metal: split copy device holder

src/backends/metal/video/copy_device.rs

@@ -0,0 +1,33 @@
+use super::*;
+
+pub struct CopyDeviceHolder {
+    pub registry: Rc<CopyDeviceRegistry>,
+    pub devnum: dev_t,
+    pub dev: OnceCell<Option<Rc<CopyDevice>>>,
+}
+
+impl Debug for CopyDeviceHolder {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("CopyDeviceHolder").finish_non_exhaustive()
+    }
+}
+
+impl CopyDeviceHolder {
+    pub fn get(&self) -> Option<Rc<CopyDevice>> {
+        self.dev
+            .get_or_init(
+                || match self.registry.get(self.devnum)?.create_device().map(Some) {
+                    Ok(d) => d,
+                    Err(e) => {
+                        log::error!(
+                            "Could not get copy device for {}: {}",
+                            self.devnum,
+                            ErrorFmt(e),
+                        );
+                        None
+                    }
+                },
+            )
+            .clone()
+    }
+}