DEV: Move discourse-chat to the core repo. (#18776)

As part of this move, we are also renaming `discourse-chat` to `chat`.

plugins/chat/lib/direct_message_channel_creator.rb

@@ -0,0 +1,111 @@
+# frozen_string_literal: true
+
+module Chat::DirectMessageChannelCreator
+  class NotAllowed < StandardError
+  end
+
+  def self.create!(acting_user:, target_users:)
+    Guardian.new(acting_user).ensure_can_create_direct_message!
+    target_users.uniq!
+    direct_messages_channel = DirectMessageChannel.for_user_ids(target_users.map(&:id))
+    if direct_messages_channel
+      chat_channel = ChatChannel.find_by!(chatable: direct_messages_channel)
+    else
+      ensure_actor_can_communicate!(acting_user, target_users)
+      direct_messages_channel = DirectMessageChannel.create!(user_ids: target_users.map(&:id))
+      chat_channel = direct_messages_channel.create_chat_channel!
+    end
+
+    update_memberships(acting_user, target_users, chat_channel.id)
+    ChatPublisher.publish_new_channel(chat_channel, target_users)
+
+    chat_channel
+  end
+
+  private
+
+  def self.update_memberships(acting_user, target_users, chat_channel_id)
+    sql_params = {
+      acting_user_id: acting_user.id,
+      user_ids: target_users.map(&:id),
+      chat_channel_id: chat_channel_id,
+      always_notification_level: UserChatChannelMembership::NOTIFICATION_LEVELS[:always],
+    }
+
+    DB.exec(<<~SQL, sql_params)
+      INSERT INTO user_chat_channel_memberships(
+        user_id,
+        chat_channel_id,
+        muted,
+        following,
+        desktop_notification_level,
+        mobile_notification_level,
+        created_at,
+        updated_at
+      )
+      VALUES(
+        unnest(array[:user_ids]),
+        :chat_channel_id,
+        false,
+        false,
+        :always_notification_level,
+        :always_notification_level,
+        NOW(),
+        NOW()
+      )
+      ON CONFLICT (user_id, chat_channel_id) DO NOTHING;
+
+      UPDATE user_chat_channel_memberships
+      SET following = true
+      WHERE user_id = :acting_user_id AND chat_channel_id = :chat_channel_id;
+    SQL
+  end
+
+  def self.ensure_actor_can_communicate!(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 }
+
+    screener =
+      UserCommScreener.new(acting_user: acting_user, target_user_ids: target_users.map(&:id))
+
+    # People blocking the actor.
+    screener.preventing_actor_communication.each do |user_id|
+      raise NotAllowed.new(
+              I18n.t(
+                "chat.errors.not_accepting_dms",
+                username: target_users.find { |user| user.id == user_id }.username,
+              ),
+            )
+    end
+
+    # The actor cannot start DMs with people if they are not allowing anyone
+    # to start DMs with them, that's no fair!
+    if screener.actor_disallowing_all_pms?
+      raise NotAllowed.new(I18n.t("chat.errors.actor_disallowed_dms"))
+    end
+
+    # People the actor is blocking.
+    target_users.each do |target_user|
+      if screener.actor_disallowing_pms?(target_user.id)
+        raise NotAllowed.new(
+                I18n.t(
+                  "chat.errors.actor_preventing_target_user_from_dm",
+                  username: target_user.username,
+                ),
+              )
+      end
+
+      if screener.actor_ignoring?(target_user.id)
+        raise NotAllowed.new(
+                I18n.t("chat.errors.actor_ignoring_target_user", username: target_user.username),
+              )
+      end
+
+      if screener.actor_muting?(target_user.id)
+        raise NotAllowed.new(
+                I18n.t("chat.errors.actor_muting_target_user", username: target_user.username),
+              )
+      end
+    end
+  end
+end