feat(core): Add rxjs operator prevent app stability until an event

This commit adds an operator to help rxjs observables important for rendering
keep the application unstable (and prevent serialization) until there is
an event (observable emits, completes, or errors, or the subscription is
unsubscribed). This helps with SSR for zoneless and also helps for when
operations are intentionally executed outside the Angular Zone but are
still important for SSR (i.e. angularfire and the zoneWrap helper hacks).

goldens/public-api/core/rxjs-interop/index.api.md

@@ -23,6 +23,9 @@ export function outputFromObservable<T>(observable: Observable<T>, opts?: Output
 // @public
 export function outputToObservable<T>(ref: OutputRef<T>): Observable<T>;
 
+// @public
+export function pendingUntilEvent<T>(injector?: Injector): MonoTypeOperatorFunction<T>;
+
 // @public
 export function rxResource<T, R>(opts: RxResourceOptions<T, R>): ResourceRef<T>;
 

packages/core/rxjs-interop/src/index.ts

@@ -15,4 +15,5 @@ export {
   toObservableMicrotask as ɵtoObservableMicrotask,
 } from './to_observable';
 export {toSignal, ToSignalOptions} from './to_signal';
+export {pendingUntilEvent} from './pending_until_event';
 export {RxResourceOptions, rxResource} from './rx_resource';

packages/core/rxjs-interop/src/pending_until_event.ts

@@ -0,0 +1,69 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {
+  assertInInjectionContext,
+  PendingTasks,
+  inject,
+  Injector,
+} from '@angular/core';
+import {MonoTypeOperatorFunction, Observable} from 'rxjs';
+
+/**
+ * Operator which makes the application unstable until the observable emits, complets, errors, or is unsubscribed.
+ *
+ * Use this operator in observables whose subscriptions are important for rendering and should be included in SSR serialization.
+ *
+ * @param injector The `Injector` to use during creation. If this is not provided, the current injection context will be used instead (via `inject`).
+ *
+ * @experimental
+ */
+export function pendingUntilEvent<T>(injector?: Injector): MonoTypeOperatorFunction<T> {
+  if (injector === undefined) {
+    assertInInjectionContext(pendingUntilEvent);
+    injector = inject(Injector);
+  }
+  const taskService = injector.get(PendingTasks);
+
+  return (sourceObservable) => {
+    return new Observable<T>((originalSubscriber) => {
+      // create a new task on subscription
+      const removeTask = taskService.add();
+
+      let cleanedUp = false;
+      function cleanupTask() {
+        if (cleanedUp) {
+          return;
+        }
+
+        removeTask();
+        cleanedUp = true;
+      }
+
+      const innerSubscription = sourceObservable.subscribe({
+        next: (v) => {
+          originalSubscriber.next(v);
+          cleanupTask();
+        },
+        complete: () => {
+          originalSubscriber.complete();
+          cleanupTask();
+        },
+        error: (e) => {
+          originalSubscriber.error(e);
+          cleanupTask();
+        },
+      });
+      innerSubscription.add(() => {
+        originalSubscriber.unsubscribe();
+        cleanupTask();
+      });
+      return innerSubscription;
+    });
+  };
+}

