fix(router-core): don't drop onRenderFinished listeners after fast path reserved

When the stream fast path was already reserved, onRenderFinished silently
dropped any listener registered afterwards. With @tanstack/react-router-ssr-query
this meant the dehydration query stream was never closed, hanging the SSR
response until the ~60s serialization timeout (#7529).

The fast path still calls setRenderFinished() once the app stream ends, so the
listener is now registered regardless and fires at the correct time instead of
being discarded.

Author: radosek

.changeset/cool-streams-close.md

@@ -0,0 +1,7 @@
+---
+'@tanstack/router-core': patch
+---
+
+fix(router-core): run `onRenderFinished` listeners registered after the stream fast path is reserved
+
+When `reserveStreamFastPath()` had already set `streamFastPathReserved = true`, a subsequently registered `onRenderFinished` listener was silently dropped. This broke SSR streaming with `@tanstack/react-router-ssr-query`: the dehydration query stream was never closed, so the response hung until the serialization timeout (~60s). The listener is now registered regardless; the fast path still calls `setRenderFinished()` when the app stream ends, so it fires at the correct time.

packages/router-core/src/ssr/ssr-server.ts

@@ -637,7 +637,10 @@ export function attachRouterServerSsrUtils({
       return () => removeListener(injectedHtmlListeners, listener)
     },
     onRenderFinished: (listener) => {
-      if (cleanupStarted || streamFastPathReserved) return
+      if (cleanupStarted) return
+      // Register even when the fast path is reserved: it still calls
+      // setRenderFinished() at the end of the app stream. Dropping listeners
+      // here left router-ssr-query's query stream open, hanging SSR (#7529).
       renderFinishedListeners.push(listener)
     },
     onSerializationFinished: (listener) => {

packages/router-core/tests/ssr-server-cleanup.test.ts

@@ -190,6 +190,36 @@ describe('serverSsr.cleanup', () => {
     router.serverSsr?.cleanup()
   })
 
+  test('onRenderFinished listener registered after fast path reserve still fires', async () => {
+    // Regression test for #7529: when the fast path is reserved before an
+    // integration (e.g. router-ssr-query) registers its onRenderFinished
+    // listener, the listener must not be dropped - otherwise the query stream
+    // is never closed and the response hangs until the serialization timeout.
+    // The fast path still calls setRenderFinished() at the end of the app
+    // stream, so the listener fires at that point.
+    const router = buildRouter()
+    attachRouterServerSsrUtils({ router, manifest: undefined })
+
+    await router.load()
+    await router.serverSsr!.dehydrate()
+    router.serverSsr!.takeBufferedScripts()
+
+    expect(router.serverSsr!.reserveStreamFastPath()).toBe(true)
+
+    let renderFinishedCalls = 0
+    router.serverSsr!.onRenderFinished(() => {
+      renderFinishedCalls++
+    })
+    // Not invoked at registration time - the fast path defers to the app
+    // stream end, mirrored here by an explicit setRenderFinished().
+    expect(renderFinishedCalls).toBe(0)
+
+    router.serverSsr!.setRenderFinished()
+    expect(renderFinishedCalls).toBe(1)
+
+    router.serverSsr?.cleanup()
+  })
+
   test('stream fast path rejects while SSR work is pending', async () => {
     const value = deferred<string>()
     const router = buildRouter({ value: value.promise })