FEATURE: introduces chat_max_direct_message_users setting

plugins/chat/config/locales/server.en.yml

@@ -17,6 +17,7 @@ en:
     default_emoji_reactions: "Default emoji reactions for chat messages. Add up to 5 emojis for quick reaction."
     direct_message_enabled_groups: "Allow users within these groups to create user-to-user Personal Chats. Note: staff can always create Personal Chats, and users will be able to reply to Personal Chats initiated by users who have permission to create them."
     chat_message_flag_allowed_groups: "Users in these groups are allowed to flag chat messages."
+    chat_max_direct_message_users: "Users cannot add more than this number of other users when creating a new direct message. Set to 0 to only allow messages to oneself. Staff are exempt from this setting."
     errors:
       chat_default_channel: "The default chat channel must be a public channel."
       direct_message_enabled_groups_invalid: "You must specify at least one group for this setting. If you do not want anyone except staff to send direct messages, choose the staff group."
@@ -56,6 +57,9 @@ en:
       actor_disallowed_dms: "You have chosen to prevent users from sending you private and direct messages, so you cannot create new direct messages."
       actor_preventing_target_user_from_dm: "You have chosen to prevent %{username} from sending you private and direct messages, so you cannot create new direct messages to them."
       user_cannot_send_direct_messages: "Sorry, you cannot send direct messages."
+      over_chat_max_direct_message_users:
+        one: "You can only create a direct message with yourself."
+        other: "You can't create a direct message with more than %{count} other users."
     reviewables:
       message_already_handled: "Thanks, but we've already reviewed this message and determined it does not need to be flagged again."
       actions:

plugins/chat/config/settings.yml

@@ -20,6 +20,11 @@ chat:
     client: true
     max: 3652 # 10 years
     min: 0
+  chat_max_direct_message_users:
+    default: 20
+    min: 0
+    max: 100
+    client: true
   chat_dm_retention_days:
     default: 0
     client: true

plugins/chat/lib/direct_message_channel_creator.rb

@@ -11,6 +11,7 @@ def self.create!(acting_user:, target_users:)
     if direct_message
       chat_channel = ChatChannel.find_by!(chatable: direct_message)
     else
+      enforce_max_direct_message_users!(acting_user, target_users)
       ensure_actor_can_communicate!(acting_user, target_users)
       direct_message = DirectMessage.create!(user_ids: target_users.map(&:id))
       chat_channel = direct_message.create_chat_channel!
@@ -24,6 +25,20 @@ def self.create!(acting_user:, target_users:)
 
   private
 
+  def self.enforce_max_direct_message_users!(acting_user, target_users)
+    # We never want to prevent the actor from communicating with themself.
+    target_users = target_users.reject { |user| user.id == acting_user.id }
+
+    if !acting_user.staff? && target_users.size > SiteSetting.chat_max_direct_message_users
+      raise NotAllowed.new(
+              I18n.t(
+                "chat.errors.over_chat_max_direct_message_users",
+                count: SiteSetting.chat_max_direct_message_users + 1, # +1 for the acting_user
+              ),
+            )
+    end
+  end
+
   def self.update_memberships(acting_user, target_users, chat_channel_id)
     sql_params = {
       acting_user_id: acting_user.id,

plugins/chat/spec/lib/direct_message_channel_creator_spec.rb

@@ -178,6 +178,62 @@
       }.by(1).and change { UserChatChannelMembership.count }.by(1)
     end
 
+    context "when number of users is over the limit" do
+      before { SiteSetting.chat_max_direct_message_users = 1 }
+
+      it "raises an error" do
+        expect {
+          subject.create!(acting_user: user_1, target_users: [user_1, user_2, user_3])
+        }.to raise_error(
+          Chat::DirectMessageChannelCreator::NotAllowed,
+          I18n.t("chat.errors.over_chat_max_direct_message_users", count: 2),
+        )
+      end
+
+      context "when acting user is staff" do
+        fab!(:admin) { Fabricate(:admin) }
+
+        it "creates a new chat channel" do
+          expect {
+            subject.create!(acting_user: admin, target_users: [admin, user_1, user_2])
+          }.to change { ChatChannel.count }.by(1)
+        end
+      end
+
+      context "when limit is zero" do
+        before { SiteSetting.chat_max_direct_message_users = 0 }
+
+        it "raises an error" do
+          expect {
+            subject.create!(acting_user: user_1, target_users: [user_1, user_2])
+          }.to raise_error(
+            Chat::DirectMessageChannelCreator::NotAllowed,
+            I18n.t("chat.errors.over_chat_max_direct_message_users", count: 1),
+          )
+        end
+      end
+    end
+
+    context "when number of users is at the limit" do
+      before { SiteSetting.chat_max_direct_message_users = 0 }
+
+      it "creates a new chat channel" do
+        expect { subject.create!(acting_user: user_1, target_users: [user_1]) }.to change {
+          ChatChannel.count
+        }.by(1)
+      end
+    end
+
+    context "when number of users is under the limit" do
+      before { SiteSetting.chat_max_direct_message_users = 1 }
+
+      it "creates a new chat channel" do
+        expect { subject.create!(acting_user: user_1, target_users: [user_1]) }.to change {
+          ChatChannel.count
+        }.by(1)
+      end
+    end
+
     context "when the user is not a member of direct_message_enabled_groups" do
       before { SiteSetting.direct_message_enabled_groups = Group::AUTO_GROUPS[:trust_level_4] }