From 5182ab601abe403a79fa5094affb793a2a82f49a Mon Sep 17 00:00:00 2001
From: Andrew Kushnir <akushnir@google.com>
Date: Thu, 12 Dec 2024 17:23:54 -0800
Subject: [PATCH] fix(core): avoid triggering `on timer` and `on idle` on the
 server

This commit updates defer block logic to avoid triggering `on idle` and `on timer` on the server for regular SSR mode (when incremental hydration is not enabled). Triggering the mentioned condition resulted in invoking `setTimeout` calls, which delayed serialization on the server during SSR (the process was waiting for the timeouts to clear).
---
 packages/core/src/defer/triggering.ts         | 10 ++++--
 .../test/full_app_hydration_spec.ts           | 35 +++++++++++++++++++
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/packages/core/src/defer/triggering.ts b/packages/core/src/defer/triggering.ts
index 68dc986b40904..ebadfe43e5259 100644
--- a/packages/core/src/defer/triggering.ts
+++ b/packages/core/src/defer/triggering.ts
@@ -73,11 +73,17 @@ export function scheduleDelayedTrigger(
 ) {
   const lView = getLView();
   const tNode = getCurrentTNode()!;
-  const injector = lView[INJECTOR];
-  const lDetails = getLDeferBlockDetails(lView, tNode);
 
   renderPlaceholder(lView, tNode);
 
+  // Exit early to avoid invoking `scheduleFn`, which would
+  // add `setTimeout` call and potentially delay serialization
+  // on the server unnecessarily.
+  if (!shouldTriggerDeferBlock(TriggerType.Regular, lView)) return;
+
+  const injector = lView[INJECTOR];
+  const lDetails = getLDeferBlockDetails(lView, tNode);
+
   const cleanupFn = scheduleFn(
     () => triggerDeferBlock(TriggerType.Regular, lView, tNode),
     injector,
diff --git a/packages/platform-server/test/full_app_hydration_spec.ts b/packages/platform-server/test/full_app_hydration_spec.ts
index 35849009ddfda..bdb275488c89f 100644
--- a/packages/platform-server/test/full_app_hydration_spec.ts
+++ b/packages/platform-server/test/full_app_hydration_spec.ts
@@ -3052,6 +3052,41 @@ describe('platform-server full application hydration integration', () => {
         expect(clientRootNode.outerHTML).toContain('<my-lazy-cmp>Hi!</my-lazy-cmp>');
       });
 
+      it('should not trigger `setTimeout` calls for `on timer` triggers on the server', async () => {
+        const setTimeoutSpy = spyOn(globalThis, 'setTimeout').and.callThrough();
+
+        @Component({
+          selector: 'my-lazy-cmp',
+          standalone: true,
+          template: 'Hi!',
+        })
+        class MyLazyCmp {}
+
+        @Component({
+          standalone: true,
+          selector: 'app',
+          imports: [MyLazyCmp],
+          template: `
+            @defer (on timer(123ms)) {
+              <my-lazy-cmp />
+            }
+          `,
+        })
+        class SimpleComponent {}
+
+        const html = await ssr(SimpleComponent);
+
+        const ssrContents = getAppContents(html);
+        expect(ssrContents).toContain('<app ngh');
+
+        // Make sure that there were no `setTimeout` calls with the # of ms
+        // defined in the `on timer` trigger.
+        for (let i = 0; i < setTimeoutSpy.calls.count(); i++) {
+          const args = setTimeoutSpy.calls.argsFor(i);
+          expect(args[1]).not.toBe(123, 'on timer was triggered during SSR unexpectedly');
+        }
+      });
+
       it('should hydrate a placeholder block', async () => {
         @Component({
           selector: 'my-lazy-cmp',