Help nodes falling-behind to catch up about dusk-blockchain

[jules]

Worth mentioning that the first paragraph denotes an issue which was solved earlier, which caused some nodes to not receive an agreed-upon block. When another node would sync with one of these nodes, they would only receive a portion of the blocks and then get stuck, waiting for the timeout.

What Tog is referring to is more like a consensus-related enhancement rather than a fix for a bug.

[jules]

Upon further discussion with Tog, this catch-up protocol requires implementation of a wire message to request agreement events for a certain round. Nodes should thus cache their received agreement events for the last round, in case any lagging node needs them.

Note that this is only necessary if the lagging node is only behind one round. If it is more than one round behind, it can simply sync it's blockchain to the tip, minus one block, and then request agreement events for the last block to catch up.

Quoting @toghrulmaharramov:

If the node falls back by more than a block, it is essentially a bootstrap process which requires the nodes to download the blocks preceeding the highest one with only the certificates and then the highest one with the certificate and the agreement messages