Remove DB transaction from Rebase operation

Instead keeping a transaction open, which can fail
if Gitaly takes more time than expected to process,
now we rescue possible Gitaly errors, cleaning up
the merge request object accordingly.

app/models/repository.rb

@@ -1052,18 +1052,19 @@ class Repository
       return rebase_deprecated(user, merge_request)
     end
 
-    MergeRequest.transaction do
-      raw.rebase(
-        user,
-        merge_request.id,
-        branch: merge_request.source_branch,
-        branch_sha: merge_request.source_branch_sha,
-        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_error: nil)
-      end
+    raw.rebase(
+      user,
+      merge_request.id,
+      branch: merge_request.source_branch,
+      branch_sha: merge_request.source_branch_sha,
+      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_error: nil)
     end
+  rescue StandardError => error
+    merge_request.update!(rebase_commit_sha: nil)
+    raise error
   end
 
   def squash(user, merge_request, message)

changelogs/unreleased/osw-remove-transaction-while-rebasing.yml

+---
+title: Remove DB transaction from Rebase operation
+merge_request: 20739
+author:
+type: fixed

spec/models/repository_spec.rb

@@ -1530,7 +1530,7 @@ describe Repository do
           expect(merge_request.reload.rebase_commit_sha).to eq(new_sha)
         end
 
-        it 'does rollback when an error is encountered in the second step' do
+        it 'does rollback when a PreReceiveError is encountered in the second step' do
           second_response = double(pre_receive_error: 'my_error', git_error: nil)
           mock_gitaly(second_response)
 
@@ -1541,6 +1541,17 @@ describe Repository do
           expect(merge_request.reload.rebase_commit_sha).to be_nil
         end
 
+        it 'does rollback when a GitError is encountered in the second step' do
+          second_response = double(pre_receive_error: nil, git_error: 'git error')
+          mock_gitaly(second_response)
+
+          expect do
+            repository.rebase(user, merge_request)
+          end.to raise_error(Gitlab::Git::Repository::GitError)
+
+          expect(merge_request.reload.rebase_commit_sha).to be_nil
+        end
+
         def mock_gitaly(second_response)
           responses = [
             double(rebase_sha: new_sha).as_null_object,