FIX: Limit new and existent staged users for email topics (#17970)

The maximum_staged_users_per_email site setting controls how many
staged users will be invited to the topic created from an incoming
email. Previously, it counted only the new staged users.

lib/email/receiver.rb

@@ -55,6 +55,7 @@ module Email
     def initialize(mail_string, opts = {})
       raise EmptyEmailError if mail_string.blank?
       @staged_users = []
+      @created_staged_users = []
       @raw_email = mail_string
 
       COMMON_ENCODINGS.each do |encoding|
@@ -92,7 +93,7 @@ module Email
           post
         rescue Exception => e
           @incoming_email.update_columns(error: e.class.name) if @incoming_email
-          delete_staged_users
+          delete_created_staged_users
           raise
         end
       end
@@ -698,11 +699,9 @@ module Email
         end
     end
 
-    def find_or_create_user(email, display_name, raise_on_failed_create: false)
-      user = nil
-
+    def find_or_create_user(email, display_name, raise_on_failed_create: false, user: nil)
       User.transaction do
-        user = User.find_by_email(email)
+        user ||= User.find_by_email(email)
 
         if user.nil? && SiteSetting.enable_staged_users
           raise EmailNotAllowed unless EmailValidator.allowed?(email)
@@ -715,12 +714,14 @@ module Email
               name: display_name.presence || User.suggest_name(email),
               staged: true
             )
-            @staged_users << user
+            @created_staged_users << user
           rescue PG::UniqueViolation, ActiveRecord::RecordNotUnique, ActiveRecord::RecordInvalid
             raise if raise_on_failed_create
             user = nil
           end
         end
+
+        @staged_users << user if user&.staged?
       end
 
       user
@@ -1358,6 +1359,8 @@ module Email
     end
 
     def add_other_addresses(post, sender, mail_object)
+      max_staged_users_post = nil
+
       %i(to cc bcc).each do |d|
         next if mail_object[d].blank?
 
@@ -1367,18 +1370,22 @@ module Email
             email = address_field.address.downcase
             display_name = address_field.display_name.try(:to_s)
             next unless email["@"]
+
             if should_invite?(email)
-              user = find_or_create_user(email, display_name)
+              user = User.find_by_email(email)
+
+              # cap number of staged users created per email
+              if (!user || user.staged) && @staged_users.count >= SiteSetting.maximum_staged_users_per_email
+                max_staged_users_post ||= post.topic.add_moderator_post(sender, I18n.t("emails.incoming.maximum_staged_user_per_email_reached"), import_mode: @opts[:import_mode])
+                next
+              end
+
+              user = find_or_create_user(email, display_name, user: user)
               if user && can_invite?(post.topic, user)
                 post.topic.topic_allowed_users.create!(user_id: user.id)
                 TopicUser.auto_notification_for_staging(user.id, post.topic_id, TopicUser.notification_reasons[:auto_watch])
                 post.topic.add_small_action(sender, "invited_user", user.username, import_mode: @opts[:import_mode])
               end
-              # cap number of staged users created per email
-              if @staged_users.count > SiteSetting.maximum_staged_users_per_email
-                post.topic.add_moderator_post(sender, I18n.t("emails.incoming.maximum_staged_user_per_email_reached"), import_mode: @opts[:import_mode])
-                return
-              end
             end
           rescue ActiveRecord::RecordInvalid, EmailNotAllowed
             # don't care if user already allowed or the user's email address is not allowed
@@ -1413,8 +1420,8 @@ module Email
       end
     end
 
-    def delete_staged_users
-      @staged_users.each do |user|
+    def delete_created_staged_users
+      @created_staged_users.each do |user|
         if @incoming_email.user&.id == user.id
           @incoming_email.update_columns(user_id: nil)
         end

spec/lib/email/receiver_spec.rb

@@ -879,7 +879,17 @@ RSpec.describe Email::Receiver do
 
     it "cap the number of staged users created per email" do
       SiteSetting.maximum_staged_users_per_email = 1
-      expect { process(:cc) }.to change(Topic, :count)
+      expect { process(:cc) }.to change(Topic, :count).by(1)
+        .and change(User, :count).by(1)
+      expect(Topic.last.ordered_posts[-1].post_type).to eq(Post.types[:moderator_action])
+    end
+
+    it "cap the number of staged users existing per email" do
+      Fabricate(:user, email: "discourse@bar.com", staged: true) # from
+      Fabricate(:user, email: "someone@else.com", staged: true) # to
+
+      SiteSetting.maximum_staged_users_per_email = 1
+      expect { process(:cc) }.to change(Topic, :count).and not_change(User, :count)
       expect(Topic.last.ordered_posts[-1].post_type).to eq(Post.types[:moderator_action])
     end