x11: fixup buffer age

The current code always acquires the buffer it just submitted,
which results in always rendering to a buffer with age "1", although
its contents are older, because the currently held buffer is already
submitted, just not to the swapchain. When it finally is submitted
its age is again set to 1, the other gets its correct age (but too late)
and the circle is repeated again. So lets fix that.

anvil/src/x11.rs

@@ -306,7 +306,9 @@ pub fn run_x11(log: Logger) {
                     }
 
                     // Submit the buffer
-                    backend_data.surface.submit();
+                    if let Err(err) = backend_data.surface.submit() {
+                        error!(log, "Error submitting buffer for display: {}", err);
+                    }
                 }
 
                 Err(err) => {

src/backend/x11/mod.rs

@@ -349,8 +349,7 @@ pub struct X11Surface {
     format: DrmFourcc,
     width: u16,
     height: u16,
-    current: Option<Slot<BufferObject<()>>>,
-    next: Option<Slot<BufferObject<()>>>,
+    buffer: Option<Slot<BufferObject<()>>>,
 }
 
 #[derive(Debug, thiserror::Error)]
@@ -467,8 +466,7 @@ impl X11Surface {
             format,
             width: size.w,
             height: size.h,
-            current: None,
-            next: None,
+            buffer: None,
             resize: recv,
         })
     }
@@ -493,8 +491,8 @@ impl X11Surface {
             self.resize(new_size)?;
         }
 
-        if self.next.is_none() {
-            self.next = Some(
+        if self.buffer.is_none() {
+            self.buffer = Some(
                 self.swapchain
                     .acquire()
                     .map_err(Into::<AllocateBuffersError>::into)?
@@ -502,7 +500,7 @@ impl X11Surface {
             );
         }
 
-        let slot = self.next.as_ref().unwrap();
+        let slot = self.buffer.as_ref().unwrap();
         let age = slot.age();
         match slot.userdata().get::<Dmabuf>() {
             Some(dmabuf) => Ok((dmabuf.clone(), age)),
@@ -515,41 +513,45 @@ impl X11Surface {
     }
 
     /// Consume and submit the buffer to the window.
-    pub fn submit(&mut self) {
+    pub fn submit(&mut self) -> Result<(), AllocateBuffersError> {
         if let Some(connection) = self.connection.upgrade() {
+            // Get a new buffer
+            let mut next = self
+                .swapchain
+                .acquire()
+                .map_err(Into::<AllocateBuffersError>::into)?
+                .ok_or(AllocateBuffersError::NoFreeSlots)?;
+
             // Swap the buffers
-            if let Some(mut next) = self.next.take() {
-                if let Some(current) = self.current.as_mut() {
-                    mem::swap(&mut next, current);
-                    self.swapchain.submitted(next);
-                } else {
-                    self.current = Some(next);
-                }
+            if let Some(current) = self.buffer.as_mut() {
+                mem::swap(&mut next, current);
             }
 
             if let Ok(pixmap) = PixmapWrapper::with_dmabuf(
                 &*connection,
                 &self.window,
-                self.current.as_ref().unwrap().userdata().get::<Dmabuf>().unwrap(),
+                next.userdata().get::<Dmabuf>().unwrap(),
             ) {
                 // Now present the current buffer
                 let _ = pixmap.present(&*connection, &self.window);
             }
+            self.swapchain.submitted(next);
 
             // Flush the connection after presenting to the window to ensure we don't run out of buffer space in the X11 connection.
             let _ = connection.flush();
         }
+        Ok(())
     }
 
     /// Resets the internal buffers, e.g. to reset age values
     pub fn reset_buffers(&mut self) {
         self.swapchain.reset_buffers();
-        self.next = None;
+        self.buffer = None;
     }
 
     fn resize(&mut self, size: Size<u16, Logical>) -> Result<(), AllocateBuffersError> {
         self.swapchain.resize(size.w as u32, size.h as u32);
-        self.next = None;
+        self.buffer = None;
 
         self.width = size.w;
         self.height = size.h;