Properly handle the case where Navigator.gpu is undefined and WebGPU is the only compiled backend (#6197)

* Properly handle the case where `Navigator.gpu` is undefined and WebGPU is the only compiled backend.

Previously, `Instance::request_adapter` would invoke a wasm binding with an undefined arg0,
thus crashing the program. Now it will cleanly return `None` instead.

Fixes #6196.

* Fix typo in `Instance::new` doc comment.
* Add note to CHANGELOG.md
* Introduce `DefinedNonNullJsValue` type.
* Assert definedness of self.gpu in surface_get_capabilities.
* Use DefinedNonNullJsValue in signature of get_browser_gpu_property().
* Clarify meaning of gpu field with a comment.

CHANGELOG.md

@@ -88,6 +88,7 @@ By @bradwerth [#6216](https://github.com/gfx-rs/wgpu/pull/6216).
 ### Bug Fixes
 
 - Fix incorrect hlsl image output type conversion. By @atlv24 in [#6123](https://github.com/gfx-rs/wgpu/pull/6123)
+- Fix JS `TypeError` exception in `Instance::request_adapter` when browser doesn't support WebGPU but `wgpu` not compiled with `webgl` support. By @bgr360 in [#6197](https://github.com/gfx-rs/wgpu/pull/6197).
 
 #### Naga
 

wgpu/src/api/instance.rs

@@ -95,7 +95,7 @@ impl Instance {
     ///   [`Backends::BROWSER_WEBGPU`] takes a special role:
     ///   If it is set and WebGPU support is detected, this instance will *only* be able to create
     ///   WebGPU adapters. If you instead want to force use of WebGL, either
-    ///   disable the `webgpu` compile-time feature or do add the [`Backends::BROWSER_WEBGPU`]
+    ///   disable the `webgpu` compile-time feature or don't add the [`Backends::BROWSER_WEBGPU`]
     ///   flag to the the `instance_desc`'s `backends` field.
     ///   If it is set and WebGPU support is *not* detected, the instance will use wgpu-core
     ///   to create adapters. Meaning that if the `webgl` feature is enabled, it is able to create
@@ -118,8 +118,9 @@ impl Instance {
         {
             let is_only_available_backend = !cfg!(wgpu_core);
             let requested_webgpu = _instance_desc.backends.contains(Backends::BROWSER_WEBGPU);
-            let support_webgpu =
-                crate::backend::get_browser_gpu_property().map_or(false, |gpu| !gpu.is_undefined());
+            let support_webgpu = crate::backend::get_browser_gpu_property()
+                .map(|maybe_gpu| maybe_gpu.is_some())
+                .unwrap_or(false);
 
             if is_only_available_backend || (requested_webgpu && support_webgpu) {
                 return Self {

wgpu/src/backend/webgpu.rs

@@ -1,5 +1,6 @@
 #![allow(clippy::type_complexity)]
 
+mod defined_non_null_js_value;
 mod ext_bindings;
 mod webgpu_sys;
 
@@ -22,6 +23,8 @@ use crate::{
     CompilationInfo, SurfaceTargetUnsafe, UncapturedErrorHandler,
 };
 
+use defined_non_null_js_value::DefinedNonNullJsValue;
+
 // We need to make a wrapper for some of the handle types returned by the web backend to make them
 // implement `Send` and `Sync` to match native.
 //
@@ -38,7 +41,10 @@ unsafe impl<T> Send for Sendable<T> {}
 #[cfg(send_sync)]
 unsafe impl<T> Sync for Sendable<T> {}
 
-pub(crate) struct ContextWebGpu(webgpu_sys::Gpu);
+pub(crate) struct ContextWebGpu {
+    /// `None` if browser does not advertise support for WebGPU.
+    gpu: Option<DefinedNonNullJsValue<webgpu_sys::Gpu>>,
+}
 #[cfg(send_sync)]
 unsafe impl Send for ContextWebGpu {}
 #[cfg(send_sync)]
@@ -189,6 +195,36 @@ impl<F, M> MakeSendFuture<F, M> {
 #[cfg(send_sync)]
 unsafe impl<F, M> Send for MakeSendFuture<F, M> {}
 
+/// Wraps a future that returns `Option<T>` and adds the ability to immediately
+/// return None.
+pub(crate) struct OptionFuture<F>(Option<F>);
+
+impl<F: Future<Output = Option<T>>, T> Future for OptionFuture<F> {
+    type Output = Option<T>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
+        // This is safe because we have no Drop implementation to violate the Pin requirements and
+        // do not provide any means of moving the inner future.
+        unsafe {
+            let this = self.get_unchecked_mut();
+            match &mut this.0 {
+                Some(future) => Pin::new_unchecked(future).poll(cx),
+                None => task::Poll::Ready(None),
+            }
+        }
+    }
+}
+
+impl<F> OptionFuture<F> {
+    fn some(future: F) -> Self {
+        Self(Some(future))
+    }
+
+    fn none() -> Self {
+        Self(None)
+    }
+}
+
 fn map_texture_format(texture_format: wgt::TextureFormat) -> webgpu_sys::GpuTextureFormat {
     use webgpu_sys::GpuTextureFormat as tf;
     use wgt::TextureFormat;
@@ -1046,27 +1082,34 @@ pub enum Canvas {
     Offscreen(web_sys::OffscreenCanvas),
 }
 