packages/core/rxjs-interop/test/pending_until_event_spec.ts

@@ -0,0 +1,249 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {EnvironmentInjector, ɵPendingTasks as PendingTasks, ApplicationRef} from '@angular/core';
+import {
+  BehaviorSubject,
+  EMPTY,
+  Subject,
+  catchError,
+  delay,
+  config,
+  finalize,
+  firstValueFrom,
+  interval,
+  of,
+} from 'rxjs';
+
+import {pendingUntilEvent} from '@angular/core/rxjs-interop';
+import {TestBed} from '@angular/core/testing';
+
+describe('pendingUntilEvent', () => {
+  let taskService: PendingTasks;
+  let injector: EnvironmentInjector;
+  let appRef: ApplicationRef;
+  beforeEach(() => {
+    taskService = TestBed.inject(PendingTasks);
+    injector = TestBed.inject(EnvironmentInjector);
+    appRef = TestBed.inject(ApplicationRef);
+  });
+
+  it('should not block stability until subscription', async () => {
+    const originalSource = new BehaviorSubject(0);
+    const delayedSource = originalSource.pipe(delay(5), pendingUntilEvent(injector));
+    expect(taskService.hasPendingTasks.value).toEqual(false);
+
+    const emitPromise = firstValueFrom(delayedSource);
+    expect(taskService.hasPendingTasks.value).toEqual(true);
+
+    await expectAsync(emitPromise).toBeResolvedTo(0);
+    await expectAsync(appRef.whenStable()).toBeResolved();
+  });
+
+  it('runs the subscription body before stability', async () => {
+    const source = of(1).pipe(pendingUntilEvent(injector));
+
+    // stable before subscription
+    expect(taskService.hasPendingTasks.value).toEqual(false);
+    source.subscribe(() => {
+      // unstable within synchronous subscription body
+      expect(taskService.hasPendingTasks.value).toBe(true);
+    });
+    // stable after above synchronous subscription execution
+    await expectAsync(appRef.whenStable()).toBeResolved();
+  });
+
+  it('only blocks stability until first emit', async () => {
+    const intervalSource = interval(5).pipe(pendingUntilEvent(injector));
+    expect(taskService.hasPendingTasks.value).toEqual(false);
+
+    await new Promise<void>(async (resolve) => {
+      const subscription = intervalSource.subscribe(async (v) => {
+        if (v === 0) {
+          expect(taskService.hasPendingTasks.value).toBe(true);
+        } else {
+          await expectAsync(appRef.whenStable()).toBeResolved();
+        }
+        if (v === 3) {
+          subscription.unsubscribe();
+          resolve();
+        }
+      });
+      expect(taskService.hasPendingTasks.value).toBe(true);
+    });
+  });
+
+  it('should unblock stability on complete (but no emit)', async () => {
+    const sub = new Subject();
+    sub.asObservable().pipe(pendingUntilEvent(injector)).subscribe();
+    expect(taskService.hasPendingTasks.value).toBe(true);
+    sub.complete();
+    await expectAsync(appRef.whenStable()).toBeResolved();
+  });
+
+  it('should unblock stability on unsubscribe before emit', async () => {
+    const sub = new Subject();
+    const subscription = sub.asObservable().pipe(pendingUntilEvent(injector)).subscribe();
+    expect(taskService.hasPendingTasks.value).toBe(true);
+    subscription.unsubscribe();
+    await expectAsync(appRef.whenStable()).toBeResolved();
+  });
+
+  // Note that we cannot execute `finalize` operators that appear _after_ ours before
+  // removing the pending task. We need to register the finalize operation on the subscription
+  // as soon as the operator executes. A `finalize` operator later on in the stream will
+  // be appear later in the finalizers list. These finalizers are both registered and executed
+  // serially. We cannot execute our finalizer after other finalizers in the pipeline.
+  it('should execute user finalize body before stability (as long as it appears first)', async () => {
+    const sub = new Subject();
+    let finalizeExecuted = false;
+    const subscription = sub
+      .asObservable()
+      .pipe(
+        finalize(() => {
+          finalizeExecuted = true;
+          expect(taskService.hasPendingTasks.value).toBe(true);
+        }),
+        pendingUntilEvent(injector),
+      )
+      .subscribe();
+    expect(taskService.hasPendingTasks.value).toBe(true);
+    subscription.unsubscribe();
+    await expectAsync(appRef.whenStable()).toBeResolved();
+    expect(finalizeExecuted).toBe(true);
+  });
+
