Merge branch 'merge-if-green' into 'master'

Merge when build succeeds

### What does this MR do?
Adds a button to a MR when the build/ci is running so it can be merged when/if the build is successfull.

### Are there points in the code the reviewer needs to double check?
English spelling and whether or not the grammer is correct.

### Why was this MR needed?
When you expect its all good, and don't want to revisit the current MR it can be accepted allready.

### What are the relevant issue numbers / Feature requests?
Fixes #2640 -- although `Merge if green` is replaced with `Merge when the build succeeds` the general idea is the same.

### Screenshots (if relevant)
![Screenshot_from_2015-12-07_10-34-39](/uploads/b90b558b6ad6c8266bc9c96120d86f96/Screenshot_from_2015-12-07_10-34-39.png)

![Screenshot_from_2015-11-02_17-26-56](/uploads/9f52f56e3c5e9ec63cb8f42978c92a52/Screenshot_from_2015-11-02_17-26-56.png)
![Screenshot_from_2015-11-02_17-27-03](/uploads/119fc18c9b15ff73dc38010ac5b6244a/Screenshot_from_2015-11-02_17-27-03.png)

![Screenshot_from_2015-12-03_10-08-14](/uploads/bad7b35c7d129981d43631877c958be0/Screenshot_from_2015-12-03_10-08-14.png)

![Screenshot_from_2015-11-02_17-27-16](/uploads/b616d750a16cc11ba72f2ca84213515e/Screenshot_from_2015-11-02_17-27-16.png)

### Further considerations
What if there are minor things needed solving, e.g. Rubocop, the current implementation will reset the approved status of the MR. It might be a consideration keep the approval for team member, or even guests. This would require an extra option in the Admin screen, though might add extra value.

#### TODO
- [x] Docs
- [x] Specs

/cc @DouweM @rspeicher

See merge request !1729

2 files changed, 37 insertions, 28 deletions

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 96b73df6af9..81bf7a8222b 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -171,6 +171,7 @@ module API
       expose :description
       expose :work_in_progress?, as: :work_in_progress
       expose :milestone, using: Entities::Milestone
+      expose :merge_when_build_succeeds
     end
 
     class MergeRequestChanges < MergeRequest
diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index e7c5f808aea..3c1c6bda260 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -195,46 +195,54 @@ module API
       # Merge MR
       #
       # Parameters:
-      #   id (required)                   - The ID of a project
-      #   merge_request_id (required)     - ID of MR
-      #   merge_commit_message (optional) - Custom merge commit message
+      #   id (required)                           - The ID of a project
+      #   merge_request_id (required)             - ID of MR
+      #   merge_commit_message (optional)         - Custom merge commit message
+      #   should_remove_source_branch (optional)  - When true, the source branch will be deleted if possible
+      #   merge_when_build_succeeds (optional)    - When true, this MR will be merged when the build succeeds
       # Example:
       #   PUT /projects/:id/merge_request/:merge_request_id/merge
       #
       put ":id/merge_request/:merge_request_id/merge" do
         merge_request = user_project.merge_requests.find(params[:merge_request_id])
 
-        allowed = ::Gitlab::GitAccess.new(current_user, user_project).
-          can_push_to_branch?(merge_request.target_branch)
+        # Merge request can not be merged
+        # because user dont have permissions to push into target branch
+        unauthorized! unless merge_request.can_be_merged_by?(current_user)
+        not_allowed! if !merge_request.open? || merge_request.work_in_progress?
 
-        if allowed
-          if merge_request.unchecked?
-            merge_request.check_if_can_be_merged
-          end
+        merge_request.check_if_can_be_merged if merge_request.unchecked?
 
-          if merge_request.open? && !merge_request.work_in_progress?
-            if merge_request.can_be_merged?
-              commit_message = params[:merge_commit_message] || merge_request.merge_commit_message
-
-              ::MergeRequests::MergeService.new(merge_request.target_project, current_user).
-                execute(merge_request, commit_message)
-
-              present merge_request, with: Entities::MergeRequest
-            else
-              render_api_error!('Branch cannot be merged', 405)
-            end
-          else
-            # Merge request can not be merged
-            # because it is already closed/merged or marked as WIP
-            not_allowed!
-          end
+        render_api_error!('Branch cannot be merged', 406) unless merge_request.can_be_merged?
+
+        merge_params = {
+          commit_message: params[:merge_commit_message],
+          should_remove_source_branch: params[:should_remove_source_branch]
+        }
+
+        if parse_boolean(params[:merge_when_build_succeeds]) && merge_request.ci_commit && merge_request.ci_commit.active?
+          ::MergeRequests::MergeWhenBuildSucceedsService.new(merge_request.target_project, current_user, merge_params).
+            execute(merge_request)
         else
-          # Merge request can not be merged
-          # because user dont have permissions to push into target branch
-          unauthorized!
+          ::MergeRequests::MergeService.new(merge_request.target_project, current_user, merge_params).
+            execute(merge_request)
         end
+
+        present merge_request, with: Entities::MergeRequest
       end
 
+      # Cancel Merge if Merge When build succeeds is enabled
+      # Parameters:
+      #   id (required)                         - The ID of a project
+      #   merge_request_id (required)           - ID of MR
+      #
+      post ":id/merge_request/:merge_request_id/cancel_merge_when_build_succeeds" do
+        merge_request = user_project.merge_requests.find(params[:merge_request_id])
+
+        unauthorized! unless merge_request.can_cancel_merge_when_build_succeeds?(current_user)
+
+        ::MergeRequest::MergeWhenBuildSucceedsService.new(merge_request.target_project, current_user).cancel(merge_request)
+      end
 
       # Get a merge request's comments
       #