Add option of rebasing before merging.

app/controllers/projects/merge_requests_controller.rb

@@ -122,6 +122,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
 
     if @merge_request.open? && @merge_request.can_be_merged?
       @merge_request.should_remove_source_branch = params[:should_remove_source_branch]
+      @merge_request.should_rebase = params[:should_rebase]
       @merge_request.automerge!(current_user, params[:commit_message])
       @status = true
     else

app/controllers/projects_controller.rb

@@ -202,7 +202,8 @@ class ProjectsController < ApplicationController
     params.require(:project).permit(
       :name, :path, :description, :issues_tracker, :tag_list,
       :issues_enabled, :merge_requests_enabled, :snippets_enabled, :issues_tracker_id, :default_branch,
-      :wiki_enabled, :visibility_level, :import_url, :last_activity_at, :namespace_id, :merge_requests_template
+      :wiki_enabled, :visibility_level, :import_url, :last_activity_at, :namespace_id, :merge_requests_template,
+      :merge_requests_rebase_enabled
     )
   end
 end

app/models/merge_request.rb

@@ -40,6 +40,9 @@ class MergeRequest < ActiveRecord::Base
 
   attr_accessor :should_remove_source_branch
 
+  # If true, merge request should rebase before merging
+  attr_accessor :should_rebase
+
   # When this attribute is true some MR validation is ignored
   # It allows us to close or modify broken merge requests
   attr_accessor :allow_broken

app/views/projects/merge_requests/show/_mr_accept.html.haml

@@ -24,6 +24,11 @@
                 = label_tag :should_remove_source_branch, class: "checkbox" do
                   = check_box_tag :should_remove_source_branch
                   Remove source-branch
+            - if @merge_request.source_project.merge_requests_rebase_enabled && !@merge_request.for_fork?
+              .remove_branch_holder.pull-left
+                = label_tag :should_rebase, class: "checkbox" do
+                  = check_box_tag :should_rebase
+                  Rebase before merge
             .js-toggle-container
               %label
                 %i.fa.fa-edit

lib/gitlab/satellite/merge_action.rb

@@ -2,6 +2,8 @@ module Gitlab
   module Satellite
     # GitLab server-side merge
     class MergeAction < Action
+      include Gitlab::Popen
+
       attr_accessor :merge_request
 
       def initialize(user, merge_request)
@@ -27,6 +29,16 @@ module Gitlab
       def merge!(merge_commit_message = nil)
         in_locked_and_timed_satellite do |merge_repo|
           prepare_satellite!(merge_repo)
+
+          if merge_request.should_rebase
+            if rebase_in_satellite!(merge_repo)
+              merge_repo.git.push(default_options, :origin, merge_request.target_branch)
+              return true
+            else
+              prepare_satellite!(merge_repo)
+            end
+          end
+
           if merge_in_satellite!(merge_repo, merge_commit_message)
             # push merge back to bare repo
             # will raise CommandFailed when push fails
@@ -133,6 +145,25 @@ module Gitlab
         handle_exception(ex)
       end
 
+      def rebase_in_satellite!(repo)
+        update_satellite_source_and_target!(repo)
+        repo.git.checkout(default_options({b: true}), merge_request.source_branch, "source/#{merge_request.source_branch}")
+        repo.git.merge(default_options({no_ff: true}), "origin/#{merge_request.target_branch}")
+
+        output, status = popen(%W(git rebase origin/#{merge_request.target_branch}), repo.working_dir)
+        if status == 0
+          Gitlab::AppLogger.info "Rebasing before merge in #{merge_request.source_project.path_with_namespace} MR!#{merge_request.id}: #{output}."
+          repo.git.checkout(default_options, merge_request.target_branch)
+          repo.git.merge(default_options, "source/#{merge_request.source_branch}")
+        else
+          repo.git.rebase(default_options, "--abort")
+          Gitlab::AppLogger.info "Rebasing in in #{merge_request.source_project.path_with_namespace} MR!#{merge_request.id} aborted, rebase manually."
+          false
+        end
+      rescue Grit::Git::CommandFailed => ex
+        handle_exception(ex)
+      end
+
       # Assumes a satellite exists that is a fresh clone of the projects repo, prepares satellite for merges, diffs etc
       def update_satellite_source_and_target!(repo)
         repo.remote_add('source', merge_request.source_project.repository.path_to_repo)