Propagate error on FF pre-receive failure

Ensure pre-receive hook errors are propagated to the GUI
when encountered on fast-forward merges.

app/services/merge_requests/merge_service.rb

@@ -27,6 +27,7 @@ module MergeRequests
           success
         end
       end
+
       log_info("Merge process finished on JID #{merge_jid} with state #{state}")
     rescue MergeError => e
       handle_merge_error(log_message: e.message, save_message_on_model: true)
@@ -89,7 +90,7 @@ module MergeRequests
       repository.merge(current_user, source, merge_request, commit_message)
     rescue Gitlab::Git::PreReceiveError => e
       raise MergeError,
-            "Something went wrong during merge pre-receive hook. #{e.message}".strip
+            "GL-HOOK-ERR: Something went wrong during merge pre-receive hook. #{e.message}".strip
     rescue => e
       handle_merge_error(log_message: e.message)
       raise_error('Something went wrong during merge')

changelogs/unreleased/rm-25228-pre-receive-error.yml

+---
+title: Propagate error on FF pre-receive failure
+merge_request: 35633
+author:
+type: fixed

lib/gitlab/gitaly_client/operation_service.rb

@@ -178,6 +178,10 @@ module Gitlab
           timeout: GitalyClient.long_timeout
         )
 
+        if response.pre_receive_error.present?
+          raise Gitlab::Git::PreReceiveError.new("GL-HOOK-ERR: pre-receive hook failed: #{response.pre_receive_error}")
+        end
+
         Gitlab::Git::OperationService::BranchUpdate.from_gitaly(response.branch_update)
       rescue GRPC::FailedPrecondition => e
         raise Gitlab::Git::CommitError, e

spec/lib/gitlab/gitaly_client/operation_service_spec.rb

@@ -191,6 +191,20 @@ RSpec.describe Gitlab::GitalyClient::OperationService do
 
       it { expect(subject).to be_nil }
     end
+
+    context "when the pre-receive hook fails" do
+      let(:response) do
+        Gitaly::UserFFBranchResponse.new(
+          branch_update: nil,
+          pre_receive_error: "pre-receive hook error message\n"
+        )
+      end
+
+      it "raises the error" do
+        # the PreReceiveError class strips the GL-HOOK-ERR prefix from this error
+        expect { subject }.to raise_error(Gitlab::Git::PreReceiveError, "pre-receive hook failed: pre-receive hook error message")
+      end
+    end
   end
 
   shared_examples 'cherry pick and revert errors' do