Implement DecryptionFailureTracker for less agressive tracking

Instead of pinging Analytics once per failed decryption, add the failure
to a list of failures and after a grace period, add it to a FIFO for
tracking. On an interval, track a single failure from the FIFO.

test/DecryptionFailureTracker-test.js

@@ -0,0 +1,128 @@
+/*
+Copyright 2018 New Vector Ltd
+
+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.
+*/
+
+import expect from 'expect';
+
+import DecryptionFailureTracker from '../src/DecryptionFailureTracker';
+
+import { MatrixEvent } from 'matrix-js-sdk';
+
+function createFailedDecryptionEvent() {
+    const event = new MatrixEvent({
+        event_id: "event-id-" + Math.random().toString(16).slice(2),
+    });
+    event._setClearData(
+        event._badEncryptedMessage(":("),
+    );
+    return event;
+}
+
+describe.only('DecryptionFailureTracker', function() {
+    it('tracks a failed decryption', function(done) {
+        const failedDecryptionEvent = createFailedDecryptionEvent();
+        let trackedFailure = null;
+        const tracker = new DecryptionFailureTracker((failure) => {
+            trackedFailure = failure;
+        });
+
+        tracker.eventDecrypted(failedDecryptionEvent);
+
+        // Pretend "now" is Infinity
+        tracker.checkFailures(Infinity);
+
+        // Immediately track the newest failure, if there is one
+        tracker.trackFailure();
+
+        expect(trackedFailure).toNotBe(null, 'should track a failure for an event that failed decryption');
+
+        done();
+    });
+
+    it('does not track a failed decryption where the event is subsequently successfully decrypted', (done) => {
+        const decryptedEvent = createFailedDecryptionEvent();
+        const tracker = new DecryptionFailureTracker((failure) => {
+            expect(true).toBe(false, 'should not track an event that has since been decrypted correctly');
+        });
+
+        tracker.eventDecrypted(decryptedEvent);
+
+        // Indicate successful decryption: clear data can be anything where the msgtype is not m.bad.encrypted
+        decryptedEvent._setClearData({});
+        tracker.eventDecrypted(decryptedEvent);
+
+        // Pretend "now" is Infinity
+        tracker.checkFailures(Infinity);
+
+        // Immediately track the newest failure, if there is one
+        tracker.trackFailure();
+        done();
+    });
+
+    it('only tracks a single failure per event, despite multiple failed decryptions for multiple events', (done) => {
+        const decryptedEvent = createFailedDecryptionEvent();
+        const decryptedEvent2 = createFailedDecryptionEvent();
+
+        let count = 0;
+        const tracker = new DecryptionFailureTracker((failure) => count++);
+
+        // Arbitrary number of failed decryptions for both events
+        tracker.eventDecrypted(decryptedEvent);
+        tracker.eventDecrypted(decryptedEvent);
+        tracker.eventDecrypted(decryptedEvent);
+        tracker.eventDecrypted(decryptedEvent);
+        tracker.eventDecrypted(decryptedEvent);
+        tracker.eventDecrypted(decryptedEvent2);
+        tracker.eventDecrypted(decryptedEvent2);
+        tracker.eventDecrypted(decryptedEvent2);
+
+        // Pretend "now" is Infinity
+        tracker.checkFailures(Infinity);
+
+        // Simulated polling of `trackFailure`, an arbitrary number ( > 2 ) times
+        tracker.trackFailure();
+        tracker.trackFailure();
+        tracker.trackFailure();
+        tracker.trackFailure();
+
+        expect(count).toBe(2, count + ' failures tracked, should only track a single failure per event');
+
+        done();
+    });
+
+    it('track failures in the order they occured', (done) => {
+        const decryptedEvent = createFailedDecryptionEvent();
+        const decryptedEvent2 = createFailedDecryptionEvent();
+
+        const failures = [];
+        const tracker = new DecryptionFailureTracker((failure) => failures.push(failure));
+
+        // Indicate decryption
+        tracker.eventDecrypted(decryptedEvent);
+        tracker.eventDecrypted(decryptedEvent2);
+
+        // Pretend "now" is Infinity
+        tracker.checkFailures(Infinity);
+
+        // Simulated polling of `trackFailure`, an arbitrary number ( > 2 ) times
+        tracker.trackFailure();
+        tracker.trackFailure();
+
+        expect(failures[0].failedEventId).toBe(decryptedEvent.getId(), 'the first failure should be tracked first');
+        expect(failures[1].failedEventId).toBe(decryptedEvent2.getId(), 'the second failure should be tracked second');
+
+        done();
+    });
+});