[rs] Merge #710

710: Implement read_buffer r=kvark a=fintelia

I'm not sure about the exact interface that makes sense, but this PR sketches out how read_buffer (and eventually also read_texture) could be implemented. 

Resolves #694

Co-authored-by: Dzmitry Malyshau <kvarkus@gmail.com>
Co-authored-by: Jonathan Behrens <fintelia@gmail.com>

wgpu/src/backend/direct.rs

@@ -1325,7 +1325,6 @@ impl crate::Context for Context {
             Ok(ptr) => BufferMappedRange {
                 ptr,
                 size: size as usize,
-                phantom: PhantomData,
             },
             Err(err) => self.handle_error_fatal(err, "Buffer::get_mapped_range"),
         }
@@ -1897,13 +1896,12 @@ fn default_error_handler(err: crate::Error) {
 }
 
 #[derive(Debug)]
-pub struct BufferMappedRange<'a> {
+pub struct BufferMappedRange {
     ptr: *mut u8,
     size: usize,
-    phantom: PhantomData<&'a ()>,
 }
 
-impl<'a> crate::BufferMappedRangeSlice for BufferMappedRange<'a> {
+impl crate::BufferMappedRangeSlice for BufferMappedRange {
     fn slice(&self) -> &[u8] {
         unsafe { slice::from_raw_parts(self.ptr, self.size) }
     }
@@ -1913,7 +1911,7 @@ impl<'a> crate::BufferMappedRangeSlice for BufferMappedRange<'a> {
     }
 }
 
-impl<'a> Drop for BufferMappedRange<'a> {
+impl Drop for BufferMappedRange {
     fn drop(&mut self) {
         // Intentionally left blank so that `BufferMappedRange` still
         // implements `Drop`, to match the web backend

wgpu/src/backend/web.rs

@@ -2,7 +2,6 @@ use std::{
     collections::HashSet,
     fmt,
     future::Future,
-    marker::PhantomData,
     ops::Range,
     pin::Pin,
     task::{self, Poll},
@@ -1436,7 +1435,6 @@ impl crate::Context for Context {
         BufferMappedRange {
             actual_mapping,
             temporary_mapping,
-            phantom: PhantomData,
         }
     }
 
@@ -1888,13 +1886,12 @@ impl crate::Context for Context {
 pub(crate) type SwapChainOutputDetail = ();
 
 #[derive(Debug)]
-pub struct BufferMappedRange<'a> {
+pub struct BufferMappedRange {
     actual_mapping: js_sys::Uint8Array,
     temporary_mapping: Vec<u8>,
-    phantom: PhantomData<&'a ()>,
 }
 
-impl<'a> crate::BufferMappedRangeSlice for BufferMappedRange<'a> {
+impl crate::BufferMappedRangeSlice for BufferMappedRange {
     fn slice(&self) -> &[u8] {
         &self.temporary_mapping
     }
@@ -1904,7 +1901,7 @@ impl<'a> crate::BufferMappedRangeSlice for BufferMappedRange<'a> {
     }
 }
 
-impl<'a> Drop for BufferMappedRange<'a> {
+impl Drop for BufferMappedRange {
     fn drop(&mut self) {
         // Copy from the temporary mapping back into the array buffer that was
         // originally provided by the browser

wgpu/src/lib.rs

@@ -1690,14 +1690,14 @@ trait BufferMappedRangeSlice {
 #[derive(Debug)]
 pub struct BufferView<'a> {
     slice: BufferSlice<'a>,
-    data: BufferMappedRange<'a>,
+    data: BufferMappedRange,
 }
 
 /// Write only view into mapped buffer.
 #[derive(Debug)]
 pub struct BufferViewMut<'a> {
     slice: BufferSlice<'a>,
-    data: BufferMappedRange<'a>,
+    data: BufferMappedRange,
     readable: bool,
 }
 

wgpu/src/util/mod.rs

@@ -6,6 +6,7 @@ mod encoder;
 
 use std::{
     borrow::Cow,
+    future::Future,
     mem::{align_of, size_of},
     ptr::copy_nonoverlapping,
 };
@@ -54,3 +55,42 @@ pub fn make_spirv<'a>(data: &'a [u8]) -> super::ShaderSource<'a> {
     );
     super::ShaderSource::SpirV(words)
 }
+
+/// CPU accessible buffer used to download data back from the GPU.
+pub struct DownloadBuffer(super::Buffer, super::BufferMappedRange);
+
+impl DownloadBuffer {
+    /// Asynchronously read the contents of a buffer.
+    pub fn read_buffer(device: &super::Device, queue: &super::Queue, buffer: &super::BufferSlice) -> impl Future<Output=Result<Self, super::BufferAsyncError>> + Send {
+        let size = match buffer.size {
+            Some(size) => size.into(),
+            None => buffer.buffer.map_context.lock().total_size - buffer.offset,
+        };
+
+        let download = device.create_buffer(&super::BufferDescriptor {
+            size,
+            usage: super::BufferUsage::COPY_DST | super::BufferUsage::MAP_READ,
+            mapped_at_creation: false,
+            label: None,
+        });
+
+        let mut encoder = device.create_command_encoder(&super::CommandEncoderDescriptor { label: None });
+        encoder.copy_buffer_to_buffer(buffer.buffer, buffer.offset, &download, 0, size);
+        let command_buffer: super::CommandBuffer = encoder.finish();
+        queue.submit(Some(command_buffer));
+
+        let fut = download.slice(..).map_async(super::MapMode::Read);
+        async move {
+            fut.await?;
+            let mapped_range = super::Context::buffer_get_mapped_range(&*download.context,  &download.id, 0..size);
+            Ok(Self(download, mapped_range))
+        }
+    }
+}
+
+impl std::ops::Deref for DownloadBuffer{
+    type Target = [u8];
+    fn deref(&self) -> &[u8] {
+        super::BufferMappedRangeSlice::slice(&self.1)
+    }
+}