Limit the number of people that can be followed from one account (#8807)

Configurable soft limit of 7,500, and above that, configurable
ratio of 1.1 * followers, controlled by:

- MAX_FOLLOWS_THRESHOLD
- MAX_FOLLOWS_RATIO

Fix #2311

app/models/follow.rb

@@ -25,6 +25,7 @@ class Follow < ApplicationRecord
   has_one :notification, as: :activity, dependent: :destroy
 
   validates :account_id, uniqueness: { scope: :target_account_id }
+  validates_with FollowLimitValidator, on: :create
 
   scope :recent, -> { reorder(id: :desc) }
 

app/models/follow_request.rb

@@ -22,6 +22,7 @@ class FollowRequest < ApplicationRecord
   has_one :notification, as: :activity, dependent: :destroy
 
   validates :account_id, uniqueness: { scope: :target_account_id }
+  validates_with FollowLimitValidator, on: :create
 
   def authorize!
     account.follow!(target_account, reblogs: show_reblogs, uri: uri)

app/validators/follow_limit_validator.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class FollowLimitValidator < ActiveModel::Validator
+  LIMIT = ENV.fetch('MAX_FOLLOWS_THRESHOLD', 7_500).to_i
+  RATIO = ENV.fetch('MAX_FOLLOWS_RATIO', 1.1).to_f
+
+  def validate(follow)
+    return if follow.account.nil? || !follow.account.local?
+    follow.errors.add(:base, I18n.t('users.follow_limit_reached', limit: self.class.limit_for_account(follow.account))) if limit_reached?(follow.account)
+  end
+
+  class << self
+    def limit_for_account(account)
+      if account.following_count < LIMIT
+        LIMIT
+      else
+        account.followers_count * RATIO
+      end
+    end
+  end
+
+  private
+
+  def limit_reached?(account)
+    account.following_count >= self.class.limit_for_account(account)
+  end
+end

app/workers/import_worker.rb

@@ -37,6 +37,8 @@ class ImportWorker
   end
 
   def import_rows
-    CSV.new(import_contents).reject(&:blank?)
+    rows = CSV.new(import_contents).reject(&:blank?)
+    rows = rows.take(FollowLimitValidator.limit_for_account(@import.account)) if @import.type == 'following'
+    rows
   end
 end