add backtracking to opt decoding

Author: crusso

packages/candid/src/idl.ts

@@ -922,7 +922,16 @@ export class OptClass<T> extends ConstructType<[T] | []> {
       case 0:
         return [];
       case 1:
-        return [this._type.decodeValue(b, opt._type)];
+        let checkpoint = b.save();
+        try {
+          let v = this._type.decodeValue(b, opt._type)
+          return [v];
+        } catch (e : any) {
+          b.restore(checkpoint);
+          // skip value at wire typ (to advance b)
+          let v = opt._type.decodeValue(b, opt._type)
+          return [];
+        };
       default:
         throw new Error('Not an option value');
     }

packages/candid/src/utils/buffer.ts

@@ -38,6 +38,20 @@ export class PipeArrayBuffer {
    */
   private _view: Uint8Array;
 
+  /**
+   * Save a checkpoint of the reading view (for backtracking)
+   */
+  public save() : Uint8Array {
+     return this._view;
+  };
+
+  /**
+   * Restore a checkpoint of the reading view (for backtracking)
+   */
+  public restore(checkPoint: Uint8Array) {
+     this._view = checkPoint;
+  };
+
   /**
    * The actual buffer containing the bytes.
    * @private