Fix clearing webpush targets after any MARKREAD

Previously, we would clear webpush targets after any MARKREAD.

Consider the following scenario (ignore any typos, this is crafted by
hand):

    <<< @time=2020-01-01T00:00:00Z PRIVMSG #foo :hi mark!
    <<< @time=2020-01-02T00:00:00Z PRIVMSG #foo :hi again mark!
    >>> MARKREAD #foo timestamp=2020-01-01T00:00:00Z
    >>> MARKREAD #foo timestamp=2020-01-02T00:00:00Z

The push target was previously cleared on the first MARKREAD, which
means that the second MARKREAD was never broadcast to Firebase, and all
devices would keep the "hi again mark!" notification indefinitely.

This changes the webpush target map so that we store a timestamp of the
last highlight we sent. We only clear the push target when sending a
MARKREAD that is at or after the last message.

downstream.go

@@ -2818,7 +2818,9 @@ func (dc *downstreamConn) handleMessageRegistered(ctx context.Context, msg *irc.
 
 		if broadcast && network.pushTargets.Has(target) {
 			// TODO: only broadcast if draft/read-marker has been negotiated
-			network.pushTargets.Del(target)
+			if !r.Timestamp.Before(network.pushTargets.Get(target)) {
+				network.pushTargets.Del(target)
+			}
 			go network.broadcastWebPush(&irc.Message{
 				Command: "MARKREAD",
 				Params:  []string{target, timestampStr},

irc.go

@@ -459,10 +459,18 @@ func (cm *monitorCasemapMap) ForEach(f func(name string, online bool)) {
 	}
 }
 
-type casemapSet struct{ casemapMap }
+type pushTargetCasemapMap struct{ casemapMap }
 
-func (cs *casemapSet) Add(name string) {
+func (cm *pushTargetCasemapMap) Get(name string) (last time.Time) {
-	cs.set(name, nil)
+	if v := cm.get(name); v == nil {
+		return time.Time{}
+	} else {
+		return v.(time.Time)
+	}
+}
+
+func (cm *pushTargetCasemapMap) Set(name string, last time.Time) {
+	cm.set(name, last)
 }
 
 func isWordBoundary(r rune) bool {

upstream.go

@@ -556,7 +556,9 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 
 		if highlight || uc.isOurNick(target) {
 			go uc.network.broadcastWebPush(msg)
-			uc.network.pushTargets.Add(bufferName)
+			if timestamp, err := time.Parse(xirc.ServerTimeLayout, string(msg.Tags["time"])); err == nil {
+				uc.network.pushTargets.Set(bufferName, timestamp)
+			}
 		}
 
 		uc.produce(bufferName, msg, downstreamID)

user.go

@@ -141,7 +141,7 @@ type network struct {
 	conn        *upstreamConn
 	channels    channelCasemapMap
 	delivered   deliveredStore
-	pushTargets casemapSet
+	pushTargets pushTargetCasemapMap
 	lastError   error
 	casemap     casemapping
 }
@@ -162,7 +162,7 @@ func newNetwork(user *user, record *database.Network, channels []database.Channe
 		stopped:     make(chan struct{}),
 		channels:    m,
 		delivered:   newDeliveredStore(),
-		pushTargets: casemapSet{newCasemapMap()},
+		pushTargets: pushTargetCasemapMap{newCasemapMap()},
 		casemap:     casemapRFC1459,
 	}
 }