net: check for empty header before calling FillBlock

Previously in debug builds, this would cause an Assume crash if
FillBlock had been called previously. This could happen when multiple
blocktxn messages were received.

Co-Authored-By: Greg Sanders <[email protected]>

Author: Crypt-iQ

src/net_processing.cpp

@@ -3329,6 +3329,16 @@ void PeerManagerImpl::ProcessCompactBlockTxns(CNode& pfrom, Peer& peer, const Bl
 
         PartiallyDownloadedBlock& partialBlock = *range_flight.first->second.second->partialBlock;
 
+        if (partialBlock.header.IsNull()) {
+            // It is possible for the header to be empty if a previous call to FillBlock wiped the header, but left
+            // the PartiallyDownloadedBlock pointer around (i.e. did not call RemoveBlockRequest). In this case, we
+            // should not call LookupBlockIndex below.
+            RemoveBlockRequest(block_transactions.blockhash, pfrom.GetId());
+            Misbehaving(peer, "invalid compact block/non-matching block transactions");
+            LogDebug(BCLog::NET, "Peer %d sent compact block transactions multiple times\n", pfrom.GetId());
+            return;
+        }
+
         // We should not have gotten this far in compact block processing unless it's attached to a known header
         const CBlockIndex* prev_block{Assume(m_chainman.m_blockman.LookupBlockIndex(partialBlock.header.hashPrevBlock))};
         ReadStatus status = partialBlock.FillBlock(*pblock, block_transactions.txn,
@@ -3340,6 +3350,9 @@ void PeerManagerImpl::ProcessCompactBlockTxns(CNode& pfrom, Peer& peer, const Bl
         } else if (status == READ_STATUS_FAILED) {
             if (first_in_flight) {
                 // Might have collided, fall back to getdata now :(
+                // We keep the failed partialBlock to disallow processing another compact block announcement from the same
+                // peer for the same block. We let the full block download below continue under the same m_downloading_since
+                // timer.
                 std::vector<CInv> invs;
                 invs.emplace_back(MSG_BLOCK | GetFetchFlags(peer), block_transactions.blockhash);
                 MakeAndPushMessage(pfrom, NetMsgType::GETDATA, invs);