Advance pindexLastCommonBlock for blocks in chainActive

This prevents an edge case where a block downloaded and pruned
in-between successive calls to FindNextBlocksToDownload could
cause the block to be unnecessarily re-requested.

Github-Pull: #6233
Rebased-From: 3e9143386a90e508c8d41719294db11264f5f0a0

src/main.cpp

@@ -434,13 +434,14 @@ void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<CBl
 
         // Iterate over those blocks in vToFetch (in forward direction), adding the ones that
         // are not yet downloaded and not in flight to vBlocks. In the mean time, update
-        // pindexLastCommonBlock as long as all ancestors are already downloaded.
+        // pindexLastCommonBlock as long as all ancestors are already downloaded, or if it's
+        // already part of our chain (and therefore don't need it even if pruned).
         BOOST_FOREACH(CBlockIndex* pindex, vToFetch) {
             if (!pindex->IsValid(BLOCK_VALID_TREE)) {
                 // We consider the chain that this peer is on invalid.
                 return;
             }
-            if (pindex->nStatus & BLOCK_HAVE_DATA) {
+            if (pindex->nStatus & BLOCK_HAVE_DATA || chainActive.Contains(pindex)) {
                 if (pindex->nChainTx)
                     state->pindexLastCommonBlock = pindex;
             } else if (mapBlocksInFlight.count(pindex->GetBlockHash()) == 0) {