PGPool is a structure used to manage and update the status of removed snapshots. It does this by maintaining two fields, cached_removed_snaps - the current removed snap set and newly_removed_snaps - newly removed snaps in the last epoch. In OSD::load_pgs the osd map is recovered from the pg’s file store and passed down to OSD::_get_pool where a PGPool object is initialised with the map.
With each new map we receive we call PGPool::update with the new map. In that function we build a list of newly removed snaps (pg_pool_t::build_removed_snaps) and merge that with our cached_removed_snaps. This function included checks to make sure we only do this update when things have changed or there has been a map gap.
When we activate the pg we initialise the snap trim queue from cached_removed_snaps and subtract the purged_snaps we have already purged leaving us with the list of snaps that need to be trimmed. Trimming is later performed asynchronously by the snap_trim_wq.