wayland: Reuse buffers and pools; check buffer release

Also updates `winit` example to redraw on resize, which seems to be
necessary.

With this resizing seems to be entirely smooth, without visual
corruption from it overwriting the buffer the server is displaying.

src/wayland/buffer.rs

@@ -0,0 +1,137 @@
+use nix::sys::memfd::{memfd_create, MemFdCreateFlag};
+use std::{
+    ffi::CStr,
+    fs::File,
+    os::unix::prelude::{AsRawFd, FileExt, FromRawFd},
+    sync::{
+        atomic::{AtomicBool, Ordering},
+        Arc,
+    },
+};
+use wayland_client::{
+    protocol::{wl_buffer, wl_shm, wl_shm_pool, wl_surface},
+    Connection, Dispatch, QueueHandle,
+};
+
+use super::State;
+
+pub(super) struct WaylandBuffer {
+    qh: QueueHandle<State>,
+    tempfile: File,
+    pool: wl_shm_pool::WlShmPool,
+    pool_size: i32,
+    buffer: wl_buffer::WlBuffer,
+    width: i32,
+    height: i32,
+    released: Arc<AtomicBool>,
+}
+
+impl WaylandBuffer {
+    pub fn new(shm: &wl_shm::WlShm, width: i32, height: i32, qh: &QueueHandle<State>) -> Self {
+        let name = unsafe { CStr::from_bytes_with_nul_unchecked("swbuf\0".as_bytes()) };
+        let tempfile_fd = memfd_create(name, MemFdCreateFlag::MFD_CLOEXEC)
+            .expect("Failed to create memfd to store buffer.");
+        let tempfile = unsafe { File::from_raw_fd(tempfile_fd) };
+        let pool_size = width * height * 4;
+        let pool = shm.create_pool(tempfile.as_raw_fd(), pool_size, &qh, ());
+        let released = Arc::new(AtomicBool::new(true));
+        let buffer = pool.create_buffer(
+            0,
+            width,
+            height,
+            width * 4,
+            wl_shm::Format::Xrgb8888,
+            &qh,
+            released.clone(),
+        );
+        Self {
+            qh: qh.clone(),
+            tempfile,
+            pool,
+            pool_size,
+            buffer,
+            width,
+            height,
+            released,
+        }
+    }
+
+    pub fn resize(&mut self, width: i32, height: i32) {
+        // If size is the same, there's nothing to do
+        if self.width != width || self.height != height {
+            // Destroy old buffer
+            self.buffer.destroy();
+
+            // Grow pool, if needed
+            let size = ((width * height * 4) as u32).next_power_of_two() as i32;
+            if size > self.pool_size {
+                let _ = self.tempfile.set_len(size as u64);
+                self.pool.resize(size);
+                self.pool_size = size;
+            }
+
+            // Create buffer with correct size
+            self.buffer = self.pool.create_buffer(
+                0,
+                width,
+                height,
+                width * 4,
+                wl_shm::Format::Xrgb8888,
+                &self.qh,
+                self.released.clone(),
+            );
+        }
+    }
+
+    pub fn write(&self, buffer: &[u32]) {
+        let buffer =
+            unsafe { std::slice::from_raw_parts(buffer.as_ptr() as *const u8, buffer.len() * 4) };
+        self.tempfile
+            .write_all_at(buffer, 0)
+            .expect("Failed to write buffer to temporary file.");
+    }
+
+    pub fn attach(&self, surface: &wl_surface::WlSurface) {
+        self.released.store(false, Ordering::SeqCst);
+        surface.attach(Some(&self.buffer), 0, 0);
+    }
+
+    pub fn released(&self) -> bool {
+        self.released.load(Ordering::SeqCst)
+    }
+}
+
+impl Drop for WaylandBuffer {
+    fn drop(&mut self) {
+        self.buffer.destroy();
+        self.pool.destroy();
+    }
+}
+
+impl Dispatch<wl_shm_pool::WlShmPool, ()> for State {
+    fn event(
+        _: &mut State,
+        _: &wl_shm_pool::WlShmPool,
+        _: wl_shm_pool::Event,
+        _: &(),
+        _: &Connection,
+        _: &QueueHandle<State>,
+    ) {
+    }
+}
+
+impl Dispatch<wl_buffer::WlBuffer, Arc<AtomicBool>> for State {
+    fn event(
+        _: &mut State,
+        _: &wl_buffer::WlBuffer,
+        event: wl_buffer::Event,
+        released: &Arc<AtomicBool>,
+        _: &Connection,
+        _: &QueueHandle<State>,
+    ) {
+        match event {
+            wl_buffer::Event::Release => released.store(true, Ordering::SeqCst),
+            _ => {}
+        }
+    }
+}