mor

frontend/test-support/checkpoint.ts

@@ -0,0 +1,45 @@
+import assert from "node:assert/strict";
+
+/** A one-shot, observable scheduling boundary. An unexercised gate is a failure. */
+export class Checkpoint<T> {
+    private readonly reached = Promise.withResolvers<T>();
+    private readonly released = Promise.withResolvers<void>();
+    public fired = false;
+    public resumed = false;
+    public value?: T;
+
+    public async enter(value: T, signal?: AbortSignal | null): Promise<void> {
+        assert(!this.fired, "Checkpoint entered twice");
+        this.fired = true;
+        this.value = structuredClone(value);
+        this.reached.resolve(this.value);
+        signal?.throwIfAborted();
+        let abort = () => {};
+        try {
+            await Promise.race([
+                this.released.promise,
+                new Promise<never>((_, reject) => {
+                    abort = () =>
+                        reject(
+                            signal?.reason ?? new Error("Checkpoint aborted")
+                        );
+                    signal?.addEventListener("abort", abort, { once: true });
+                })
+            ]);
+        } finally {
+            signal?.removeEventListener("abort", abort);
+        }
+    }
+
+    public wait(): Promise<T> {
+        return this.reached.promise;
+    }
+    public resume(): void {
+        this.resumed = true;
+        this.released.resolve();
+    }
+    public assertConsumed(): void {
+        assert(this.fired, "Checkpoint never reached");
+        assert(this.resumed, "Checkpoint never released");
+    }
+}