Respond to getheaders if we have sufficient chainwork

Previously, we would check to see if we were in IBD and ignore getheaders
requests accordingly. However, the IBD criteria -- an optimization mostly
targeted at behavior when we have peers serving us many blocks we need to
download -- is difficult to reason about in edge-case scenarios, such as if the
network were to go a long time without any blocks found and nodes getting
restarted during that time.

To make things simpler to reason about, just use nMinimumChainWork as our
anti-DoS threshold; as long as our chain has that much work, it should be fine
to respond to a peer asking for our headers (and this should allow such a peer
to request blocks from us if needed).

Author: sdaftuar

src/net_processing.cpp

@@ -3189,9 +3189,23 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
             return;
         }
 
+        if (fImporting || fReindex) {
+            LogPrint(BCLog::NET, "Ignoring getheaders from peer=%d while importing/reindexing\n", pfrom.GetId());
+            return;
+        }
+
         LOCK(cs_main);
-        if (m_chainman.ActiveChainstate().IsInitialBlockDownload() && !pfrom.HasPermission(NetPermissionFlags::Download)) {
-            LogPrint(BCLog::NET, "Ignoring getheaders from peer=%d because node is in initial block download\n", pfrom.GetId());
+
+        // Note that if we were to be on a chain that forks from the checkpointed
+        // chain, then serving those headers to a peer that has seen the
+        // checkpointed chain would cause that peer to disconnect us. Requiring
+        // that our chainwork exceed nMinimumChainWork is a protection against
+        // being fed a bogus chain when we started up for the first time and
+        // getting partitioned off the honest network for serving that chain to
+        // others.
+        if (m_chainman.ActiveTip() == nullptr ||
+                (m_chainman.ActiveTip()->nChainWork < nMinimumChainWork && !pfrom.HasPermission(NetPermissionFlags::Download))) {
+            LogPrint(BCLog::NET, "Ignoring getheaders from peer=%d because active chain has too little work\n", pfrom.GetId());
             return;
         }