Limit the number of people that can be followed from one account

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 @@ def relationship_type
   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

config/locales/en.yml

@@ -917,6 +917,7 @@ en:
       tips: Tips
       title: Welcome aboard, %{name}!
   users:
+    follow_limit_reached: You cannot follow more than %{limit} people
     invalid_email: The e-mail address is invalid
     invalid_otp_token: Invalid two-factor code
     otp_lost_help_html: If you lost access to both, you may get in touch with %{email}

spec/models/follow_spec.rb

@@ -23,6 +23,20 @@
       follow.valid?
       expect(follow).to model_have_error_on_field(:target_account)
     end
+
+    it 'is invalid if account already follows too many people' do
+      alice.update(following_count: FollowLimitValidator::LIMIT)
+
+      expect(subject).to_not be_valid
+      expect(subject).to model_have_error_on_field(:base)
+    end
+
+    it 'is valid if account is only on the brink of following too many people' do
+      alice.update(following_count: FollowLimitValidator::LIMIT - 1)
+
+      expect(subject).to be_valid
+      expect(subject).to_not model_have_error_on_field(:base)
+    end
   end
 
   describe 'recent' do