clear

Drop every span this exporter holds and persist the emptied set (#2208).

Removal, not ORSet.empty: an ORSet removal retains causal.context, so the retired dots stay witnessed and a peer re-merging the pre-clear adds is dominated rather than resurrecting them. An emptied-by-reset set would re-mint dots this replica has already used, and a peer whose context already holds one would treat the new span as seen-and-removed — swallowing it silently.

The key is rewritten rather than deleted, because the retained context is what the paragraph above rests on and it lives in those bytes.

One ORSet.removeAll, not a per-element fold. Absorbing a patch is a causal join over the whole set, so removing one element at a time pays a join per element: measured here at DEFAULT_MAX_SPANS that fold was quadratic and cost seconds, which is why the bulk form exists (#2245). The two are otherwise indistinguishable — same elements gone, same dots retired, same retained context, same bytes.

A configured WarpCausalClock's frontier is emptied here too, and its seq left alone. The frontier would otherwise name dots of spans this call just removed, so the next auto-stamped span would carry predecessors that can never resolve — links pointing at deliberately forgotten spans, which is noise rather than causality.

Shares ioMutex with export and merge so a concurrent export cannot land a stale encoded snapshot after the clear.