/* Allocate a unique recirculation id for the given set of flow metadata.
* The ID space is 2^^32, so there should never be a situation in which all
* the IDs are used up. We loop until we find a free one.
/* Allocate a unique recirculation id for the given set of flow metadata.
* The ID space is 2^^32, so there should never be a situation in which all
* the IDs are used up. We loop until we find a free one.