feat(@angular-devkit/core): add a reuse JobStrategy

It allows a job to be reused if it's still running. This includes redirecting
the inputs to the new job.

packages/angular_devkit/core/src/experimental/jobs/strategy.ts

@@ -5,10 +5,16 @@
  * 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 { Observable, concat, of } from 'rxjs';
-import { ignoreElements, share, shareReplay } from 'rxjs/operators';
+import { Observable, Subject, concat, of } from 'rxjs';
+import { finalize, ignoreElements, share, shareReplay, tap } from 'rxjs/operators';
 import { JsonValue } from '../../json';
-import { JobDescription, JobHandler, JobHandlerContext, JobOutboundMessage } from './api';
+import {
+  JobDescription,
+  JobHandler,
+  JobHandlerContext, JobInboundMessage,
+  JobOutboundMessage,
+  JobOutboundMessageKind,
+} from './api';
 
 const stableStringify = require('fast-json-stable-stringify');
 
@@ -49,6 +55,64 @@ export namespace strategy {
   }
 
 
+  /**
+   * Creates a JobStrategy that will always reuse a running job, and restart it if the job ended.
+   * @param replayMessages Replay ALL messages if a job is reused, otherwise just hook up where it
+   *        is.
+   */
+  export function reuse<
+    A extends JsonValue = JsonValue,
+    I extends JsonValue = JsonValue,
+    O extends JsonValue = JsonValue,
+  >(replayMessages = false): JobStrategy<A, I, O> {
+    let inboundBus = new Subject<JobInboundMessage<I>>();
+    let runContext: JobHandlerContext | null = null;
+    let run: Observable<JobOutboundMessage<O>> | null = null;
+    let state: JobOutboundMessage<O> | null = null;
+
+    return (handler, options) => {
+      const newHandler = (argument: A, context: JobHandlerContext<A, I, O>) => {
+        // Forward inputs.
+        const subscription = context.inboundBus.subscribe(inboundBus);
+
+        if (run) {
+          return concat(
+            // Update state.
+            of(state),
+            run,
+          ).pipe(
+            finalize(() => subscription.unsubscribe()),
+          );
+        }
+
+        run = handler(argument, { ...context, inboundBus: inboundBus.asObservable() }).pipe(
+          tap(
+            message => {
+              if (message.kind == JobOutboundMessageKind.Start
+                  || message.kind == JobOutboundMessageKind.OnReady
+                  || message.kind == JobOutboundMessageKind.End) {
+                state = message;
+              }
+            },
+            undefined,
+            () => {
+              subscription.unsubscribe();
+              inboundBus = new Subject<JobInboundMessage<I>>();
+              run = null;
+            },
+          ),
+          replayMessages ? shareReplay() : share(),
+        );
+        runContext = context;
+
+        return run;
+      };
+
+      return Object.assign(newHandler, handler, options || {});
+    };
+  }
+
+
   /**
    * Creates a JobStrategy that will reuse a running job if the argument matches.
    * @param replayMessages Replay ALL messages if a job is reused, otherwise just hook up where it

packages/angular_devkit/core/src/experimental/jobs/strategy_spec.ts

@@ -131,6 +131,73 @@ describe('strategy.serialize()', () => {
   });
 });
 
+describe('strategy.reuse()', () => {
+  let registry: SimpleJobRegistry;
+  let scheduler: SimpleScheduler;
+
+  beforeEach(() => {
+    registry = new SimpleJobRegistry();
+    scheduler = new SimpleScheduler(registry);
+  });
+
+  it('works', async () => {
+    let started = 0;
+    let finished = 0;
+
+    registry.register(strategy.reuse()(createJobHandler((input: number[]) => {
+      started++;
+
+      return new Promise<number>(
+        resolve => setTimeout(() => {
+          finished++;
+          resolve(input.reduce((a, c) => a + c, 0));
+        }, 10),
+      );
+    })), {
+      argument: { items: { type: 'number' } },
+      output: { type: 'number' },
+      name: 'add',
+    });
+
+    const job1 = await scheduler.schedule('add', [1, 2, 3, 4]);
+    const job2 = await scheduler.schedule('add', []);
+    expect(started).toBe(0);
+    expect(finished).toBe(0);
+
+    job1.output.subscribe();
+    expect(started).toBe(1);
+    expect(finished).toBe(0);
+
+    job2.output.subscribe();
+    expect(started).toBe(1);  // job2 is reusing job1.
+    expect(finished).toBe(0);
+
+    let result = await job1.output.toPromise();
+    expect(result).toBe(10);
+    expect(started).toBe(1);
+    expect(finished).toBe(1);
+    expect(job1.state).toBe(JobState.Ended);
+    expect(job2.state).toBe(JobState.Ended);
+
+    const job3 = await scheduler.schedule('add', [1, 2, 3, 4, 5]);
+    const job4 = await scheduler.schedule('add', []);
+    job3.output.subscribe();
+    expect(started).toBe(2);
+    expect(finished).toBe(1);
+
+    job4.output.subscribe();
+    expect(started).toBe(2);  // job4 is reusing job3.
+    expect(finished).toBe(1);
+
+    result = await job3.output.toPromise();
+    expect(result).toBe(15);
+    expect(started).toBe(2);
+    expect(finished).toBe(2);
+    expect(job3.state).toBe(JobState.Ended);
+    expect(job4.state).toBe(JobState.Ended);
+  });
+});
+
 describe('strategy.memoize()', () => {
   let registry: SimpleJobRegistry;
   let scheduler: SimpleScheduler;