+ } else if last.ModifiedAt == nil {
+ return fmt.Errorf("BUG: Last collection on the page (%s) has no modified_at timestamp; cannot make progress", last.UUID)
+ } else if len(page.Items) > 0 && *last.ModifiedAt == filterTime {
+ // If we requested time>=X and never got a
+ // time>X then we might not have received all
+ // items with time==X yet. Switch to
+ // gettingExactTimestamp mode (if we're not
+ // there already), advancing our UUID
+ // threshold with each request, until we get
+ // an empty page.
+ gettingExactTimestamp = true
+ params.Filters = []arvados.Filter{{
+ Attr: "modified_at",
+ Operator: "=",
+ Operand: filterTime,
+ }, {
+ Attr: "uuid",
+ Operator: ">",
+ Operand: last.UUID,
+ }}
+ } else if gettingExactTimestamp {
+ // This must be an empty page (in this mode,
+ // an unequal timestamp is impossible) so we
+ // can start getting pages of newer
+ // collections.
+ gettingExactTimestamp = false
+ params.Filters = []arvados.Filter{{
+ Attr: "modified_at",
+ Operator: ">",
+ Operand: filterTime,
+ }}
+ } else {
+ // In the normal case, we know we have seen
+ // all collections with modtime<filterTime,
+ // but we might not have seen all that have
+ // modtime=filterTime. Hence we use >= instead
+ // of > and skip the obvious overlapping item,
+ // i.e., the last item on the previous
+ // page. In some edge cases this can return
+ // collections we have already seen, but
+ // avoiding that would add overhead in the
+ // overwhelmingly common cases, so we don't
+ // bother.
+ filterTime = *last.ModifiedAt
+ params.Filters = []arvados.Filter{{
+ Attr: "modified_at",
+ Operator: ">=",
+ Operand: filterTime,
+ }, {
+ Attr: "uuid",
+ Operator: "!=",
+ Operand: last.UUID,
+ }}