feat(platform-server): wait on returned BEFORE_APP_SERIALIZED promises (#29120)

This update gives external tooling the ability for async providers to
finish resolving before the document is serialized. This is not a
breaking change since render already returns a promise. All returned
promises from `BEFORE_APP_SERIALIZED` providers will wait to be
resolved or rejected. Any rejected promises will only console.warn().

PR Close #29120

packages/platform-server/src/utils.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {ApplicationRef, NgModuleFactory, NgModuleRef, PlatformRef, StaticProvider, Type} from '@angular/core';
+import {ApplicationRef, NgModuleFactory, NgModuleRef, PlatformRef, StaticProvider, Type, ɵisPromise} from '@angular/core';
 import {ɵTRANSITION_ID} from '@angular/platform-browser';
 import {first} from 'rxjs/operators';
 
@@ -45,12 +45,18 @@ the server-rendered app can be properly bootstrapped into a client app.`);
         .then(() => {
           const platformState = platform.injector.get(PlatformState);
 
+          const asyncPromises: Promise<any>[] = [];
+
           // Run any BEFORE_APP_SERIALIZED callbacks just before rendering to string.
           const callbacks = moduleRef.injector.get(BEFORE_APP_SERIALIZED, null);
           if (callbacks) {
             for (const callback of callbacks) {
               try {
-                callback();
+                const callbackResult = callback();
+                if (ɵisPromise(callbackResult)) {
+                  asyncPromises.push(callbackResult);
+                }
+
               } catch (e) {
                 // Ignore exceptions.
                 console.warn('Ignoring BEFORE_APP_SERIALIZED Exception: ', e);
@@ -58,9 +64,22 @@ the server-rendered app can be properly bootstrapped into a client app.`);
             }
           }
 
-          const output = platformState.renderToString();
-          platform.destroy();
-          return output;
+          const complete = () => {
+            const output = platformState.renderToString();
+            platform.destroy();
+            return output;
+          };
+
+          if (asyncPromises.length === 0) {
+            return complete();
+          }
+
+          return Promise
+              .all(asyncPromises.map(asyncPromise => {
+                return asyncPromise.catch(
+                    e => { console.warn('Ignoring BEFORE_APP_SERIALIZED Exception: ', e); });
+              }))
+              .then(complete);
         });
   });
 }