FIX: staff should not be able to PM groups that "Nobody" can message (#16163)

If a group's messageable_level is set to nobody then staff can't should not be able to send PMs to it.

Co-authored-by: Martin Brennan <martin@discourse.org>

lib/guardian.rb

@@ -47,6 +47,9 @@ class Guardian
     def silenced?
       false
     end
+    def is_system_user?
+      false
+    end
     def secure_category_ids
       []
     end
@@ -422,6 +425,7 @@ class Guardian
   def can_send_private_message?(target, notify_moderators: false)
     is_user = target.is_a?(User)
     is_group = target.is_a?(Group)
+    from_system = @user.is_system_user?
 
     (is_group || is_user) &&
     # User is authenticated
@@ -435,7 +439,7 @@ class Guardian
     # Can't send PMs to suspended users
     (is_staff? || is_group || !target.suspended?) &&
     # Check group messageable level
-    (is_staff? || is_user || Group.messageable(@user).where(id: target.id).exists? || notify_moderators) &&
+    (from_system || is_user || Group.messageable(@user).where(id: target.id).exists? || notify_moderators) &&
     # Silenced users can only send PM to staff
     (!is_silenced? || target.staff?)
   end

spec/lib/guardian_spec.rb

@@ -336,14 +336,13 @@ describe Guardian do
     it "respects the group's messageable_level" do
       Group::ALIAS_LEVELS.each do |level, _|
         group.update!(messageable_level: Group::ALIAS_LEVELS[level])
-        output = level == :everyone ? true : false
+        user_output = level == :everyone ? true : false
+        admin_output = level != :nobody
+        mod_output = [:nobody, :only_admins].exclude?(level)
 
-        expect(Guardian.new(user).can_send_private_message?(group)).to eq(output)
-      end
-
-      Group::ALIAS_LEVELS.each do |level, _|
-        group.update!(messageable_level: Group::ALIAS_LEVELS[level])
-        expect(Guardian.new(admin).can_send_private_message?(group)).to eq(true)
+        expect(Guardian.new(user).can_send_private_message?(group)).to eq(user_output)
+        expect(Guardian.new(admin).can_send_private_message?(group)).to eq(admin_output)
+        expect(Guardian.new(moderator).can_send_private_message?(group)).to eq(mod_output)
       end
     end
 

spec/lib/topic_query_spec.rb

@@ -1079,7 +1079,7 @@ describe TopicQuery do
     end
 
     let(:group_with_user) do
-      group = Fabricate(:group)
+      group = Fabricate(:group, messageable_level: Group::ALIAS_LEVELS[:everyone])
       group.add(user)
       group.save
       group