feat(shell-api): add checkpoint method to InterruptFlag

Author: addaleax

packages/shell-api/src/interruptor.ts

@@ -21,6 +21,17 @@ export class InterruptFlag {
     return this.interrupted;
   }
 
+  /**
+   * Perform a checkpoint; reject immediately if an interruption has already
+   * occurred, and resolve immediately otherwise. This is useful to insert
+   * in operations consisting of multiple asynchronous steps.
+   */
+  public async checkpoint(): Promise<void> {
+    if (this.interrupted) {
+      await this.asPromise().promise;
+    }
+  }
+
   /**
    * The returned promise will never be resolved but is rejected
    * when the interrupt is set. The rejection happens with an

packages/shell-evaluator/src/shell-evaluator.ts

@@ -63,7 +63,11 @@ class ShellEvaluator<EvaluationResultType = ShellResult> {
       rewrittenInput = supportCode + ';\n' + rewrittenInput;
     }
 
-    return await originalEval(rewrittenInput, context, filename);
+    try {
+      return await originalEval(rewrittenInput, context, filename);
+    } catch (err) {
+      throw this.internalState.transformError(err);
+    }
   }
 
   /**