Retrieve Store Messages when resuming connectivity

[D4nte]

Problem

When a dApp using js-waku is offline, relay messages are not received.
Same for filter messages.

Solution

To ensure that no message were missed, a store query should be done once a device resume connectivity.

Definition of Done

A guide that demonstrate how to watch loss of connectivity to then do a store query to retrieve any missed messages.
Can start with filter as it may be easier to detected disconnectivity.

Notes

This guide may be accompanied by a set of helpful API to make it easier to enable/use.

The loss of connectivity refers to relay nodes, that forward messages.
From testing js-libp2p behaviour, it does not seem that the peer:disconnect event on the connectManager is triggered when there is a loss of connection.
Most likely, only a failed attempt to contact a peer allows the node to learn that the peer is not reachable anymore.
libp2p-interfaces/pubsub handles this by calling _onPeerDisconnected if a message is failed to send to a peer.

One could either hook this method or watch the number of available relay peers to know whether we are connected.
Then, the start/end time field should be used in the store query to only retrieve messages in the timeframe where we are offline.

Inspire yourself to what status is doing: https://rfc.vac.dev/spec/27/#peer-connectivity

Card to write RFC for relay ping: https://github.com/status-im/js-waku/projects/1#card-65707839

[fryorcraken]

Something to think about is how to handle duplicates (e.g. message from filter and store), see waku-org/pm#9

[fryorcraken]

Duplicate handling could be done via the Deterministic Message Hashing as specified in vacp2p/rfc#573