+            self.egl.buffer_age().map(|x| x as usize)
         } else {
-            0
+            Some(0)
         }
     }