Allow setting content for resolutions

When reading conflicts:

1. Add a `type` field. `text` works as before, and has `sections`;
   `text-editor` is a file with ambiguous conflict markers that can only
   be resolved in an editor.
2. Add a `content_path` field pointing to a JSON representation of the
   file's content for a single file.
3. Hitting `content_path` returns a similar datastructure to the `file`,
   but without the `content_path` and `sections` fields, and with a
   `content` field containing the full contents of the file (with
   conflict markers).

When writing conflicts:

1. Instead of `sections` being at the top level, they are now in a
   `files` array. This matches the read format better.
2. The `files` array contains file hashes, each of which must contain:
   a. `new_path`
   b. `old_path`
   c. EITHER `sections` (which works as before) or `content` (with the
      full content of the resolved file).

2 files changed, 21 insertions, 6 deletions

diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index b3455e04c29..bf37421771f 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -114,7 +114,12 @@ class ApplicationController < ActionController::Base
   end
 
   def render_404
-    render file: Rails.root.join("public", "404"), layout: false, status: "404"
+    respond_to do |format|
+      format.json { head :not_found }
+      format.any do
+        render file: Rails.root.join("public", "404"), layout: false, status: "404"
+      end
+    end
   end
 
   def no_cache_headers
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 869d96b86f4..2c7a0062f2b 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -9,15 +9,15 @@ class Projects::MergeRequestsController < Projects::ApplicationController
 
   before_action :module_enabled
   before_action :merge_request, only: [
-    :edit, :update, :show, :diffs, :commits, :conflicts, :builds, :pipelines, :merge, :merge_check,
+    :edit, :update, :show, :diffs, :commits, :conflicts, :conflict_for_path, :builds, :pipelines, :merge, :merge_check,
     :ci_status, :toggle_subscription, :cancel_merge_when_build_succeeds, :remove_wip, :resolve_conflicts, :assign_related_issues
   ]
   before_action :validates_merge_request, only: [:show, :diffs, :commits, :builds, :pipelines]
-  before_action :define_show_vars, only: [:show, :diffs, :commits, :conflicts, :builds, :pipelines]
+  before_action :define_show_vars, only: [:show, :diffs, :commits, :conflicts, :conflict_for_path, :builds, :pipelines]
   before_action :define_widget_vars, only: [:merge, :cancel_merge_when_build_succeeds, :merge_check]
   before_action :define_commit_vars, only: [:diffs]
   before_action :define_diff_comment_vars, only: [:diffs]
-  before_action :ensure_ref_fetched, only: [:show, :diffs, :commits, :builds, :conflicts, :pipelines]
+  before_action :ensure_ref_fetched, only: [:show, :diffs, :commits, :builds, :conflicts, :conflict_for_path, :pipelines]
   before_action :close_merge_request_without_source_project, only: [:show, :diffs, :commits, :builds, :pipelines]
   before_action :apply_diff_view_cookie!, only: [:new_diffs]
   before_action :build_merge_request, only: [:new, :new_diffs]
@@ -33,7 +33,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
 
   before_action :authenticate_user!, only: [:assign_related_issues]
 
-  before_action :authorize_can_resolve_conflicts!, only: [:conflicts, :resolve_conflicts]
+  before_action :authorize_can_resolve_conflicts!, only: [:conflicts, :conflict_for_path, :resolve_conflicts]
 
   def index
     @merge_requests = merge_requests_collection
@@ -170,6 +170,16 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     end
   end
 
+  def conflict_for_path
+    return render_404 unless @merge_request.conflicts_can_be_resolved_in_ui?
+
+    file = @merge_request.conflicts.file_for_path(params[:old_path], params[:new_path])
+
+    return render_404 unless file
+
+    render json: file, full_content: true
+  end
+
   def resolve_conflicts
     return render_404 unless @merge_request.conflicts_can_be_resolved_in_ui?
 
@@ -184,7 +194,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       flash[:notice] = 'All merge conflicts were resolved. The merge request can now be merged.'
 
       render json: { redirect_to: namespace_project_merge_request_url(@project.namespace, @project, @merge_request, resolved_conflicts: true) }
-    rescue Gitlab::Conflict::File::MissingResolution => e
+    rescue Gitlab::Conflict::ResolutionError => e
       render status: :bad_request, json: { message: e.message }
     end
   end