Prevent fast forward merge when rebase is required

app/services/merge_requests/merge_service.rb

@@ -18,15 +18,7 @@ module MergeRequests
 
       @merge_request = merge_request
 
-      unless @merge_request.mergeable?
-        return handle_merge_error(log_message: 'Merge request is not mergeable', save_message_on_model: true)
-      end
-
-      @source = find_merge_source
-
-      unless @source
-        return handle_merge_error(log_message: 'No source for merge', save_message_on_model: true)
-      end
+      error_check!
 
       merge_request.in_locked_state do
         if commit
@@ -41,6 +33,19 @@ module MergeRequests
 
     private
 
+    def error_check!
+      error =
+        if @merge_request.should_be_rebased?
+          'Only fast-forward merge is allowed for your project. Please update your source branch'
+        elsif !@merge_request.mergeable?
+          'Merge request is not mergeable'
+        elsif !source
+          'No source for merge'
+        end
+
+      raise MergeError, error if error
+    end
+
     def commit
       message = params[:commit_message] || merge_request.merge_commit_message
 
@@ -91,8 +96,8 @@ module MergeRequests
       merge_request.to_reference(full: true)
     end
 
-    def find_merge_source
-      merge_request.diff_head_sha
+    def source
+      @source ||= @merge_request.diff_head_sha
     end
   end
 end

changelogs/unreleased/issue_39773_be.yml

@@ -0,0 +1,5 @@
+---
+title: Show error message when fast-forward merge is not possible
+merge_request:
+author:
+type: fixed

spec/services/merge_requests/merge_service_spec.rb

@@ -248,6 +248,28 @@ describe MergeRequests::MergeService do
         expect(merge_request.merge_error).to include(error_message)
         expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
       end
+
+      context "when fast-forward merge is not allowed" do
+        before do
+          allow_any_instance_of(Repository).to receive(:ancestor?).and_return(nil)
+        end
+
+        %w(semi-linear ff).each do |merge_method|
+          it "logs and saves error if merge is #{merge_method} only" do
+            merge_method = 'rebase_merge' if merge_method == 'semi-linear'
+            merge_request.project.update(merge_method: merge_method)
+            error_message = 'Only fast-forward merge is allowed for your project. Please update your source branch'
+            allow(service).to receive(:execute_hooks)
+
+            service.execute(merge_request)
+
+            expect(merge_request).to be_open
+            expect(merge_request.merge_commit_sha).to be_nil
+            expect(merge_request.merge_error).to include(error_message)
+            expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
+          end
+        end
+      end
     end
   end
 end