FIX: min_personal_message_post_length not applying to first post (#23531)

* FIX: min_personal_message_post_length not applying to first post

Due to the way PostCreator is wired, we were not applying min_personal_message_post_length
to the first post.

This meant that admins could not configure it so PMs have different
limits.

The code was already pretending that this works, but had no reliable way
of figuring out if we were dealing with a private message

config/locales/server.en.yml

@@ -1510,9 +1510,9 @@ en:
     allow_user_locale: "Allow users to choose their own language interface preference"
     set_locale_from_accept_language_header: "set interface language for anonymous users from their web browser's language headers"
     support_mixed_text_direction: "Support mixed left-to-right and right-to-left text directions."
-    min_post_length: "Minimum allowed post length in characters"
-    min_first_post_length: "Minimum allowed first post (topic body) length in characters"
-    min_personal_message_post_length: "Minimum allowed post length in characters for messages"
+    min_post_length: "Minimum allowed post length in characters (excluding personal messages)"
+    min_first_post_length: "Minimum allowed first post (topic body) length (excluding personal messages)"
+    min_personal_message_post_length: "Minimum allowed post length in characters for messages (both first post and replies)"
     max_post_length: "Maximum allowed post length in characters"
     topic_featured_link_enabled: "Enable posting a link with topics."
     show_topic_featured_link_in_digest: "Show the topic featured link in the digest email."

lib/post_creator.rb

@@ -167,7 +167,11 @@ class PostCreator
     DiscourseEvent.trigger :before_create_post, @post, @opts
     DiscourseEvent.trigger :validate_post, @post
 
-    post_validator = PostValidator.new(skip_topic: true)
+    post_validator =
+      PostValidator.new(
+        skip_topic: true,
+        private_message: @opts[:archetype] == Archetype.private_message,
+      )
     post_validator.validate(@post)
 
     valid = @post.errors.blank?

lib/validators/post_validator.rb

@@ -212,7 +212,7 @@ class PostValidator < ActiveModel::Validator
   end
 
   def private_message?(post)
-    post.topic.try(:private_message?)
+    post.topic.try(:private_message?) || options[:private_message]
   end
 
   def add_error_if_count_exceeded(

spec/lib/post_creator_spec.rb

@@ -208,7 +208,6 @@ RSpec.describe PostCreator do
         cat.save
 
         created_post = nil
-        other_user_tracking_topic = nil
 
         messages =
           MessageBus.track_publish do
@@ -1099,6 +1098,26 @@ RSpec.describe PostCreator do
       )
     end
 
+    it "respects min_personal_message_post_length" do
+      SiteSetting.min_personal_message_post_length = 5
+      SiteSetting.min_first_post_length = 20
+      SiteSetting.min_post_length = 25
+      SiteSetting.body_min_entropy = 20
+      user.update!(trust_level: 3)
+      Group.refresh_automatic_groups!
+
+      expect {
+        PostCreator.create!(
+          user,
+          title: "hi there welcome to my PM",
+          raw: "sorry",
+          archetype: Archetype.private_message,
+          target_usernames: [target_user1.username, target_user2.username].join(","),
+          category: 1,
+        )
+      }.not_to raise_error
+    end
+
     it "acts correctly" do
       freeze_time