test(e2e): deterministically wait for event stream in logout propagation test (#38535)

The test raced the server-side SSE channel registration: a logout
emitted before registration is silently dropped ([example
flake](https://github.com/go-gitea/gitea/actions/runs/29699349255/job/88225654080)).
The 500ms wait from https://github.com/go-gitea/gitea/pull/37403 only
made this unlikely.

The server now registers the channel before sending the initial response
bytes, so an open connection implies registration. The shared worker
forwards the built-in `open` event (replaying it to late-attaching ports
via `EventSource.readyState`), the page exposes it as a
`data-user-events-connected` attribute, and the test waits for that
attribute instead of a fixed timeout.

routers/web/events/events.go

@@ -38,7 +38,6 @@ func Events(ctx *context.Context) {
 
 	// Listen to connection close and un-register messageChan
 	notify := ctx.Done()
-	ctx.Resp.Flush()
 
 	shutdownCtx := graceful.GetManager().ShutdownContext()
 
@@ -57,11 +56,14 @@ func Events(ctx *context.Context) {
 		}
 	}
 
+	// send the initial response bytes only after registering messageChan, so a client whose
+	// connection is open can rely on receiving all subsequent events
 	if _, err := ctx.Resp.Write([]byte("\n")); err != nil {
 		log.Error("Unable to write to EventStream: %v", err)
 		unregister()
 		return
 	}
+	ctx.Resp.Flush()
 
 	timer := time.NewTicker(30 * time.Second)
 

tests/e2e/events.test.ts

@@ -66,12 +66,9 @@ test.describe('events', () => {
     // Verify page2 is logged in
     await expect(page2.getByRole('link', {name: 'Sign In'})).toBeHidden();
 
-    // Give page2's SharedWorker time to register its SSE connection on the
-    // server — otherwise the logout event can race the connection and be
-    // silently dropped. See https://github.com/go-gitea/gitea/pull/37403
-    // In the future, we can set an attribute to HTML page when the connection is established,
-    // then here we can just wait for that attribute (it should also work for the planned WebSocket SharedWorker)
-    await page2.waitForTimeout(500); // eslint-disable-line playwright/no-wait-for-timeout
+    // Wait until the server has registered page2's event stream, otherwise the logout
+    // event can race the connection and be silently dropped.
+    await expect(page2.locator('html[data-user-events-connected]')).toBeAttached();
 
     // Logout from page1 — this sends a logout event to all tabs
     await page1.goto('/user/logout');

web_src/js/eventsource.sharedworker.ts

@@ -26,6 +26,11 @@ class Source {
       type: 'status',
       message: `registered to ${this.url}`,
     });
+
+    // replay the "open" event to ports attaching to an already-open source
+    if (this.eventSource?.readyState === EventSource.OPEN) {
+      port.postMessage({type: 'open'});
+    }
   }
 
   deregister(port: MessagePort) {

web_src/js/modules/worker.ts

@@ -54,6 +54,9 @@ export class UserEventsSharedWorker {
       } else if (event.data.type === 'close') {
         this.sharedWorker.port.postMessage({type: 'close'});
         this.sharedWorker.port.close();
+      } else if (event.data.type === 'open') {
+        // e2e tests wait for this attribute to know events cannot be missed anymore
+        document.documentElement.setAttribute('data-user-events-connected', 'true');
       }
       listener(event);
     });