FEATURE: Add new setting to force user edit last post.

config/locales/server.en.yml

@@ -218,6 +218,10 @@ en:
     one: "We're sorry, but new users are temporarily limited to 1 reply in the same topic."
     other: "We're sorry, but new users are temporarily limited to %{count} replies in the same topic."
 
+  max_consecutive_replies:
+    one: "No more than 1 reply is allowed. Please edit your previous reply instead, or wait for someone to reply to you."
+    other: "No more than %{count} consecutive replies are allowed. Please edit your previous reply instead, or wait for someone to reply to you."
+
   embed:
     start_discussion: "Start Discussion"
     continue: "Continue Discussion"
@@ -1536,6 +1540,7 @@ en:
     title_min_entropy: "The minimum entropy (unique characters, non-english count for more) required for a topic title."
     body_min_entropy: "The minimum entropy (unique characters, non-english count for more) required for a post body."
     allow_uppercase_posts: "Allow all caps in a topic title or a post body."
+    max_consecutive_replies: "Force users to edit their last post instead of replying"
 
     title_fancy_entities: "Convert common ASCII characters to fancy HTML entities in topic titles, ala SmartyPants <a href='https://daringfireball.net/projects/smartypants/' target='_blank'>https://daringfireball.net/projects/smartypants/</a>"
 

config/site_settings.yml

@@ -594,6 +594,8 @@ posting:
     default: false
     locale_default:
       ja: true
+  max_consecutive_replies:
+    default: 0
   title_prettify:
     default: true
     locale_default:

lib/validators/post_validator.rb

@@ -17,6 +17,7 @@ def validate(record)
     max_attachments_validator(record)
     can_post_links_validator(record)
     unique_post_validator(record)
+    force_edit_last_validator(record)
   end
 
   def presence(post)
@@ -141,6 +142,32 @@ def unique_post_validator(post)
     end
   end
 
+  def force_edit_last_validator(post)
+    return if SiteSetting.max_consecutive_replies == 0 || post.id || post.acting_user&.staff? || private_message?(post)
+
+    topic = post.topic
+
+    last_posts_count = DB.query_single(<<~SQL, topic_id: post.topic_id, user_id: post.acting_user.id, max_replies: SiteSetting.max_consecutive_replies).first
+      SELECT COUNT(*)
+      FROM (
+        SELECT user_id
+          FROM posts
+         WHERE deleted_at IS NULL
+           AND NOT hidden
+           AND topic_id = :topic_id
+         ORDER BY post_number DESC
+         LIMIT :max_replies
+      ) c
+      WHERE c.user_id = :user_id
+    SQL
+    return if last_posts_count < SiteSetting.max_consecutive_replies
+
+    guardian = Guardian.new(post.acting_user)
+    if guardian.can_edit?(topic.posts.last)
+      post.errors.add(:base, I18n.t(:max_consecutive_replies, count: SiteSetting.max_consecutive_replies))
+    end
+  end
+
   private
 
   def acting_user_is_trusted?(post, level = 1)

spec/components/validators/post_validator_spec.rb

@@ -219,6 +219,43 @@
     end
   end
 
+  context "force_edit_last_validator" do
+
+    let(:user) { Fabricate(:user) }
+    let(:other_user) { Fabricate(:user) }
+    let(:topic) { Fabricate(:topic) }
+
+    before do
+      SiteSetting.max_consecutive_replies = 2
+    end
+
+    it "should not allow posting more than 2 consecutive replies" do
+      1.upto(3).each do |i|
+        post = Post.new(user: user, topic: topic, raw: "post number #{i}")
+        validator.force_edit_last_validator(post)
+        expect(post.errors.count).to eq(i > SiteSetting.max_consecutive_replies ? 1 : 0)
+        post.save
+      end
+    end
+
+    it "should always allow editing" do
+      post = Fabricate(:post, user: user, topic: topic)
+      post = Fabricate(:post, user: user, topic: topic)
+
+      revisor = PostRevisor.new(post)
+      revisor.revise!(post.user, raw: 'hello world123456789')
+    end
+
+    it "should allow posting more than 2 replies" do
+      3.times do
+        post = Fabricate(:post, user: user, topic: topic)
+        Fabricate(:post, user: other_user, topic: topic)
+        validator.force_edit_last_validator(post)
+        expect(post.errors.count).to eq(0)
+      end
+    end
+  end
+
   shared_examples "almost no validations" do
     it "skips most validations" do
       validator.expects(:stripped_length).never
@@ -229,6 +266,7 @@
       validator.expects(:max_attachments_validator).never
       validator.expects(:newuser_links_validator).never
       validator.expects(:unique_post_validator).never
+      validator.expects(:force_edit_last_validator).never
       validator.validate(post)
     end
   end