net: check for empty header before calling FillBlock

Crypt-iQ · fanquake · commit 569ceb0df46f · 2025-09-12T15:44:03.000+01:00
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 <gsanders87@gmail.com> Github-Pull: #33296 Rebased-From: 5e585a0
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
@@ -3315,6 +3315,16 @@ 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, "previous compact block reconstruction attempt failed");
+            LogDebug(BCLog::NET, "Peer %d sent compact block transactions multiple times", 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,
@@ -3326,6 +3336,9 @@ 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);
