Module registry 

In-memory SyncRegistry keyed by user_id.

The registry is the optimistic layer of the “one running sync per user” rule (the authoritative layer is the DB query). Insertion is atomic — two threads racing through try_register will see exactly one win.

Structs

AlreadyRunning

Returned by SyncRegistry::try_register when the user already has a running sync. Carries a snapshot of the existing run so the caller can build the 409 conflict response.

RunningSync

One in-flight sync. Stored behind an Arc so snapshot_for does not need the entire entry to live across the call.

RunningSyncSnapshot

Read-only snapshot of a running sync, used by handlers and conflict responses.

SyncRegistry

In-memory registry: user_id → RunningSync.