netsync: process inv announcements when no syncPeer is set

handleInvMsg early-returned for any inv from a non-syncPeer whenever
sm.current() was false, with the comment that it prevents fetching a
mass of orphans. That guard assumes a syncPeer is already fetching
blocks; when syncPeer is nil, the assumption breaks down and the
early return becomes a deadlock.

The deadlock is reachable whenever two nodes connect at equal heights:
startSync exits without picking a syncPeer (no peer is "higher"), and
nothing later promotes the freshly-mined blocks the peer announces via
inv. The pre-verack disconnect and sync-race regression tests in
integration/sync_race_test.go fail consistently because of this.

Only skip the inv when we actually have a syncPeer. When syncPeer is
nil, fall through and let the normal request path queue the block --
the inv is the only signal that there are blocks to fetch.

Author: kcalvinalvin

netsync/manager.go

@@ -1124,9 +1124,10 @@ func (sm *SyncManager) handleInvMsg(imsg *invMsg) {
 		peer.UpdateLastAnnouncedBlock(&invVects[lastBlock].Hash)
 	}
 
-	// Ignore invs from peers that aren't the sync if we are not current.
-	// Helps prevent fetching a mass of orphans.
-	if peer != sm.syncPeer && !sm.current() {
+	// Ignore invs from peers that aren't the sync peer if we are not
+	// current. Helps prevent fetching a mass of orphans. When syncPeer
+	// is nil, accept invs from any peer.
+	if sm.syncPeer != nil && peer != sm.syncPeer && !sm.current() {
 		return
 	}