metal: check if in the main thread when calling create_surface (#2736)

wgpu-hal/src/metal/surface.rs

@@ -9,7 +9,7 @@ use objc::{
     declare::ClassDecl,
     msg_send,
     rc::autoreleasepool,
-    runtime::{Class, Object, Sel, BOOL, YES},
+    runtime::{Class, Object, Sel, BOOL, NO, YES},
     sel, sel_impl,
 };
 use parking_lot::Mutex;
@@ -74,6 +74,7 @@ impl super::Surface {
         }
     }
 
+    /// If not called on the main thread, this will panic.
     #[allow(clippy::transmute_ptr_to_ref)]
     pub unsafe fn from_view(
         view: *mut c_void,
@@ -84,6 +85,11 @@ impl super::Surface {
             panic!("window does not have a valid contentView");
         }
 
+        let is_main_thread: BOOL = msg_send![class!(NSThread), isMainThread];
+        if is_main_thread == NO {
+            panic!("create_surface cannot be called in non-ui thread.");
+        }
+
         let main_layer: *mut Object = msg_send![view, layer];
         let class = class!(CAMetalLayer);
         let is_valid_layer: BOOL = msg_send![main_layer, isKindOfClass: class];

wgpu/src/lib.rs

@@ -1708,6 +1708,7 @@ impl Instance {
     ///
     /// - Raw Window Handle must be a valid object to create a surface upon and
     ///   must remain valid for the lifetime of the returned surface.
+    /// - If not called on the main thread, metal backend will panic.
     pub unsafe fn create_surface<W: raw_window_handle::HasRawWindowHandle>(
         &self,
         window: &W,