Merge bitcoin/bitcoin#24178: p2p: Respond to getheaders if we have sufficient chainwork

a35f963edf1a14ee572abea106fb0147575e4694 Add test for getheaders behavior (Suhas Daftuar)
ef6dbe6863d92710fd2da7781e5b2aac87578751 Respond to getheaders if we have sufficient chainwork (Suhas Daftuar)

Pull request description:

  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 are getting restarted during that time.

  To make things simpler to reason about, just use `nMinimumChainWork` as our anti-DoS threshold for responding to a getheaders request; 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).

ACKs for top commit:
  klementtan:
    crACK a35f963edf1a14ee572abea106fb0147575e4694
  naumenkogs:
    ACK a35f963edf1a14ee572abea106fb0147575e4694
  MarcoFalke:
    review ACK a35f963edf1a14ee572abea106fb0147575e4694 🗯

Tree-SHA512: 131e3872e7fe80382ea9c1ec202d6c2dc59c006355c69000aa3f4ce6bccd02a6c689c8cb8f3542b5d9bc48bfa61edcbd1a78535c0b79018971d02bed2655d284

1 files changed, 16 insertions, 2 deletions

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 502bbb5d99..6d17ab4be8 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -3300,9 +3300,23 @@ 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;
         }