[util] allow scheduler to be mocked

Add MockForward method to the scheduler that mimics going into the future by rescheduling all items on the taskQueue to be sooner.

src/scheduler.cpp

@@ -114,6 +114,28 @@ void CScheduler::scheduleFromNow(CScheduler::Function f, int64_t deltaMilliSecon
     schedule(f, boost::chrono::system_clock::now() + boost::chrono::milliseconds(deltaMilliSeconds));
 }
 
+void CScheduler::MockForward(boost::chrono::seconds delta_seconds)
+{
+    assert(delta_seconds.count() > 0 && delta_seconds < boost::chrono::hours{1});
+
+    {
+        boost::unique_lock<boost::mutex> lock(newTaskMutex);
+
+        // use temp_queue to maintain updated schedule
+        std::multimap<boost::chrono::system_clock::time_point, Function> temp_queue;
+
+        for (const auto& element : taskQueue) {
+            temp_queue.emplace_hint(temp_queue.cend(), element.first - delta_seconds, element.second);
+        }
+
+        // point taskQueue to temp_queue
+        taskQueue = std::move(temp_queue);
+    }
+
+    // notify that the taskQueue needs to be processed
+    newTaskScheduled.notify_one();
+}
+
 static void Repeat(CScheduler* s, CScheduler::Function f, int64_t deltaMilliSeconds)
 {
     f();

src/scheduler.h

@@ -55,6 +55,13 @@ public:
     // need more accurate scheduling, don't use this method.
     void scheduleEvery(Function f, int64_t deltaMilliSeconds);
 
+    /**
+     * Mock the scheduler to fast forward in time.
+     * Iterates through items on taskQueue and reschedules them
+     * to be delta_seconds sooner.
+     */
+    void MockForward(boost::chrono::seconds delta_seconds);
+
     // To keep things as simple as possible, there is no unschedule.
 
     // Services the queue 'forever'. Should be run in a thread,