Add `sha` parameter to MR accept API

The `sha` parameter is optional, and when present, must match the
current HEAD SHA of the source branch. Otherwise, the API call fails
with a 409 Conflict and a message containing the current HEAD for the
source branch.

Also tidy up some doc wording.

1 files changed, 5 insertions, 0 deletions

diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index 4e7de8867b4..50baf4c09ad 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -218,6 +218,7 @@ module API
         #   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
+        #   sha (optional)                          - When present, must have the HEAD SHA of the source branch
         # Example:
         #   PUT /projects/:id/merge_requests/:merge_request_id/merge
         #
@@ -233,6 +234,10 @@ module API
 
           render_api_error!('Branch cannot be merged', 406) unless merge_request.can_be_merged?
 
+          if params[:sha] && merge_request.source_sha != params[:sha]
+            render_api_error!("SHA does not match HEAD of source branch: #{merge_request.target_sha}", 409)
+          end
+
           merge_params = {
             commit_message: params[:merge_commit_message],
             should_remove_source_branch: params[:should_remove_source_branch]