Detect rebase-to-no-op in CreateRefService

Marc Shawrequested to merge
598820-detect-collapsed-rebase into master

What does this MR do and why?

Closes part of https://gitlab.com/gitlab-org/gitlab/-/work_items/598820 — Site 1 of three (root-cause detection).

When a project has automatic_rebase_enabled=true and a fast-forward / semi-linear merge method, MergeRequests::CreateRefService#maybe_rebase! calls repository.rebase_to_ref. If the source contains no unique changes against the target (e.g. the MR was branched off another MR whose changes are now on target), the rebase output equals first_parent_sha — the source contributes nothing.

Previously this was undetected. MergeService#commit (app/services/merge_requests/merge_service.rb:94-100) then recorded the MR as merged with:

  • merged_commit_sha = current target tip (inherited from another MR)
  • merge_commit_sha = nil (stripped via .compact, so the column held its prior nil value)

With force_remove_source_branch=true (the common configuration), DeleteSourceBranchWorker then deleted the source branch and the MR's unique commits became unreachable. This is data loss, not just a UX bug.

This MR adds a guard in maybe_rebase!: if the rebase output equals first_parent_sha, raise CreateRefError. MergeService#try_merge converts that into a user-visible MergeError, leaving the MR in opened state with merge_error populated. No DB lie, no source-branch deletion, no orphaned commits.

Customer evidence

Two customer reports on the auto-rebase rollout issue #524048:

  • hij1nx on 18.11 — provided rails-console output showing MR B with merge_commit_sha = nil and merged_commit_sha equal to MR A's merge_commit_sha.
  • joshk0 on 18.10 — "+1, similar experience of a disappearing commit."

Reproduction

Repro script tmp/issue-524048-toggle-collapse.rb produces all three of the customer's DB signatures (A, B, C) end-to-end through MergeService.execute on local GDK. Before this MR, B's signature reproduces (state=merged, mcs=nil, mcsd=A_mcs). After this MR, B stays opened (state=opened, mcs=nil, mcsd=nil), and the script prints B signature: false / ALL THREE: false.

Site 1 of three

The issue documents three independent fix sites, each defending a different layer. This MR implements Site 1 only — the earliest, most root-cause-targeted guard. Sites 2 and 3 will follow as defense-in-depth:

  • Site 1 (this MR): CreateRefService#maybe_rebase! raises when rebase collapses to first_parent_sha. Closes the data-loss path.
  • Site 2 (planned): from_source_branch.rb#fast_forward! verifies the target ref actually advanced (mirrors from_train_ref.rb:48-50).
  • Site 3 (planned): MergeService#commit drops .compact so nils get written truthfully.

Site 1 alone is sufficient to prevent the data loss. Sites 2 and 3 add belt-and-braces protection and schema honesty.

Backports

To be requested for 18.10 and 18.11 stable once this lands on master.

Test plan

  • New spec: spec/services/merge_requests/create_ref_service_spec.rb — "when the source contains no unique changes against the target" returns :error.
  • Full file: 22 examples pass (was 21 before).
  • Related: ee/spec/services/ee/merge_requests/create_ref_service_spec.rb (4 examples), spec/services/merge_requests/merge_strategies/from_source_branch_spec.rb (18 examples) — all green.
  • E2E: re-running tmp/issue-524048-toggle-collapse.rb against the fixed code, B no longer matches the bug signature.

MR acceptance checklist

  • Tests added
  • Changelog trailer present (Changelog: fixed)
  • Backport MRs for 18.10 and 18.11 (follow-up)

Merge request reports