A pinned syncer is an ActiveSyncer that is configured to always remain
active for the lifetime of the connection. Pinned syncers do not count
towards the total NumActiveSyncer count, which are rotated periodically.
This features allows nodes to more tightly synchronize their routing
tables by ensuring they are always receiving gossip from distinguished
subset of peers.
We move from our legacy way of interpreting ReplyChannelRange messages
which was incorrect. Previously, we'd rely on the Complete field of the
ReplyChannelRange message to determine when our peer had sent all of
their replies. Now, we properly adhere to the specification by
interpreting the block ranges of these messages as intended.
Due to the large number of nodes deployed with the previous method, we
still maintain and detect when we are communicating with them, such that
we are still able to sync with them for backwards compatibility.
This ensures that the graph synced status is marked true at some point
once a historical sync has completed. Before this commit, a stalled
historical sync could cause us to never mark the graph as synced.
TestSyncManagerHistoricalSyncOnReconnect tests that the sync manager will
re-trigger a historical sync when a new peer connects after a historical
sync has completed, but we have lost all peers.
In this commit, we begin to queue any active syncers until the initial
historical sync has completed. We do this to ensure we can properly
handle any new channel updates at tip. This is required for fresh nodes
that are syncing the channel graph for the first time. If we begin
accepting updates at tip while the initial historical sync is still
ongoing, then we risk not processing certain updates since we've yet to
learn of the channels themselves.
In this commit, we add logic to handle a peer with whom we're performing
an initial historical sync disconnecting. This is required to ensure we
get as much of the graph as possible when starting a fresh node. It will
also serve useful to ensure we do not get stalled once we prevent active
GossipSyncers from starting until the initial historical sync has
completed.
Now that the roundRobinHandler is no longer present, this commit aims to
clean up and simplify some of the logic surrounding initializing/tearing
down new/stale GossipSyncers from the SyncManager. Along the way, we
also synchronize these calls with the syncerHandler, which will serve
useful in future work that allows us to recovery from initial historical
sync disconnections.
Since ActiveSync GossipSyncers no longer synchronize our state with the
remote peers, none of the logic surrounding the round-robin is required
within the SyncManager.
In this commit, we introduce a new subsystem for the gossiper: the
SyncManager. This subsystem is a major overhaul on the way the daemon
performs the graph query sync state machine with peers.
Along with this subsystem, we also introduce the concept of an active
syncer. An active syncer is simply a GossipSyncer currently operating
under an ActiveSync sync type. Before this commit, all GossipSyncer's
would act as active syncers, which means that we were receiving new
graph updates from all of them. This isn't necessary, as it greatly
increases bandwidth usage as the network grows. The SyncManager changes
this by requiring a specific number of active syncers. Once we reach
this specified number, any future peers will have a GossipSyncer with a
PassiveSync sync type.
It is responsible for three main things:
1. Choosing different peers randomly to receive graph updates from to
ensure we don't only receive them from the same set of peers.
2. Choosing different peers to force a historical sync with to ensure we
have as much of the public network as possible. The first syncer
registered with the manager will also attempt a historical sync.
3. Managing an in-order queue of active syncers where the next cannot be
started until the current one has completed its state machine to ensure
they don't overlap and request the same set of channels, which
significantly reduces bandwidth usage and addresses a number of issues.
