A sliding window cache that stores published messages and their first seen time. The module also keeps track of the number of accesses to a message by a peer, thus indirectly tracking the number of IWant requests a peer makes for the same message between two heartbeats.

The module assumes that no two different messages have the same message id. However, the cache stores duplicates; for instance, if add_message id msg topic is called exactly twice, then msg will appear twice in get_message_ids_to_gossip's result (assuming not more than gossip_slots shifts have been executed in the meanwhile).

create ~history_slots ~gossip_slots ~seen_message_slots creates two sliding window caches, one with length history_slots for storing message contents and another with length seen_message for storing seen messages and their first seen times.

When queried for messages to advertise, the cache only returns messages in the last gossip_slots. The gossip_slots must be smaller or equal to history_slots.

The slack between gossip_slots and history_slots accounts for the reaction time between when a message is advertised via IHave gossip, and when the peer pulls it via an IWant command. To see this, if say gossip_slot = history_slots then the messages inserted in cache history_slots heartbeat ticks ago and advertised now will not be available after the next tick, because they are removed from the cache. This means IWant requests from the remote peer for such messages would be unfulfilled and potentially penalizing.

• raises Assert_failure

  when gossip_slots <= 0 || gossip_slots > history_slots

TODO: https://gitlab.com/tezos/tezos/-/issues/5129 Error handling.

Add message to the most recent cache slot. If the message already exists in the cache, the message is not overridden, instead a duplicate message id is stored (the message itself is only stored once).

Get the message associated to the given message id, increase the access counter for the peer requesting the message, and also returns the updated counter.

Get the message ids for the given topic in the last gossip_slots slots of the cache. If there were duplicates added in the cache, then there will be duplicates in the output. There is no guarantee about the order of messages in the output.

get_first_seen_time message_id t returns the time the message with message_id was first seen. Returns None if the message was not seen during the period covered by the sliding window.

seen_message message_id t returns true if the message was seen during the period covered by the sliding window and returns false if otherwise.

Shift the sliding window by one slot (usually corresponding to one heartbeat tick).