Merge bitcoin#8054: net: Avoid duplicate getheaders requests.

f93c2a1 net: Avoid duplicate getheaders requests. (Daniel Kraft)

src/main.cpp

@@ -5084,6 +5084,13 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             return true;
         }
 
+        // If we already know the last header in the message, then it contains
+        // no new information for us.  In this case, we do not request
+        // more headers later.  This prevents multiple chains of redundant
+        // getheader requests from running in parallel if triggered by incoming
+        // blocks while the node is still in initial headers sync.
+        const bool hasNewHeaders = (mapBlockIndex.count(headers.back().GetHash()) == 0);
+
         CBlockIndex *pindexLast = NULL;
         BOOST_FOREACH(const CBlockHeader& header, headers) {
             CValidationState state;
@@ -5104,7 +5111,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         if (pindexLast)
             UpdateBlockAvailability(pfrom->GetId(), pindexLast->GetBlockHash());
 
-        if (nCount == MAX_HEADERS_RESULTS && pindexLast) {
+        if (nCount == MAX_HEADERS_RESULTS && pindexLast && hasNewHeaders) {
             // Headers message had its maximum size; the peer may have more headers.
             // TODO: optimize: if pindexLast is an ancestor of chainActive.Tip or pindexBestHeader, continue
             // from there instead.