fix: serialize session lifecycle transitions

backend/src/sse/sse-hub.ts

@@ -1,5 +1,11 @@
 type Send = (event: string, data: object, id: number) => void;
 
+interface Subscriber {
+  send: Send;
+  close: () => void;
+  closed: boolean;
+}
+
 /**
  * Per-session ring buffer of recent events.
  *
@@ -17,6 +23,7 @@ interface BufferedEvent {
 interface SubscribeOptions {
   afterId?: number;
   pending?: object | null;
+  close?: () => void;
 }
 
 function descriptorId(value: unknown): string | null {
@@ -32,7 +39,7 @@ function bufferedGateId(item: BufferedEvent): string | null {
 }
 
 export class SseHub {
-  private subs = new Map<string, Set<Send>>();
+  private subs = new Map<string, Set<Subscriber>>();
   private buffers = new Map<string, BufferedEvent[]>();
   private lastIds = new Map<string, number>();
 
@@ -42,7 +49,8 @@ export class SseHub {
     options: SubscribeOptions = {},
   ): () => void {
     if (!this.subs.has(sessionId)) this.subs.set(sessionId, new Set());
-    this.subs.get(sessionId)!.add(send);
+    const subscriber = { send, close: options.close ?? (() => undefined), closed: false };
+    this.subs.get(sessionId)!.add(subscriber);
 
     const afterId = Number.isSafeInteger(options.afterId) && (options.afterId ?? 0) >= 0
       ? options.afterId ?? 0
@@ -63,12 +71,14 @@ export class SseHub {
       send(item.event, item.data, item.id);
     }
 
-    return () => this.subs.get(sessionId)?.delete(send);
+    return () => this.subs.get(sessionId)?.delete(subscriber);
   }
 
   publish(sessionId: string, event: string, data: object): number {
     const item = this.buffer(sessionId, event, data);
-    for (const send of this.subs.get(sessionId) ?? []) send(event, data, item.id);
+    for (const subscriber of this.subs.get(sessionId) ?? []) {
+      subscriber.send(event, data, item.id);
+    }
     return item.id;
   }
 
@@ -88,6 +98,13 @@ export class SseHub {
 
   /** Clear buffered/runtime bindings while retaining session event-id monotonicity. */
   clear(sessionId: string): void {
+    // Snapshot because ending an HTTP response can synchronously/asynchronously unsubscribe it.
+    // Mark before invoking callbacks so even a re-entrant clear cannot close a response twice.
+    for (const subscriber of [...(this.subs.get(sessionId) ?? [])]) {
+      if (subscriber.closed) continue;
+      subscriber.closed = true;
+      try { subscriber.close(); } catch { /* disconnect every remaining subscriber */ }
+    }
     this.buffers.delete(sessionId);
     this.subs.delete(sessionId);
   }