Merge pull request mozilla#1503 from nesium/fix-swift-callback-interf…

…ace-race

Fix race condition affecting callback interfaces in generated Swift code

uniffi_bindgen/src/bindings/swift/templates/CallbackInterfaceTemplate.swift

@@ -101,18 +101,15 @@ fileprivate let {{ foreign_callback }} : ForeignCallback =
 
 // FfiConverter protocol for callback interfaces
 fileprivate struct {{ ffi_converter_name }} {
-    // Initialize our callback method with the scaffolding code
-    private static var callbackInitialized = false
-    private static func initCallback() {
+    private static let initCallbackOnce: () = {
+        // Swift ensures this initializer code will once run once, even when accessed by multiple threads.
         try! rustCall { (err: UnsafeMutablePointer<RustCallStatus>) in
-                {{ cbi.ffi_init_callback().name() }}({{ foreign_callback }}, err)
+            {{ cbi.ffi_init_callback().name() }}({{ foreign_callback }}, err)
         }
-    }
+    }()
+
     private static func ensureCallbackinitialized() {
-        if !callbackInitialized {
-            initCallback()
-            callbackInitialized = true
-        }
+        _ = initCallbackOnce
     }
 
     static func drop(handle: UniFFICallbackHandle) {