Reduce getheaders spam by serializing getheader requests per peer

Introduces a counter for getheader requests that have been sent to
a peer but are pending response, reducing the number of parallel
requests a node pushes out to its peers when needing to sync large
amounts of headers. All getheader requests are serialized during
initial sync, except when a non-connecting header is received,
allowing the node to resolve issues with peers sending faulty
blocks using the DoS mechanism, and when we get an inv for a block
that we do not know, because it's possible we're only connected to
legacy nodes that do not implement header announcement properly.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/net.h b/src/net.h
index 868be7fd9..f5841a314 100644
--- a/src/net.h
+++ b/src/net.h
@@ -681,6 +681,9 @@ public:
     CAmount lastSentFeeFilter;
     int64_t nextSendTimeFeeFilter;
 
+    // Counts getheaders requests sent to this peer
+    std::atomic<int64_t> nPendingHeaderRequests;
+
     // Alert relay
     std::vector<CAlert> vAlertToSend;