Implement fast-forward merge

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/controllers/projects/merge_requests_controller.rb

@@ -2,7 +2,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   before_action :module_enabled
   before_action :merge_request, only: [
     :edit, :update, :show, :diffs, :commits, :merge, :merge_check,
-    :ci_status, :toggle_subscription, :approve
+    :ci_status, :toggle_subscription, :approve, :ff_merge
   ]
   before_action :closes_issues, only: [:edit, :update, :show, :diffs, :commits]
   before_action :validates_merge_request, only: [:show, :diffs, :commits]
@@ -218,6 +218,19 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     redirect_to merge_request_path(@merge_request)
   end
 
+  def ff_merge
+    return access_denied! unless @merge_request.can_be_merged_by?(current_user)
+    return render_404 unless @merge_request.approved?
+
+    if @merge_request.ff_merge_possible?
+      MergeRequests::FfMergeService.new(merge_request.target_project, current_user).
+        execute(merge_request)
+      @status = true
+    else
+      @status = false
+    end
+  end
+
   protected
 
   def selected_target_project

app/models/repository.rb

@@ -435,6 +435,18 @@ class Repository
     end
   end
 
+  def ff_merge(user, source_sha, target_branch, options = {})
+    our_commit = rugged.branches[target_branch].target
+    their_commit = rugged.lookup(source_sha)
+
+    raise "Invalid merge target" if our_commit.nil?
+    raise "Invalid merge source" if their_commit.nil?
+
+    commit_with_hooks(user, target_branch) do |ref|
+      source_sha
+    end
+  end
+
   def merge(user, source_sha, target_branch, options = {})
     our_commit = rugged.branches[target_branch].target
     their_commit = rugged.lookup(source_sha)

app/services/merge_requests/ff_merge_service.rb

+module MergeRequests
+  # MergeService class
+  #
+  # Do git merge and in case of success
+  # mark merge request as merged and execute all hooks and notifications
+  # Executed when you do merge via GitLab UI
+  #
+  class FfMergeService < MergeRequests::BaseService
+    attr_reader :merge_request
+
+    def execute(merge_request)
+      @merge_request = merge_request
+
+      unless @merge_request.ff_merge_possible?
+        return error('Merge request is not mergeable')
+      end
+
+      merge_request.in_locked_state do
+        if update_head
+          after_merge
+          success
+        else
+          error('Can not merge changes')
+        end
+      end
+    end
+
+    private
+
+    def update_head
+      repository.ff_merge(current_user, merge_request.source_sha, merge_request.target_branch)
+    end
+
+    def after_merge
+      MergeRequests::PostMergeService.new(project, current_user).execute(merge_request)
+    end
+  end
+end

app/views/projects/merge_requests/ff_merge.js.haml

+- if @status
+  :plain
+    location.reload()
+- else
+  :plain
+    $('.mr-widget-body').html("#{escape_javascript(render('projects/merge_requests/widget/open/reload'))}");

app/views/projects/merge_requests/widget/open/_ff_accept.html.haml

@@ -17,8 +17,8 @@
         .accept-action
           = f.button class: "btn btn-reopen rebase-mr" do
             Rebase from #{@merge_request.target_branch}
-        .accept-control
-          Fast-forward merge is not possible. Branch must be rebased first
+      .accept-control
+        Fast-forward merge is not possible. Branch must be rebased first
 
 :coffeescript
   $('.accept-mr-form').on 'ajax:before', ->