Fix original account being unfollowed on migration before the follow request could be sent (#21957)

app/services/follow_migration_service.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+class FollowMigrationService < FollowService
+  # Follow an account with the same settings as another account, and unfollow the old account once the request is sent
+  # @param [Account] source_account From which to follow
+  # @param [Account] target_account Account to follow
+  # @param [Account] old_target_account Account to unfollow once the follow request has been sent to the new one
+  # @option [Boolean] bypass_locked Whether to immediately follow the new account even if it is locked
+  def call(source_account, target_account, old_target_account, bypass_locked: false)
+    @old_target_account = old_target_account
+
+    follow    = source_account.active_relationships.find_by(target_account: old_target_account)
+    reblogs   = follow&.show_reblogs?
+    notify    = follow&.notify?
+
+    super(source_account, target_account, reblogs: reblogs, notify: notify, bypass_locked: bypass_locked, bypass_limit: true)
+  end
+
+  private
+
+  def request_follow!
+    follow_request = @source_account.request_follow!(@target_account, **follow_options.merge(rate_limit: @options[:with_rate_limit], bypass_limit: @options[:bypass_limit]))
+
+    if @target_account.local?
+      LocalNotificationWorker.perform_async(@target_account.id, follow_request.id, follow_request.class.name, 'follow_request')
+      UnfollowService.new.call(@source_account, @old_target_account, skip_unmerge: true)
+    elsif @target_account.activitypub?
+      ActivityPub::MigratedFollowDeliveryWorker.perform_async(build_json(follow_request), @source_account.id, @target_account.inbox_url, @old_target_account.id)
+    end
+
+    follow_request
+  end
+
+  def direct_follow!
+    follow = super
+    UnfollowService.new.call(@source_account, @old_target_account, skip_unmerge: true)
+    follow
+  end
+
+  def follow_options
+    @options.slice(:reblogs, :notify)
+  end
+end

app/workers/activitypub/migrated_follow_delivery_worker.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class ActivityPub::MigratedFollowDeliveryWorker < ActivityPub::DeliveryWorker
+  def perform(json, source_account_id, inbox_url, old_target_account_id, options = {})
+    super(json, source_account_id, inbox_url, options)
+    unfollow_old_account!(old_target_account_id)
+  end
+
+  private
+
+  def unfollow_old_account!(old_target_account_id)
+    old_target_account = Account.find(old_target_account_id)
+    UnfollowService.new.call(@source_account, old_target_account, skip_unmerge: true)
+  rescue StandardError
+    true
+  end
+end

app/workers/unfollow_follow_worker.rb

@@ -10,12 +10,7 @@ class UnfollowFollowWorker
     old_target_account = Account.find(old_target_account_id)
     new_target_account = Account.find(new_target_account_id)
 
-    follow  = follower_account.active_relationships.find_by(target_account: old_target_account)
-    reblogs = follow&.show_reblogs?
-    notify  = follow&.notify?
-
-    FollowService.new.call(follower_account, new_target_account, reblogs: reblogs, notify: notify, bypass_locked: bypass_locked, bypass_limit: true)
-    UnfollowService.new.call(follower_account, old_target_account, skip_unmerge: true)
+    FollowMigrationService.new.call(follower_account, new_target_account, old_target_account, bypass_locked: bypass_locked)
   rescue ActiveRecord::RecordNotFound, Mastodon::NotPermittedError
     true
   end