Use race-condition free following method.

lib/pleroma/user.ex

@@ -343,18 +343,17 @@ defmodule Pleroma.User do
           Websub.subscribe(follower, followed)
         end
 
-        following =
+        q =
-          [ap_followers | follower.following]
+          from(u in User,
-          |> Enum.uniq()
+            where: u.id == ^follower.id,
+            update: [push: [following: ^ap_followers]]
+          )
 
-        follower =
+        {1, [follower]} = Repo.update_all(q, [], returning: true)
-          follower
-          |> follow_changeset(%{following: following})
-          |> update_and_set_cache
 
         {:ok, _} = update_follower_count(followed)
 
-        follower
+        set_cache(follower)
     end
   end
 
@@ -362,17 +361,18 @@ defmodule Pleroma.User do
     ap_followers = followed.follower_address
 
     if following?(follower, followed) and follower.ap_id != followed.ap_id do
-      following =
+      q =
-        follower.following
+        from(u in User,
-        |> List.delete(ap_followers)
+          where: u.id == ^follower.id,
+          update: [pull: [following: ^ap_followers]]
+        )
 
-      {:ok, follower} =
+      {1, [follower]} = Repo.update_all(q, [], returning: true)
-        follower
-        |> follow_changeset(%{following: following})
-        |> update_and_set_cache
 
       {:ok, followed} = update_follower_count(followed)
 
+      set_cache(follower)
+
       {:ok, follower, Utils.fetch_latest_follow(follower, followed)}
     else
       {:error, "Not subscribed!"}
@@ -423,12 +423,16 @@ defmodule Pleroma.User do
     get_by_nickname(nickname)
   end
 
-  def update_and_set_cache(changeset) do
+  def set_cache(user) do
-    with {:ok, user} <- Repo.update(changeset) do
     Cachex.put(:user_cache, "ap_id:#{user.ap_id}", user)
     Cachex.put(:user_cache, "nickname:#{user.nickname}", user)
     Cachex.put(:user_cache, "user_info:#{user.id}", user_info(user))
     {:ok, user}
+  end
+
+  def update_and_set_cache(changeset) do
+    with {:ok, user} <- Repo.update(changeset) do
+      set_cache(user)
     else
       e -> e
     end