Make sure that the search scheduler doesn't run to many tasks in parallel

Signed-off-by: Oliver Sand <oliver.sand@sda-se.com>

.changeset/tasty-ghosts-raise.md

@@ -0,0 +1,8 @@
+---
+'@backstage/plugin-search-backend-node': patch
+---
+
+Change search scheduler from starting indexing in a fixed interval (for example
+every 60 seconds), to wait a fixed time between index runs.
+This makes sure that no second index process for the same document type is
+started when the previous one is still running.

plugins/search-backend-node/src/Scheduler.ts

@@ -15,6 +15,7 @@
  */
 
 import { Logger } from 'winston';
+import { runPeriodically } from './runPeriodically';
 
 type TaskEnvelope = {
   task: Function;
@@ -28,7 +29,7 @@ type TaskEnvelope = {
 export class Scheduler {
   private logger: Logger;
   private schedule: TaskEnvelope[];
-  private intervalTimeouts: NodeJS.Timeout[] = [];
+  private runningTasks: Function[] = [];
 
   constructor({ logger }: { logger: Logger }) {
     this.logger = logger;
@@ -40,7 +41,7 @@ export class Scheduler {
    *
    */
   addToSchedule(task: Function, interval: number) {
-    if (this.intervalTimeouts.length) {
+    if (this.runningTasks.length) {
       throw new Error(
         'Cannot add task to schedule that has already been started.',
       );
@@ -56,11 +57,7 @@ export class Scheduler {
     this.schedule.forEach(({ task, interval }) => {
       // Fire the task immediately, then schedule it.
       task();
-      this.intervalTimeouts.push(
-        setInterval(() => {
-          task();
-        }, interval),
-      );
+      this.runningTasks.push(runPeriodically(() => task(), interval));
     });
   }
 
@@ -69,9 +66,9 @@ export class Scheduler {
    */
   stop() {
     this.logger.info('Stopping all scheduled search tasks.');
-    this.intervalTimeouts.forEach(timeout => {
-      clearInterval(timeout);
+    this.runningTasks.forEach(cancel => {
+      cancel();
     });
-    this.intervalTimeouts = [];
+    this.runningTasks = [];
   }
 }

plugins/search-backend-node/src/runPeriodically.ts

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2020 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Runs a function repeatedly, with a fixed wait between invocations.
+ *
+ * Supports async functions, and silently ignores exceptions and rejections.
+ *
+ * @param fn The function to run. May return a Promise.
+ * @param delayMs The delay between a completed function invocation and the
+ *                next.
+ * @returns A function that, when called, stops the invocation loop.
+ */
+export function runPeriodically(fn: () => any, delayMs: number): () => void {
+  let cancel: () => void;
+  let cancelled = false;
+  const cancellationPromise = new Promise<void>(resolve => {
+    cancel = () => {
+      resolve();
+      cancelled = true;
+    };
+  });
+
+  const startRefresh = async () => {
+    while (!cancelled) {
+      try {
+        await fn();
+      } catch {
+        // ignore intentionally
+      }
+
+      await Promise.race([
+        new Promise(resolve => setTimeout(resolve, delayMs)),
+        cancellationPromise,
+      ]);
+    }
+  };
+  startRefresh();
+
+  return cancel!;
+}