Add jitter support to BackOff class

ts/test-both/util/BackOff_test.ts

@@ -42,4 +42,32 @@ describe('BackOff', () => {
     assert.strictEqual(b.getAndIncrement(), 1);
     assert.strictEqual(b.getAndIncrement(), 2);
   });
+
+  it('should apply jitter', () => {
+    const b = new BackOff([1, 2, 3], {
+      jitter: 1,
+      random: () => 0.5,
+    });
+
+    assert.strictEqual(b.getIndex(), 0);
+    assert.strictEqual(b.isFull(), false);
+
+    assert.strictEqual(b.get(), 1);
+    assert.strictEqual(b.getAndIncrement(), 1);
+    assert.strictEqual(b.get(), 2.5);
+    assert.strictEqual(b.getIndex(), 1);
+    assert.strictEqual(b.isFull(), false);
+
+    assert.strictEqual(b.getAndIncrement(), 2.5);
+    assert.strictEqual(b.getIndex(), 2);
+    assert.strictEqual(b.isFull(), true);
+
+    assert.strictEqual(b.getAndIncrement(), 3.5);
+    assert.strictEqual(b.getIndex(), 2);
+    assert.strictEqual(b.isFull(), true);
+
+    assert.strictEqual(b.getAndIncrement(), 3.5);
+    assert.strictEqual(b.getIndex(), 2);
+    assert.strictEqual(b.isFull(), true);
+  });
 });

ts/textsecure/SocketManager.ts

@@ -32,6 +32,8 @@ const TEN_SECONDS = 10 * durations.SECOND;
 
 const FIVE_MINUTES = 5 * durations.MINUTE;
 
+const JITTER = 5 * durations.SECOND;
+
 export type SocketManagerOptions = Readonly<{
   url: string;
   certificateAuthority: string;
@@ -54,7 +56,9 @@ export type SocketManagerOptions = Readonly<{
 // Incoming requests on unauthenticated resource are not currently supported.
 // WebSocketResource is responsible for their immediate termination.
 export class SocketManager extends EventListener {
-  private backOff = new BackOff(FIBONACCI_TIMEOUTS);
+  private backOff = new BackOff(FIBONACCI_TIMEOUTS, {
+    jitter: JITTER,
+  });
 
   private authenticated?: AbortableProcess<WebSocketResource>;
 
@@ -199,7 +203,7 @@ export class SocketManager extends EventListener {
           return;
         }
 
-        if (code !== 500 && code !== -1) {
+        if (!(code >= 500 && code <= 599) && code !== -1) {
           // No reconnect attempt should be made
           return;
         }

ts/util/BackOff.ts

@@ -15,13 +15,33 @@ export const FIBONACCI_TIMEOUTS: ReadonlyArray<number> = [
   55 * SECOND,
 ];
 
+export type BackOffOptionsType = Readonly<{
+  jitter?: number;
+
+  // Testing
+  random?: () => number;
+}>;
+
+const DEFAULT_RANDOM = () => Math.random();
+
 export class BackOff {
   private count = 0;
 
-  constructor(private readonly timeouts: ReadonlyArray<number>) {}
+  constructor(
+    private readonly timeouts: ReadonlyArray<number>,
+    private readonly options: BackOffOptionsType = {}
+  ) {}
 
   public get(): number {
-    return this.timeouts[this.count];
+    let result = this.timeouts[this.count];
+    const { jitter = 0, random = DEFAULT_RANDOM } = this.options;
+
+    // Do not apply jitter larger than the timeout value. It is supposed to be
+    // activated for longer timeouts.
+    if (jitter < result) {
+      result += random() * jitter;
+    }
+    return result;
   }
 
   public getAndIncrement(): number {