-/// Returns the browsers gpu object or `None` if the current context is neither the main thread nor a dedicated worker.
+#[derive(Debug, Clone, Copy)]
+pub struct BrowserGpuPropertyInaccessible;
+
+/// Returns the browser's gpu object or `Err(BrowserGpuPropertyInaccessible)` if
+/// the current context is neither the main thread nor a dedicated worker.
 ///
-/// If WebGPU is not supported, the Gpu property is `undefined` (but *not* necessarily `None`).
+/// If WebGPU is not supported, the Gpu property is `undefined`, and so this
+/// function will return `Ok(None)`.
 ///
 /// See:
 /// * <https://developer.mozilla.org/en-US/docs/Web/API/Navigator/gpu>
 /// * <https://developer.mozilla.org/en-US/docs/Web/API/WorkerNavigator/gpu>
-pub fn get_browser_gpu_property() -> Option<webgpu_sys::Gpu> {
+pub fn get_browser_gpu_property(
+) -> Result<Option<DefinedNonNullJsValue<webgpu_sys::Gpu>>, BrowserGpuPropertyInaccessible> {
     let global: Global = js_sys::global().unchecked_into();
 
-    if !global.window().is_undefined() {
+    let maybe_undefined_gpu: webgpu_sys::Gpu = if !global.window().is_undefined() {
         let navigator = global.unchecked_into::<web_sys::Window>().navigator();
-        Some(ext_bindings::NavigatorGpu::gpu(&navigator))
+        ext_bindings::NavigatorGpu::gpu(&navigator)
     } else if !global.worker().is_undefined() {
         let navigator = global
             .unchecked_into::<web_sys::WorkerGlobalScope>()
             .navigator();
-        Some(ext_bindings::NavigatorGpu::gpu(&navigator))
+        ext_bindings::NavigatorGpu::gpu(&navigator)
     } else {
-        None
-    }
+        return Err(BrowserGpuPropertyInaccessible);
+    };
+    Ok(DefinedNonNullJsValue::new(maybe_undefined_gpu))
 }
 
 impl crate::context::Context for ContextWebGpu {
@@ -1096,9 +1139,11 @@ impl crate::context::Context for ContextWebGpu {
     type SubmissionIndexData = ();
     type PipelineCacheData = ();
 
-    type RequestAdapterFuture = MakeSendFuture<
-        wasm_bindgen_futures::JsFuture,
-        fn(JsFutureResult) -> Option<Self::AdapterData>,
+    type RequestAdapterFuture = OptionFuture<
+        MakeSendFuture<
+            wasm_bindgen_futures::JsFuture,
+            fn(JsFutureResult) -> Option<Self::AdapterData>,
+        >,
     >;
     type RequestDeviceFuture = MakeSendFuture<
         wasm_bindgen_futures::JsFuture,
@@ -1115,12 +1160,13 @@ impl crate::context::Context for ContextWebGpu {
     >;
 
     fn init(_instance_desc: wgt::InstanceDescriptor) -> Self {
-        let Some(gpu) = get_browser_gpu_property() else {
+        let Ok(gpu) = get_browser_gpu_property() else {
             panic!(
                 "Accessing the GPU is only supported on the main thread or from a dedicated worker"
             );
         };
-        ContextWebGpu(gpu)
+
+        ContextWebGpu { gpu }
     }
 
     unsafe fn instance_create_surface(
@@ -1190,12 +1236,16 @@ impl crate::context::Context for ContextWebGpu {
         if let Some(mapped_pref) = mapped_power_preference {
             mapped_options.power_preference(mapped_pref);
         }
-        let adapter_promise = self.0.request_adapter_with_options(&mapped_options);
-
-        MakeSendFuture::new(
-            wasm_bindgen_futures::JsFuture::from(adapter_promise),
-            future_request_adapter,
-        )
+        if let Some(gpu) = &self.gpu {
+            let adapter_promise = gpu.request_adapter_with_options(&mapped_options);
+            OptionFuture::some(MakeSendFuture::new(
+                wasm_bindgen_futures::JsFuture::from(adapter_promise),
+                future_request_adapter,
+            ))
+        } else {
+            // Gpu is undefined; WebGPU is not supported in this browser.
+            OptionFuture::none()
+        }
     }
 
     fn adapter_request_device(
@@ -1316,7 +1366,11 @@ impl crate::context::Context for ContextWebGpu {
         let mut mapped_formats = formats.iter().map(|format| map_texture_format(*format));
         // Preferred canvas format will only be either "rgba8unorm" or "bgra8unorm".
         // https://www.w3.org/TR/webgpu/#dom-gpu-getpreferredcanvasformat
-        let preferred_format = self.0.get_preferred_canvas_format();
+        let gpu = self
+            .gpu
+            .as_ref()
+            .expect("Caller could not have created an adapter if gpu is undefined.");
+        let preferred_format = gpu.get_preferred_canvas_format();
         if let Some(index) = mapped_formats.position(|format| format == preferred_format) {
             formats.swap(0, index);
         }

wgpu/src/backend/webgpu/defined_non_null_js_value.rs

@@ -0,0 +1,46 @@
+use std::ops::{Deref, DerefMut};
+
+use wasm_bindgen::JsValue;
+
+/// Derefs to a [`JsValue`] that's known not to be `undefined` or `null`.
+#[derive(Debug)]
+pub struct DefinedNonNullJsValue<T>(T);
+
+impl<T> DefinedNonNullJsValue<T>
+where
+    T: AsRef<JsValue>,
+{
+    pub fn new(value: T) -> Option<Self> {
+        if value.as_ref().is_undefined() || value.as_ref().is_null() {
+            None
+        } else {
+            Some(Self(value))
+        }
+    }
+}
+
+impl<T> Deref for DefinedNonNullJsValue<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<T> DerefMut for DefinedNonNullJsValue<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+impl<T> AsRef<T> for DefinedNonNullJsValue<T> {
+    fn as_ref(&self) -> &T {
+        &self.0
+    }
+}
+
+impl<T> AsMut<T> for DefinedNonNullJsValue<T> {
+    fn as_mut(&mut self) -> &mut T {
+        &mut self.0
+    }
+}