event/TimerWheel: add a "ready" list as a special case

This reduces delays of zero-duration timers from up to 1 second to
zero.  libavahi-client schedules zero-duration timers often.

src/event/Loop.cxx

@@ -145,7 +145,7 @@ EventLoop::AbandonFD(SocketEvent &event)  noexcept
 void
 EventLoop::Insert(CoarseTimerEvent &t) noexcept
 {
-	coarse_timers.Insert(t);
+	coarse_timers.Insert(t, SteadyNow());
 	again = true;
 }
 

src/event/TimerWheel.cxx

@@ -46,14 +46,18 @@ TimerWheel::TimerWheel() noexcept
 TimerWheel::~TimerWheel() noexcept = default;
 
 void
-TimerWheel::Insert(CoarseTimerEvent &t) noexcept
+TimerWheel::Insert(CoarseTimerEvent &t,
+		   Event::TimePoint now) noexcept
 {
-	/* if this timer's due time is already in the past, don't
+	assert(now >= last_time);
-	   insert it into an older bucket because Run() won't look at
-	   it in this iteration */
-	const auto due = std::max(t.GetDue(), last_time);
 
-	buckets[BucketIndexAt(due)].push_back(t);
+	auto &list = t.GetDue() > now
+		? buckets[BucketIndexAt(t.GetDue())]
+		/* if this timer is already due, insert it into the
+		   "ready" list to be invoked without delay */
+		: ready;
+
+	list.push_back(t);
 
 	empty = false;
 }
@@ -101,6 +105,10 @@ TimerWheel::GetNextDue(const std::size_t bucket_index,
 inline Event::Duration
 TimerWheel::GetSleep(Event::TimePoint now) const noexcept
 {
+	/* note: not checking the "ready" list here because this
+	   method gets called only from Run() after the "ready" list
+	   has been processed already */
+
 	if (empty)
 		return Event::Duration(-1);
 
@@ -117,6 +125,11 @@ TimerWheel::GetSleep(Event::TimePoint now) const noexcept
 Event::Duration
 TimerWheel::Run(const Event::TimePoint now) noexcept
 {
+	/* invoke the "ready" list unconditionally */
+	ready.clear_and_dispose([&](auto *t){
+		t->Run();
+	});
+
 	/* check all buckets between the last time we were invoked and
 	   now */
 	const std::size_t start_bucket = BucketIndexAt(last_time);

src/event/TimerWheel.hxx

@@ -64,6 +64,13 @@ class TimerWheel final {
 	 */
 	std::array<List, N_BUCKETS> buckets;
 
+	/**
+	 * A list of timers which are already ready.  This can happen
+	 * if they are scheduled with a zero duration or scheduled in
+	 * the past.
+	 */
+	List ready;
+
 	/**
 	 * The last time Run() was invoked.  This is needed to
 	 * determine the range of buckets to be checked, because we
@@ -87,13 +94,15 @@ public:
 	~TimerWheel() noexcept;
 
 	bool IsEmpty() const noexcept {
-		return std::all_of(buckets.begin(), buckets.end(),
+		return ready.empty() &&
-				   [](const auto &list){
+			std::all_of(buckets.begin(), buckets.end(),
-					   return list.empty();
+				    [](const auto &list){
-				   });
+					    return list.empty();
+				    });
 	}
 
-	void Insert(CoarseTimerEvent &t) noexcept;
+	void Insert(CoarseTimerEvent &t,
+		    Event::TimePoint now) noexcept;
 
 	/**
 	 * Invoke all expired #CoarseTimerEvent instances and return