+  it('should not throw if application is destroyed before emit', async () => {
+    const sub = new Subject<void>();
+    sub.asObservable().pipe(pendingUntilEvent(injector)).subscribe();
+    expect(taskService.hasPendingTasks.value).toBe(true);
+    TestBed.resetTestingModule();
+    await expectAsync(appRef.whenStable()).toBeResolved();
+    sub.next();
+    await expectAsync(appRef.whenStable()).toBeResolved();
+  });
+
+  it('should unblock stability on error before emit', async () => {
+    const sub = new Subject<void>();
+    sub
+      .asObservable()
+      .pipe(
+        pendingUntilEvent(injector),
+        catchError(() => EMPTY),
+      )
+      .subscribe();
+    expect(taskService.hasPendingTasks.value).toBe(true);
+    sub.error(new Error('error in pipe'));
+    await expectAsync(appRef.whenStable()).toBeResolved();
+    sub.next();
+    await expectAsync(appRef.whenStable()).toBeResolved();
+  });
+
+  it('should unblock stability on error in subscription', async () => {
+    function nextUncaughtError() {
+      return new Promise((resolve) => {
+        config.onUnhandledError = (e) => {
+          config.onUnhandledError = null;
+          resolve(e);
+        };
+      });
+    }
+    const sub = new Subject<void>();
+    sub
+      .asObservable()
+      .pipe(pendingUntilEvent(injector))
+      .subscribe({
+        next: () => {
+          throw new Error('oh noes');
+        },
+      });
+    expect(taskService.hasPendingTasks.value).toBe(true);
+    const errorPromise = nextUncaughtError();
+    sub.next();
+    await expectAsync(errorPromise).toBeResolved();
+    await expectAsync(appRef.whenStable()).toBeResolved();
+
+    const errorPromise2 = nextUncaughtError();
+    sub.next();
+    await expectAsync(appRef.whenStable()).toBeResolved();
+    await expectAsync(errorPromise2).toBeResolved();
+  });
+
+  it('finalize and complete are delivered correctly', () => {
+    const sub = new Subject<void>();
+    let log: string[] = [];
+    const obs1 = sub.asObservable().pipe(
+      pendingUntilEvent(injector),
+      finalize(() => {
+        log.push('finalize');
+      }),
+    );
+
+    // complete after subscription
+    obs1.subscribe({
+      complete: () => {
+        log.push('complete');
+      },
+    });
+    sub.complete();
+    expect(log).toEqual(['complete', 'finalize']);
+
+    // already completed before subscription
+    log.length = 0;
+    obs1.subscribe({
+      complete: () => {
+        log.push('complete');
+      },
+    });
+    expect(log).toEqual(['complete', 'finalize']);
+
+    log.length = 0;
+    new Subject()
+      .asObservable()
+      .pipe(
+        pendingUntilEvent(injector),
+        finalize(() => {
+          log.push('finalize');
+        }),
+      )
+      .subscribe({
+        complete: () => {
+          log.push('complete');
+        },
+      })
+      .unsubscribe();
+    expect(log).toEqual(['finalize']);
+  });
+
+  it('should block stability for each new subscriber', async () => {
+    const sub = new Subject<void>();
+    const observable = sub.asObservable().pipe(delay(5), pendingUntilEvent(injector));
+
+    observable.subscribe();
+    expect(taskService.hasPendingTasks.value).toBe(true);
+    sub.next();
+    observable.subscribe();
+    // first subscription unblocks
+    await new Promise((r) => setTimeout(r, 5));
+    // still pending because the other subscribed after the emit
+    expect(taskService.hasPendingTasks.value).toBe(true);
+
+    sub.next();
+    await new Promise((r) => setTimeout(r, 3));
+    observable.subscribe();
+    sub.next();
+    // second subscription unblocks
+    await new Promise((r) => setTimeout(r, 2));
+    // still pending because third subscription delay not finished
+    expect(taskService.hasPendingTasks.value).toBe(true);
+
+    // finishes third subscription
+    await new Promise((r) => setTimeout(r, 3));
+    await expectAsync(appRef.whenStable()).toBeResolved();
+  });
+});

packages/core/src/change_detection/scheduling/zoneless_scheduling_impl.ts

@@ -219,7 +219,7 @@ export class ChangeDetectionSchedulerImpl implements ChangeDetectionScheduler {
   }
 
   private shouldScheduleTick(force: boolean): boolean {
-    if (this.disableScheduling && !force) {
+    if ((this.disableScheduling && !force) || this.appRef.destroyed) {
       return false;
     }
     // already scheduled or running