Trigger full sync only on old GossipTimestampFilters

Previously, upon receipt of a GossipTimestampFilter message, we would
immediately start unloading the entire network graph on our
unsuspecting peer.

This commit modifies our behavior to only do so if the timestamp of
the filter message is at least six hour old. Otherwise, we only send
updated sync data as it comes in.

Author: arik-so

lightning/src/ln/peer_handler.rs

@@ -509,9 +509,18 @@ impl fmt::Display for PeerHandleError {
 	}
 }
 
+/// Internal struct for keeping track of the gossip syncing progress with a given peer
 enum InitSyncTracker{
+	/// Only sync ad-hoc gossip as it comes in, do not send historical gossip.
+	/// Upon receipt of a GossipTimestampFilter message, this is the default initial state if the
+	/// contained timestamp is less than 6 hours old.
 	NoSyncRequested,
+	/// Send historical gossip starting at the given channel id, which gets incremented as the
+	/// gossiping progresses.
+	/// Upon receipt of a GossipTimestampFilter message, this is the default initial state if the
+	/// contained timestamp is at least 6 hours old, and the initial channel id is set to 0.
 	ChannelsSyncing(u64),
+	/// Once the channel announcements and updates finish syncing, the node announcements are synced.
 	NodesSyncing(NodeId),
 }
 
@@ -1727,7 +1736,24 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, OM: Deref, L: Deref, CM
 			if peer_lock.their_features.as_ref().unwrap().supports_gossip_queries() &&
 				!peer_lock.sent_gossip_timestamp_filter {
 				peer_lock.sent_gossip_timestamp_filter = true;
-				peer_lock.sync_status = InitSyncTracker::ChannelsSyncing(0);
+
+				#[allow(unused_mut)]
+				let mut should_do_full_sync = true;
+				#[cfg(feature = "std")]
+				{
+					// Forward ad-hoc gossip if the timestamp range is less than six hours ago.
+					// Otherwise, do a full sync.
+					use std::time::{SystemTime, UNIX_EPOCH};
+					let full_sync_threshold = SystemTime::now().duration_since(UNIX_EPOCH).expect("Time must be > 1970").as_secs() - 6 * 3600;
+					if (_msg.first_timestamp as u64) > full_sync_threshold {
+						should_do_full_sync = false;
+					}
+				}
+				if should_do_full_sync {
+					peer_lock.sync_status = InitSyncTracker::ChannelsSyncing(0);
+				} else {
+					peer_lock.sync_status = InitSyncTracker::NoSyncRequested;
+				}
 			}
 			return Ok(None);
 		}