Record that the buffer is mapped when its size is zero. (#2877)

* Record that the buffer is mapped when its size is zero.

* Avoid internally trying to map a zero-sized buffer if mapped_at_creation is true.

* Add an entry in the changelog.

CHANGELOG.md

@@ -50,6 +50,7 @@ Bottom level categories:
 - Fix bind group / pipeline deduplication not taking into account RenderBundle execution resetting these values by @shoebe [#2867](https://github.com/gfx-rs/wgpu/pull/2867)
 - Fix panics that occur when using `as_hal` functions when the hal generic type does not match the hub being looked up in by @i509VCB [#2871](https://github.com/gfx-rs/wgpu/pull/2871)
 - Add some validation in map_async by @nical in [#2876](https://github.com/gfx-rs/wgpu/pull/2876)
+- Fix bugs when mapping/unmapping zero-sized buffers and ranges by @nical in [#2877](https://github.com/gfx-rs/wgpu/pull/2877)
 
 #### DX12
 - `DownlevelCapabilities::default()` now returns the `ANISOTROPIC_FILTERING` flag set to true so DX12 lists `ANISOTROPIC_FILTERING` as true again by @cwfitzgerald in [#2851](https://github.com/gfx-rs/wgpu/pull/2851)

wgpu-core/src/device/life.rs

@@ -896,6 +896,11 @@ impl<A: HalApi> LifetimeTracker<A> {
                         }
                     }
                 } else {
+                    buffer.map_state = resource::BufferMapState::Active {
+                        ptr: std::ptr::NonNull::dangling(),
+                        range: mapping.range,
+                        host: mapping.op.host,
+                    };
                     resource::BufferMapAsyncStatus::Success
                 };
                 pending_callbacks.push((mapping.op, status));

wgpu-core/src/device/mod.rs

@@ -3247,14 +3247,19 @@ impl<G: GlobalIdentityHandlerFactory> Global<G> {
             } else if desc.usage.contains(wgt::BufferUsages::MAP_WRITE) {
                 // buffer is mappable, so we are just doing that at start
                 let map_size = buffer.size;
-                let ptr = match map_buffer(&device.raw, &mut buffer, 0, map_size, HostMap::Write) {
-                    Ok(ptr) => ptr,
-                    Err(e) => {
-                        let raw = buffer.raw.unwrap();
-                        device
-                            .lock_life(&mut token)
-                            .schedule_resource_destruction(queue::TempResource::Buffer(raw), !0);
-                        break e.into();
+                let ptr = if map_size == 0 {
+                    std::ptr::NonNull::dangling()
+                } else {
+                    match map_buffer(&device.raw, &mut buffer, 0, map_size, HostMap::Write) {
+                        Ok(ptr) => ptr,
+                        Err(e) => {
+                            let raw = buffer.raw.unwrap();
+                            device.lock_life(&mut token).schedule_resource_destruction(
+                                queue::TempResource::Buffer(raw),
+                                !0,
+                            );
+                            break e.into();
+                        }
                     }
                 };
                 buffer.map_state = resource::BufferMapState::Active {