fix: make session resume atomic across restarts

frontend/src/stream/useSessionStream.ts

@@ -1,29 +1,51 @@
-import { useEffect, useRef, useState } from "react";
+import { useLayoutEffect, useRef, useState } from "react";
 import { BASE } from "../api/client";
 import { joinBackendPath } from "../api/runtime-config";
 import { useSessionStore } from "../store/sessionStore";
 import type { StreamEvent } from "../api/types";
 
-export function useSessionStream(sessionId: string | null, generation = 0) {
+export function useSessionStream(
+  sessionId: string | null,
+  generation = 0,
+  cursorResetEpoch = 0,
+) {
   const [connected, setConnected] = useState(false);
   const applyEvent = useSessionStore((s) => s.applyEvent);
-  const cursor = useRef({ sessionId: null as string | null, lastEventId: "" });
+  const cursor = useRef({
+    sessionId: null as string | null,
+    cursorResetEpoch,
+    lastEventId: "",
+  });
+  const activeSource = useRef<object | null>(null);
 
-  if (cursor.current.sessionId !== sessionId) {
-    cursor.current = { sessionId, lastEventId: "" };
-  }
-
-  useEffect(() => {
-    if (!sessionId) return;
+  useLayoutEffect(() => {
+    if (
+      cursor.current.sessionId !== sessionId
+      || cursor.current.cursorResetEpoch !== cursorResetEpoch
+    ) {
+      cursor.current = { sessionId, cursorResetEpoch, lastEventId: "" };
+    }
+    if (!sessionId) {
+      activeSource.current = null;
+      setConnected(false);
+      return;
+    }
 
     const query = cursor.current.lastEventId
       ? `?lastEventId=${encodeURIComponent(cursor.current.lastEventId)}`
       : "";
     const es = new EventSource(joinBackendPath(BASE, `/sessions/${sessionId}/events${query}`));
-    es.onopen = () => setConnected(true);
-    es.onerror = () => setConnected(false);
+    const identity = { source: es, sessionId, cursorResetEpoch };
+    activeSource.current = identity;
+    es.onopen = () => {
+      if (activeSource.current === identity) setConnected(true);
+    };
+    es.onerror = () => {
+      if (activeSource.current === identity) setConnected(false);
+    };
 
     const handle = (ev: MessageEvent) => {
+      if (activeSource.current !== identity) return;
       if (ev.lastEventId) cursor.current.lastEventId = ev.lastEventId;
       try {
         applyEvent(JSON.parse(ev.data) as StreamEvent);
@@ -47,10 +69,14 @@ export function useSessionStream(sessionId: string | null, generation = 0) {
 
     return () => {
       for (const name of namedEvents) es.removeEventListener(name, handle);
+      es.onmessage = null;
       es.close();
-      setConnected(false);
+      if (activeSource.current === identity) {
+        activeSource.current = null;
+        setConnected(false);
+      }
     };
-  }, [sessionId, generation, applyEvent]);
+  }, [sessionId, generation, cursorResetEpoch, applyEvent]);
 
   return { connected };
 }