Properly clear the merge error upon rebase failure

If `merge_error` is ever set after a failure, it is never cleared, even
after a subsequent, successful rebase. We now clear this field whenever
a successful rebase occurs.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56139

app/models/repository.rb

@@ -1050,7 +1050,7 @@ class Repository
 
     # To support the full deprecated behaviour, set the
     # `rebase_commit_sha` for the merge_request here and return the value
-    merge_request.update(rebase_commit_sha: rebase_sha)
+    merge_request.update(rebase_commit_sha: rebase_sha, merge_error: nil)
 
     rebase_sha
   end
@@ -1069,7 +1069,7 @@ class Repository
         remote_repository: merge_request.target_project.repository.raw,
         remote_branch: merge_request.target_branch
       ) do |commit_id|
-        merge_request.update!(rebase_commit_sha: commit_id)
+        merge_request.update!(rebase_commit_sha: commit_id, merge_error: nil)
       end
     end
   end

changelogs/unreleased/sh-fix-rebase-error-clearing.yml

+---
+title: Properly clear the merge error upon rebase failure
+merge_request: 28319
+author:
+type: fixed

spec/services/merge_requests/rebase_service_spec.rb

@@ -38,6 +38,32 @@ describe MergeRequests::RebaseService do
       end
     end
 
+    shared_examples 'sequence of failure and success' do
+      it 'properly clears the error message' do
+        allow(repository).to receive(:gitaly_operation_client).and_raise('Something went wrong')
+
+        service.execute(merge_request)
+
+        expect(merge_request.reload.merge_error).to eq described_class::REBASE_ERROR
+
+        allow(repository).to receive(:gitaly_operation_client).and_call_original
+
+        service.execute(merge_request)
+
+        expect(merge_request.reload.merge_error).to eq nil
+      end
+    end
+
+    it_behaves_like 'sequence of failure and success'
+
+    context 'with deprecated step rebase feature' do
+      before do
+        allow(Feature).to receive(:disabled?).with(:two_step_rebase, anything).and_return(true)
+      end
+
+      it_behaves_like 'sequence of failure and success'
+    end
+
     context 'when unexpected error occurs' do
       before do
         allow(repository).to receive(:gitaly_operation_client).and_raise('Something went wrong')