Clean up reblog tracking keys, related improvements (#5428)

* Clean up reblog-tracking sets from FeedManager

Builds on #5419, with a few minor optimizations and cleanup of sets
after they are no longer needed.

* Update tests, fix multiply-reblogged case

Previously, we would have lost the fact that a given status was
reblogged if the displayed reblog of it was removed, now we don't.

Also added tests to make sure FeedManager#trim cleans up our reblog
tracking keys, fixed up FeedCleanupScheduler to use the right loop,
and fixed the test for it.

2 files changed, 54 insertions, 23 deletions

diff --git a/app/lib/feed_manager.rb b/app/lib/feed_manager.rb
index 7f95d7784..58650efb6 100644
--- a/app/lib/feed_manager.rb
+++ b/app/lib/feed_manager.rb
@@ -56,7 +56,17 @@ class FeedManager
     falloff_rank = FeedManager::REBLOG_FALLOFF - 1
     falloff_range = redis.zrevrange(timeline_key, falloff_rank, falloff_rank, with_scores: true)
     falloff_score = falloff_range&.first&.last&.to_i || 0
-    redis.zremrangebyscore(reblog_key, 0, falloff_score)
+
+    # Get any reblogs we might have to clean up after.
+    redis.zrangebyscore(reblog_key, 0, falloff_score).each do |reblogged_id|
+      # Remove it from the set of reblogs we're tracking *first* to avoid races.
+      redis.zrem(reblog_key, reblogged_id)
+      # Just drop any set we might have created to track additional reblogs.
+      # This means that if this reblog is deleted, we won't automatically insert
+      # another reblog, but also that any new reblog can be inserted into the
+      # feed.
+      redis.del(key(type, account_id, "reblogs:#{reblogged_id}"))
+    end
   end
 
   def push_update_required?(timeline_type, account_id)
@@ -177,23 +187,28 @@ class FeedManager
     reblog_key   = key(timeline_type, account.id, 'reblogs')
 
     if status.reblog?
-      reblog_set_key = key(timeline_type, account.id, "reblogs:#{status.reblog_of_id}")
-
       # If the original status or a reblog of it is within
       # REBLOG_FALLOFF statuses from the top, do not re-insert it into
       # the feed
       rank = redis.zrevrank(timeline_key, status.reblog_of_id)
 
-      redis.sadd(reblog_set_key, status.reblog_of_id) unless rank.nil?
-      redis.sadd(reblog_set_key, status.id)
-
       return false if !rank.nil? && rank < FeedManager::REBLOG_FALLOFF
 
       reblog_rank = redis.zrevrank(reblog_key, status.reblog_of_id)
-      return false unless reblog_rank.nil?
-
-      redis.zadd(timeline_key, status.id, status.id)
-      redis.zadd(reblog_key, status.id, status.reblog_of_id)
+      if reblog_rank.nil?
+        # This is not something we've already seen reblogged, so we
+        # can just add it to the feed (and note that we're
+        # reblogging it).
+        redis.zadd(timeline_key, status.id, status.id)
+        redis.zadd(reblog_key, status.id, status.reblog_of_id)
+      else
+        # Another reblog of the same status was already in the
+        # REBLOG_FALLOFF most recent statuses, so we note that this
+        # is an "extra" reblog, by storing it in reblog_set_key.
+        reblog_set_key = key(timeline_type, account.id, "reblogs:#{status.reblog_of_id}")
+        redis.sadd(reblog_set_key, status.id)
+        return false
+      end
     else
       redis.zadd(timeline_key, status.id, status.id)
     end
@@ -207,23 +222,21 @@ class FeedManager
   # do so if appropriate.
   def remove_from_feed(timeline_type, account, status)
     timeline_key = key(timeline_type, account.id)
-    reblog_key   = key(timeline_type, account.id, 'reblogs')
 
     if status.reblog?
       # 1. If the reblogging status is not in the feed, stop.
       status_rank = redis.zrevrank(timeline_key, status.id)
       return false if status_rank.nil?
 
-      # 2. Remove the reblogged status from the `:reblogs` zset.
-      redis.zrem(reblog_key, status.reblog_of_id)
-
-      # 3. Remove reblog from set of this status's reblogs, and
-      # re-insert another reblog or original into the feed if
-      # one remains in the set
+      # 2. Remove reblog from set of this status's reblogs.
       reblog_set_key = key(timeline_type, account.id, "reblogs:#{status.reblog_of_id}")
 
       redis.srem(reblog_set_key, status.id)
-      other_reblog = redis.srandmember(reblog_set_key)
+      # 3. Re-insert another reblog or original into the feed if one
+      # remains in the set. We could pick a random element, but this
+      # set should generally be small, and it seems ideal to show the
+      # oldest potential such reblog.
+      other_reblog = redis.smembers(reblog_set_key).map(&:to_i).sort.first
 
       redis.zadd(timeline_key, other_reblog, other_reblog) if other_reblog
 
diff --git a/app/workers/scheduler/feed_cleanup_scheduler.rb b/app/workers/scheduler/feed_cleanup_scheduler.rb
index 222f5ed84..cfa2d31a4 100644
--- a/app/workers/scheduler/feed_cleanup_scheduler.rb
+++ b/app/workers/scheduler/feed_cleanup_scheduler.rb
@@ -5,18 +5,36 @@ class Scheduler::FeedCleanupScheduler
   include Sidekiq::Worker
 
   def perform
+    reblogged_id_sets = {}
+    feedmanager = FeedManager.instance
+
+    redis.pipelined do
+      inactive_user_ids.each do |account_id|
+        redis.del(feedmanager.key(:home, account_id))
+        reblog_key = feedmanager.key(:home, account_id, 'reblogs')
+        # We collect a future for this: we don't block while getting
+        # it, but we can iterate over it later.
+        reblogged_id_sets[account_id] = redis.zrange(reblog_key, 0, -1)
+        redis.del(reblog_key)
+      end
+    end
+
+    # Remove all of the reblog tracking keys we just removed the
+    # references to.
     redis.pipelined do
-      inactive_users.each do |account_id|
-        redis.del(FeedManager.instance.key(:home, account_id))
-        redis.del(FeedManager.instance.key(:home, account_id, 'reblogs'))
+      reblogged_id_sets.each do |account_id, future|
+        future.value.each do |reblogged_id|
+          reblog_set_key = feedmanager.key(:home, account_id, "reblogs:#{reblogged_id}")
+          redis.del(reblog_set_key)
+        end
       end
     end
   end
 
   private
 
-  def inactive_users
-    @inactive_users ||= User.confirmed.inactive.pluck(:account_id)
+  def inactive_user_ids
+    @inactive_user_ids ||= User.confirmed.inactive.pluck(:account_id)
   end
 
   def redis