backend, web: support for resuming streams

server/src/active-streams.ts

@@ -0,0 +1,59 @@
+export type SseStreamEvent = {
+  event: string;
+  data: unknown;
+};
+
+type SseStreamListener = (event: SseStreamEvent) => void;
+
+export class ActiveSseStream {
+  private readonly events: SseStreamEvent[] = [];
+  private readonly listeners = new Set<SseStreamListener>();
+  private completed = false;
+  private resolveDone!: () => void;
+
+  readonly done: Promise<void>;
+
+  constructor() {
+    this.done = new Promise((resolve) => {
+      this.resolveDone = resolve;
+    });
+  }
+
+  get isCompleted() {
+    return this.completed;
+  }
+
+  emit(event: string, data: unknown) {
+    if (this.completed) return;
+    const entry = { event, data };
+    this.events.push(entry);
+    for (const listener of this.listeners) {
+      listener(entry);
+    }
+  }
+
+  complete(finalEvent?: SseStreamEvent) {
+    if (this.completed) return;
+    if (finalEvent) {
+      this.emit(finalEvent.event, finalEvent.data);
+    }
+    this.completed = true;
+    this.listeners.clear();
+    this.resolveDone();
+  }
+
+  subscribe(listener: SseStreamListener) {
+    for (const event of this.events) {
+      listener(event);
+    }
+
+    if (this.completed) {
+      return () => {};
+    }
+
+    this.listeners.add(listener);
+    return () => {
+      this.listeners.delete(listener);
+    };
+  }
+}