Set the SHA to be merged from push options

When we set `merge_when_pipeline_succeeds` in the push options, we
also need to provide the sha that we'll be merging.

We take the cha from the changes list that was passed into the push
options, and pass it onwards to the Create or Update service.

app/services/merge_requests/push_options_handler_service.rb

@@ -4,14 +4,14 @@ module MergeRequests
   class PushOptionsHandlerService
     LIMIT = 10
 
-    attr_reader :branches, :changes_by_branch, :current_user, :errors,
+    attr_reader :current_user, :errors, :changes,
                 :project, :push_options, :target_project
 
     def initialize(project, current_user, changes, push_options)
       @project = project
       @target_project = @project.default_merge_request_target
       @current_user = current_user
-      @branches = get_branches(changes)
+      @changes = Gitlab::ChangesList.new(changes)
       @push_options = push_options
       @errors = []
     end
@@ -34,8 +34,12 @@ module MergeRequests
 
     private
 
-    def get_branches(raw_changes)
-      Gitlab::ChangesList.new(raw_changes).map do |changes|
+    def branches
+      changes_by_branch.keys
+    end
+
+    def changes_by_branch
+      @changes_by_branch ||= changes.each_with_object({}) do |changes, result|
         next unless Gitlab::Git.branch_ref?(changes[:ref])
 
         # Deleted branch
@@ -45,8 +49,8 @@ module MergeRequests
         branch_name = Gitlab::Git.branch_name(changes[:ref])
         next if branch_name == target_project.default_branch
 
-        branch_name
-      end.compact.uniq
+        result[branch_name] = changes
+      end
     end
 
     def validate_service
@@ -112,7 +116,7 @@ module MergeRequests
       merge_request = ::MergeRequests::UpdateService.new(
         target_project,
         current_user,
-        update_params
+        update_params(merge_request)
       ).execute(merge_request)
 
       collect_errors_from_merge_request(merge_request) unless merge_request.valid?
@@ -130,19 +134,22 @@ module MergeRequests
 
       params.compact!
 
-      if push_options.key?(:merge_when_pipeline_succeeds)
-        params.merge!(
-          merge_when_pipeline_succeeds: push_options[:merge_when_pipeline_succeeds],
-          merge_user: current_user
-        )
-      end
-
       params[:add_labels] = params.delete(:label).keys if params.has_key?(:label)
       params[:remove_labels] = params.delete(:unlabel).keys if params.has_key?(:unlabel)
 
       params
     end
 
+    def merge_params(branch)
+      return {} unless push_options.key?(:merge_when_pipeline_succeeds)
+
+      {
+        merge_when_pipeline_succeeds: push_options[:merge_when_pipeline_succeeds],
+        merge_user: current_user,
+        sha: changes_by_branch.dig(branch, :newrev)
+      }
+    end
+
     def create_params(branch)
       params = base_params
 
@@ -153,13 +160,15 @@ module MergeRequests
         target_project: target_project
       )
 
+      params.merge!(merge_params(branch))
+
       params[:target_branch] ||= target_project.default_branch
 
       params
     end
 
-    def update_params
-      base_params
+    def update_params(merge_request)
+      base_params.merge(merge_params(merge_request.source_branch))
     end
 
     def collect_errors_from_merge_request(merge_request)

changelogs/unreleased/bvl-fix-merging-through-push-options.yml

+---
+title: Fix merging merge requests from push options
+merge_request: 20639
+author:
+type: fixed

spec/services/merge_requests/push_options_handler_service_spec.rb

@@ -101,17 +101,15 @@ describe MergeRequests::PushOptionsHandlerService do
   shared_examples_for 'a service that can set the merge request to merge when pipeline succeeds' do
     subject(:last_mr) { MergeRequest.last }
 
+    let(:change) { Gitlab::ChangesList.new(changes).changes.first }
+
     it 'sets auto_merge_enabled' do
       service.execute
 
       expect(last_mr.auto_merge_enabled).to eq(true)
       expect(last_mr.auto_merge_strategy).to eq(AutoMergeService::STRATEGY_MERGE_WHEN_PIPELINE_SUCCEEDS)
-    end
-
-    it 'sets merge_user to the user' do
-      service.execute
-
       expect(last_mr.merge_user).to eq(user)
+      expect(last_mr.merge_params['sha']).to eq(change[:newrev])
     end
   end