unpinAll method
Releases every pin held by by, e.g. when an account logs out. Returns
the number of blobs that lost a pin.
Implementation
@override
Future<int> unpinAll(String by) => _required.transaction((txn) async {
var released = 0;
for (final snapshot in await _meta.find(txn)) {
final holders = holdersOf(snapshot.value);
if (!holders.remove(by)) continue;
await snapshot.ref.put(txn, withHolders(snapshot.value, holders));
released++;
}
return released;
});