FIX: TL0 could not message group with everyone messageable_level (#19525)

The commits e62e93f83a77adfa80b38fbfecf82bbee14e12fe and
d6bd4ad7ee1d3bf43ba0a2bb1948f8f306b6177a caused a regression
to the behaviour added for
https://meta.discourse.org/t/allow-tl0-to-write-messages-to-staff-group-not-to-other-members-or-non-staff/124335,
which allowed a user to message a group with the messageable_level
set to Everyone even if they were TL0 (or otherwise did not reach
the appropriate trust level). This commit fixes the issue and adjusts
the spec to reflect the real scenario.

c.f. https://meta.discourse.org/t/tl0-cant-message-groups-with-messageable-level-everyone-recession/249205

lib/guardian.rb

@@ -472,15 +472,20 @@ class Guardian
     from_system = @user.is_system_user?
 
     # Must be a valid target
-    (target_is_group || target_is_user) &&
+    return false if !(target_is_group || target_is_user)
+
+    # Users can send messages to certain groups with the `everyone` messageable_level
+    # even if they are not in personal_message_enabled_groups
+    group_is_messageable = target_is_group && Group.messageable(@user).where(id: target.id).exists?
+
     # User is authenticated and can send PMs, this can be covered by trust levels as well via AUTO_GROUPS
-    can_send_private_messages?(notify_moderators: notify_moderators) &&
+    (can_send_private_messages?(notify_moderators: notify_moderators) || group_is_messageable) &&
     # User disabled private message
     (is_staff? || target_is_group || target.user_option.allow_private_messages) &&
     # Can't send PMs to suspended users
     (is_staff? || target_is_group || !target.suspended?) &&
     # Check group messageable level
-    (from_system || target_is_user || Group.messageable(@user).where(id: target.id).exists? || notify_moderators) &&
+    (from_system || target_is_user || group_is_messageable || notify_moderators) &&
     # Silenced users can only send PM to staff
     (!is_silenced? || target.staff?)
   end

spec/lib/guardian_spec.rb

@@ -364,9 +364,9 @@ RSpec.describe Guardian do
       end
     end
 
-    it "allows TL0 to message group with messageable_level = everyone" do
+    it "allows TL0 to message group with messageable_level = everyone regardless of personal_message_enabled_groups" do
       group.update!(messageable_level: Group::ALIAS_LEVELS[:everyone])
-      SiteSetting.personal_message_enabled_groups = Group::AUTO_GROUPS[:trust_level_0]
+      SiteSetting.personal_message_enabled_groups = Group::AUTO_GROUPS[:trust_level_1]
       expect(Guardian.new(trust_level_0).can_send_private_message?(group)).to eq(true)
       expect(Guardian.new(user).can_send_private_message?(group)).to eq(true)
     end

spec/requests/groups_controller_spec.rb

@@ -639,6 +639,16 @@ RSpec.describe GroupsController do
       get "/groups/#{group.name}/messageable.json"
       expect(response.status).to eq(200)
 
+      body = response.parsed_body
+      expect(body["messageable"]).to eq(true)
+
+      group.update!(
+        messageable_level: Group::ALIAS_LEVELS[:only_admins],
+      )
+
+      get "/groups/#{group.name}/messageable.json"
+      expect(response.status).to eq(200)
+
       body = response.parsed_body
       expect(body["messageable"]).to eq(false)
     end