Allow replying to an individual note in the API

If you can do this in the UI, you should be able to do it in the API. If
a discussion is not a single note discussion, or it is replyable, you
can reply to it.

changelogs/unreleased/allow-replying-to-individual-notes-from-api.yml

+---
+title: Allow replying to individual notes from API
+merge_request:
+author:
+type: fixed

doc/api/discussions.md

@@ -153,7 +153,8 @@ curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab
 
 ### Add note to existing issue discussion
 
-Adds a new note to the discussion.
+Adds a new note to the discussion. This can also
+[create a discussion from a single comment](../user/discussions/#start-a-discussion-by-replying-to-a-standard-comment).
 
 ```
 POST /projects/:id/issues/:issue_iid/discussions/:discussion_id/notes
@@ -652,7 +653,8 @@ curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab.
 
 ### Add note to existing merge request discussion
 
-Adds a new note to the discussion.
+Adds a new note to the discussion. This can also
+[create a discussion from a single comment](../user/discussions/#start-a-discussion-by-replying-to-a-standard-comment).
 
 ```
 POST /projects/:id/merge_requests/:merge_request_iid/discussions/:discussion_id/notes

lib/api/discussions.rb

@@ -134,9 +134,13 @@ module API
         post ":id/#{noteables_path}/:noteable_id/discussions/:discussion_id/notes" do
           noteable = find_noteable(parent_type, noteables_str, params[:noteable_id])
           notes = readable_discussion_notes(noteable, params[:discussion_id])
+          first_note = notes.first
 
           break not_found!("Discussion") if notes.empty?
-          break bad_request!("Discussion is an individual note.") unless notes.first.part_of_discussion?
+
+          unless first_note.part_of_discussion? || first_note.to_discussion.can_convert_to_discussion?
+            break bad_request!("Discussion can not be replied to.")
+          end
 
           opts = {
             note: params[:body],

spec/requests/api/discussions_spec.rb

@@ -13,7 +13,7 @@ describe API::Discussions do
     let!(:issue) { create(:issue, project: project, author: user) }
     let!(:issue_note) { create(:discussion_note_on_issue, noteable: issue, project: project, author: user) }
 
-    it_behaves_like 'discussions API', 'projects', 'issues', 'iid' do
+    it_behaves_like 'discussions API', 'projects', 'issues', 'iid', can_reply_to_invididual_notes: true do
       let(:parent) { project }
       let(:noteable) { issue }
       let(:note) { issue_note }
@@ -37,7 +37,7 @@ describe API::Discussions do
     let!(:diff_note) { create(:diff_note_on_merge_request, noteable: noteable, project: project, author: user) }
     let(:parent) { project }
 
-    it_behaves_like 'discussions API', 'projects', 'merge_requests', 'iid'
+    it_behaves_like 'discussions API', 'projects', 'merge_requests', 'iid', can_reply_to_invididual_notes: true
     it_behaves_like 'diff discussions API', 'projects', 'merge_requests', 'iid'
     it_behaves_like 'resolvable discussions API', 'projects', 'merge_requests', 'iid'
   end

spec/support/shared_examples/requests/api/discussions.rb

-shared_examples 'discussions API' do |parent_type, noteable_type, id_name|
+shared_examples 'discussions API' do |parent_type, noteable_type, id_name, can_reply_to_invididual_notes: false|
   describe "GET /#{parent_type}/:id/#{noteable_type}/:noteable_id/discussions" do
     it "returns an array of discussions" do
       get api("/#{parent_type}/#{parent.id}/#{noteable_type}/#{noteable[id_name]}/discussions", user)
@@ -136,15 +136,27 @@ shared_examples 'discussions API' do |parent_type, noteable_type, id_name|
       expect(response).to have_gitlab_http_status(400)
     end
 
-    it "returns a 400 bad request error if discussion is individual note" do
-      note.update_attribute(:type, nil)
+    context 'when the discussion is an individual note' do
+      before do
+        note.update!(type: nil)
 
         post api("/#{parent_type}/#{parent.id}/#{noteable_type}/#{noteable[id_name]}/"\
                  "discussions/#{note.discussion_id}/notes", user), params: { body: 'hi!' }
+      end
 
+      if can_reply_to_invididual_notes
+        it 'creates a new discussion' do
+          expect(response).to have_gitlab_http_status(201)
+          expect(json_response['body']).to eq('hi!')
+          expect(json_response['type']).to eq('DiscussionNote')
+        end
+      else
+        it 'returns 400 bad request' do
           expect(response).to have_gitlab_http_status(400)
         end
       end
+    end
+  end
 
   describe "PUT /#{parent_type}/:id/#{noteable_type}/:noteable_id/discussions/:discussion_id/notes/:note_id" do
     it 'returns modified note' do