# ADR-003: Force-with-Lease for Forward Sync

**Status:** Accepted
**Date:** 2026-01

## Context

Forward sync pushes monorepo changes to the subrepo. If someone pushes directly to the subrepo between when josh-sync reads its HEAD and when josh-sync pushes, a naive `git push` would overwrite their work. A `git push --force` would be worse — it would silently destroy concurrent changes.

## Decision

Use `git push --force-with-lease=refs/heads/<branch>:<expected-sha>` for all forward sync pushes. The expected SHA is recorded at the start of the sync operation (the "lease").

### How it works

1. Record subrepo HEAD SHA before any operations: `subrepo_sha=$(subrepo_ls_remote "$branch")`
2. Perform merge of monorepo changes onto subrepo state
3. Push with explicit lease: `--force-with-lease=refs/heads/main:<subrepo_sha>`
4. If the subrepo HEAD changed since step 1, git rejects the push
5. Josh-sync reports `lease-rejected` and retries on the next run

## Consequences

**Positive:**
- Never overwrites concurrent changes — git atomically checks the expected SHA
- Explicit SHA lease (not just "current tracking ref") prevents stale-ref bugs
- Failed leases are retried on the next sync run — no data loss, just delay
- Works correctly with josh-proxy's SHA mapping

**Negative:**
- Lease-rejected means the sync run did work that gets discarded (clone, merge, etc.)
- Persistent lease failures indicate a concurrent push pattern that needs investigation
- Requires the `--force-with-lease` flag with explicit SHA — the shorthand form (`--force-with-lease` without `=`) is unsafe because it uses the local tracking ref, which may be stale