Problem

S3 has no native compare-and-swap (CAS) operation. The current S3 backend’s put_ref is inherently racy — concurrent writers can silently overwrite each other’s ref updates, potentially losing commits.

Impact

• Data loss in multi-writer scenarios
• Cannot safely serve as a push target with concurrent users
• Documented limitation but unresolved

Acceptance Criteria

• External coordination mechanism for S3 ref atomicity (DynamoDB, Redis, or similar)
• CAS semantics match filesystem backend behavior
• Configurable: users can opt into the coordination layer
• Graceful degradation if coordination service is unavailable
• Tests covering concurrent ref update race conditions
• Documentation on setup and configuration