Add ManualFence to wgpu-rs

ManualFence is returned by Queue::submit.
It allows a user to wait for the fence created therein,
i.e. the one stored in the associated wgpu_native::ActiveSubmission.

This is a hackish solution to allow Buffer::map_read_async usage
(since it may read too soon otherwise, i.e. obtain corrupt data).

wgpu-native/src/device.rs

@@ -129,6 +129,7 @@ impl<B: hal::Backend> DestroyedResources<B> {
             value: buffer,
             ref_count,
         });
+        self.ready_to_map.push(buffer); // See https://github.com/gfx-rs/wgpu/pull/123
     }
 
     /// Returns the last submission index that is done.
@@ -1087,7 +1088,7 @@ pub extern "C" fn wgpu_queue_submit(
     queue_id: QueueId,
     command_buffer_ptr: *const CommandBufferId,
     command_buffer_count: usize,
-) {
+) -> usize {
     let command_buffer_ids =
         unsafe { slice::from_raw_parts(command_buffer_ptr, command_buffer_count) };
 
@@ -1237,6 +1238,23 @@ pub extern "C" fn wgpu_queue_submit(
         let cmd_buf = HUB.command_buffers.unregister(cmb_id);
         device.com_allocator.after_submit(cmd_buf, submit_index);
     }
+
+    submit_index
+}
+
+pub fn queue_wait_for_fence(
+    queue_id: QueueId,
+    submit_index: usize,
+) {
+    let device_guard = HUB.devices.read();
+    let device = &device_guard[queue_id];
+
+    let mut destroyed = device.destroyed.lock();
+    if let Some(active_submission) = destroyed.active.iter_mut().find(|a| a.index == submit_index) {
+        unsafe {
+            device.raw.wait_for_fence(&active_submission.fence, !0).unwrap();
+        }
+    }
 }
 
 pub fn device_create_render_pipeline(

wgpu-rs/src/lib.rs

@@ -192,6 +192,17 @@ pub struct SwapChainOutput<'a> {
     swap_chain_id: &'a wgn::SwapChainId,
 }
 
+pub struct ManualFence {
+    queue_id: wgn::QueueId,
+    submit_index: usize,
+}
+
+impl ManualFence {
+    pub fn wait(&self) {
+        wgn::queue_wait_for_fence(self.queue_id, self.submit_index);
+    }
+}
+
 pub struct BufferCopyView<'a> {
     pub buffer: &'a Buffer,
     pub offset: u32,
@@ -856,17 +867,19 @@ impl<'a> Drop for ComputePass<'a> {
 }
 
 impl<'a> Queue<'a> {
-    pub fn submit(&mut self, command_buffers: &[CommandBuffer]) {
+    pub fn submit(&mut self, command_buffers: &[CommandBuffer]) -> ManualFence {
         self.temp.command_buffers.clear();
         self.temp
             .command_buffers
             .extend(command_buffers.iter().map(|cb| cb.id));
 
-        wgn::wgpu_queue_submit(
+        let submit_index = wgn::wgpu_queue_submit(
             self.id,
             self.temp.command_buffers.as_ptr(),
             command_buffers.len(),
         );
+
+        ManualFence { queue_id: self.id, submit_index }
     }
 }