1 files changed, 63 insertions, 0 deletions
diff --git a/db/migrate/20170920032311_fix_reblogs_in_feeds.rb b/db/migrate/20170920032311_fix_reblogs_in_feeds.rb
new file mode 100644
index 000000000..c813ecd46
--- /dev/null
+++ b/db/migrate/20170920032311_fix_reblogs_in_feeds.rb
@@ -0,0 +1,63 @@
+class FixReblogsInFeeds < ActiveRecord::Migration[5.1]
+  def up
+    redis = Redis.current
+    fm = FeedManager.instance
+
+    # find_each is batched on the database side.
+    User.includes(:account).find_each do |user|
+      account = user.account
+
+      # Old scheme:
+      # Each user's feed zset had a series of score:value entries,
+      # where "regular" statuses had the same score and value (their
+      # ID). Reblogs had a score of the reblogging status' ID, and a
+      # value of the reblogged status' ID.
+
+      # New scheme:
+      # The feed contains only entries with the same score and value.
+      # Reblogs result in the reblogging status being added to the
+      # feed, with an entry in a reblog tracking zset (where the score
+      # is once again set to the reblogging status' ID, and the value
+      # is set to the reblogged status' ID). This is safe for Redis'
+      # float coersion because in this reblog tracking zset, we only
+      # need the rebloggging status' ID to be able to stop tracking
+      # entries after they have gotten too far down the feed, which
+      # does not require an exact value.
+
+      # So, first, we iterate over the user's feed to find any reblogs.
+      timeline_key = fm.key(:home, account.id)
+      reblog_key = fm.key(:home, account.id, 'reblogs')
+      redis.zrange(timeline_key, 0, -1, with_scores: true).each do |entry|
+        next if entry[0] == entry[1]
+
+        # The score and value don't match, so this is a reblog.
+        # (note that we're transitioning from IDs < 53 bits so we
+        # don't have to worry about the loss of precision)
+
+        reblogged_id, reblogging_id = entry
+
+        # Remove the old entry
+        redis.zrem(timeline_key, reblogged_id)
+
+        # Add a new one for the reblogging status
+        redis.zadd(timeline_key, reblogging_id, reblogging_id)
+
+        # Track the fact that this was a reblog
+        redis.zadd(reblog_key, reblogging_id, reblogged_id)
+      end
+    end
+  end
+
+  def down
+    # We *deliberately* do nothing here. This means that reverting
+    # this and the associated changes to the FeedManager code could
+    # allow one superfluous reblog of any given status, but in the case
+    # where things have gone wrong and a revert is necessary, this
+    # appears preferable to requiring a database hit for every status
+    # in every users' feed simply to revert.
+
+    # Note that this is operating under the assumption that entries
+    # with >53-bit IDs have already been entered. Otherwise, we could
+    # just use the data in Redis to reverse this transition.
+  end
+end