Maintain a list of GUIDs per feed and use them to de-dupe

Some RSS feeds will edit the published time of an item AFTER putting it out,
which resulted in RSS Bot sending the same article twice. We now remember the
"GUID" field for each item and de-dupe based on that. Normal timestamp
algorithm still applies.

src/github.com/matrix-org/go-neb/services/rssbot/rssbot.go

@@ -29,6 +29,7 @@ type rssBotService struct {
 		Rooms                    []string `json:"rooms"`
 		NextPollTimestampSecs    int64    // Internal: When we should poll again
 		FeedUpdatedTimestampSecs int64    // Internal: The last time the feed was updated
+		RecentGUIDs              []string // Internal: The most recently seen GUIDs. Sized to the number of items in the feed.
 	} `json:"feeds"`
 }
 
@@ -175,14 +176,7 @@ func (s *rssBotService) queryFeed(feedURL string) (*gofeed.Feed, []gofeed.Item,
 	// If the TS is 0 then this is the first ever poll, so let's not send 10s of events
 	// into the room and just do new ones from this point onwards.
 	if s.Feeds[feedURL].FeedUpdatedTimestampSecs != 0 {
-		for _, i := range feed.Items {
-			if i == nil || i.PublishedParsed == nil {
-				continue
-			}
-			if i.PublishedParsed.Unix() > s.Feeds[feedURL].FeedUpdatedTimestampSecs {
-				items = append(items, *i)
-			}
-		}
+		items = s.newItems(feedURL, feed.Items)
 	}
 
 	now := time.Now().Unix() // Second resolution
@@ -206,11 +200,42 @@ func (s *rssBotService) queryFeed(feedURL string) (*gofeed.Feed, []gofeed.Item,
 	// TODO: Handle the 'sy' Syndication extension to control update interval.
 	// See http://www.feedforall.com/syndication.htm and http://web.resource.org/rss/1.0/modules/syndication/
 
-	s.updateFeedInfo(feedURL, nextPollTsSec, feedLastUpdatedTs)
+	s.updateFeedInfo(feedURL, feed.Items, nextPollTsSec, feedLastUpdatedTs)
 	return feed, items, nil
 }
 
-func (s *rssBotService) updateFeedInfo(feedURL string, nextPollTs, feedUpdatedTs int64) {
+func (s *rssBotService) newItems(feedURL string, allItems []*gofeed.Item) (items []gofeed.Item) {
+	for _, i := range allItems {
+		if i == nil || i.PublishedParsed == nil {
+			continue
+		}
+
+		if i.PublishedParsed.Unix() > s.Feeds[feedURL].FeedUpdatedTimestampSecs {
+			// if we've seen this guid before, we've sent it before (even if the timestamp is newer)
+			seenBefore := false
+			for _, guid := range s.Feeds[feedURL].RecentGUIDs {
+				if guid == i.GUID {
+					seenBefore = true
+					break
+				}
+			}
+			if seenBefore {
+				continue
+			}
+
+			items = append(items, *i)
+		}
+	}
+	return
+}
+
+func (s *rssBotService) updateFeedInfo(feedURL string, allFeedItems []*gofeed.Item, nextPollTs, feedUpdatedTs int64) {
+	// map items to guid strings
+	var guids []string
+	for _, i := range allFeedItems {
+		guids = append(guids, i.GUID)
+	}
+
 	for u := range s.Feeds {
 		if u != feedURL {
 			continue
@@ -218,6 +243,7 @@ func (s *rssBotService) updateFeedInfo(feedURL string, nextPollTs, feedUpdatedTs
 		f := s.Feeds[u]
 		f.NextPollTimestampSecs = nextPollTs
 		f.FeedUpdatedTimestampSecs = feedUpdatedTs
+		f.RecentGUIDs = guids
 		s.Feeds[u] = f
 	}
 }