Similar to SERVER-117612 (ConvertToCappedCoordinator) and SERVER-117613 (CreateCollectionCoordinator), RefineCollectionShardKeyCoordinator may have a gap in its _mustAlwaysMakeProgress() logic that could allow the coordinator to exit without resuming migrations.

Suspected Issue:

The current implementation uses > instead of >=:

bool _mustAlwaysMakeProgress() override {    
    return _doc.getPhase() >  Phase::kRemoteIndexValidation;
}

Migrations are stopped at kRemoteIndexValidation. However, because the check uses > (greater than) instead of >= (greater than or equal), _mustAlwaysMakeProgress() returns false during the exact phase where migrations are frozen.

If a non-retriable error occurs at kRemoteIndexValidation after stopMigrations() is called and the cleanup fails before resumeMigrations() executes, the coordinator may exit leaving migrations frozen.