FIX: draft not clearing when replying to new topic

This amends our API so we provide it with the draft key when saving a post
this means post creator can clean up the draft consistently even if we are
doing fancy stuff like replying to a new topic or new pm or whatever.

There will be some followup work to clean it up so client never calls destroy
on draft during normal operation and the #create/#update endpoints takes care of it
every time

app/assets/javascripts/discourse/models/composer.js.es6

@@ -57,7 +57,8 @@ const CLOSED = "closed",
     tags: "tags",
     featured_link: "featuredLink",
     shared_draft: "sharedDraft",
-    no_bump: "noBump"
+    no_bump: "noBump",
+    draft_key: "draftKey"
   },
   _edit_topic_serializer = {
     title: "topic.title",

app/controllers/posts_controller.rb

@@ -671,7 +671,8 @@ class PostsController < ApplicationController
       :auto_track,
       :typing_duration_msecs,
       :composer_open_duration_msecs,
-      :visible
+      :visible,
+      :draft_key
     ]
 
     Post.plugin_permitted_create_params.each do |key, plugin|

lib/post_creator.rb

@@ -35,6 +35,7 @@ class PostCreator
   #                             call `enqueue_jobs` after the transaction is comitted.
   #   hidden_reason_id        - Reason for hiding the post (optional)
   #   skip_validations        - Do not validate any of the content in the post
+  #   draft_key               - the key of the draft we are creating (will be deleted on success)
   #
   #   When replying to a topic:
   #     topic_id              - topic we're replying to
@@ -180,7 +181,9 @@ class PostCreator
         update_uploads_secure_status
         ensure_in_allowed_users if guardian.is_staff?
         unarchive_message
-        @post.advance_draft_sequence unless @opts[:import_mode]
+        if !@opts[:import_mode]
+          DraftSequence.next!(@user, draft_key)
+        end
         @post.save_reply_relationships
       end
     end
@@ -292,10 +295,13 @@ class PostCreator
 
   protected
 
+  def draft_key
+    @draft_key ||= @opts[:draft_key]
+    @draft_key ||= @topic ? "topic_#{@topic.id}" : "new_topic"
+  end
+
   def build_post_stats
     if PostCreator.track_post_stats
-      draft_key = @topic ? "topic_#{@topic.id}" : "new_topic"
-
       sequence = DraftSequence.current(@user, draft_key)
       revisions = Draft.where(sequence: sequence,
                               user_id: @user.id,

spec/requests/posts_controller_spec.rb

@@ -862,18 +862,25 @@ describe PostsController do
         it "doesn't enqueue posts when user first creates a topic" do
           user.user_stat.update_column(:topic_count, 1)
 
+          Draft.set(user, "should_clear", 0, "{'a' : 'b'}")
+
           post "/posts.json", params: {
             raw: 'this is the test content',
             title: 'this is the test title for the topic',
             composer_open_duration_msecs: 204,
             typing_duration_msecs: 100,
-            topic_id: topic.id
+            topic_id: topic.id,
+            draft_key: "should_clear"
           }
 
           expect(response.status).to eq(200)
           parsed = ::JSON.parse(response.body)
 
           expect(parsed["action"]).not_to be_present
+
+          expect {
+            Draft.get(user, "should_clear", 0)
+          }.to raise_error(Draft::OutOfSequence)
         end
 
         it "doesn't enqueue replies when the topic is closed" do