Merge bitcoin#33296: net: check for empty header before calling FillBlock

Backport of bitcoin#33296 (commits 5e585a0, 8b62647), slimmed for Dash. Guards ProcessCompactBlockTxns against re-entering PartiallyDownloadedBlock::FillBlock when a prior reconstruction attempt returned READ_STATUS_FAILED and left the (now header-less) partial block in flight; such a second blocktxn now discourages and disconnects the peer with a clear reason. Adds the test_multiple_blocktxn_response regression test to p2p_compactblocks.py.

Dash adaptations: no LookupBlockIndex/DEPLOYMENT_SEGWIT (Dash's FillBlock takes no segwit_active parameter), and Misbehaving(pfrom.GetId(), 100, ...).

Note on the double-blocktxn SIGABRT: on this branch that crash is already eliminated by bitcoin#26898 (FillBlock returns READ_STATUS_INVALID instead of asserting on a null header, which the caller then handles by discouraging and removing the block request). Upstream's guard exists to avoid a LookupBlockIndex/Assume() on the wiped header, which Dash does not perform. This guard is therefore a faithful belt-and-suspenders port -- it rejects the duplicate earlier with a clearer message and skips a wasted reconstruction pass -- rather than the fix for a still-reachable crash.

Author: PastaPastaPasta

src/net_processing.cpp

@@ -3706,6 +3706,17 @@ 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. a prior reconstruction attempt returned
+            // READ_STATUS_FAILED without calling RemoveBlockRequest). Don't attempt to reconstruct again.
+            RemoveBlockRequest(block_transactions.blockhash, pfrom.GetId());
+            Misbehaving(pfrom.GetId(), 100, "previous compact block reconstruction attempt failed");
+            LogPrint(BCLog::NET, "Peer %d sent compact block transactions multiple times\n", pfrom.GetId());
+            return;
+        }
+
         ReadStatus status = partialBlock.FillBlock(*pblock, block_transactions.txn);
         if (status == READ_STATUS_INVALID) {
             RemoveBlockRequest(block_transactions.blockhash, pfrom.GetId()); // Reset in-flight state in case Misbehaving does not result in a disconnect

test/functional/p2p_compactblocks.py

@@ -556,6 +556,45 @@ def test_incorrect_blocktxn_response(self, test_node):
         test_node.send_and_ping(msg_block(block))
         assert_equal(int(node.getbestblockhash(), 16), block.sha256)
 
+    # Multiple blocktxn responses for the same in-flight block must not cause a
+    # re-entrant FillBlock (which previously hit an assert); the second response
+    # should instead get the peer disconnected (see bitcoin#33296).
+    def test_multiple_blocktxn_response(self, test_node):
+        node = self.nodes[0]
+        utxo = self.utxos[0]
+
+        block = self.build_block_with_transactions(node, utxo, 2)
+
+        # Send compact block, prefilling only the coinbase so the node has to
+        # request the other two transactions via getblocktxn.
+        comp_block = HeaderAndShortIDs()
+        comp_block.initialize_from_block(block, prefill_list=[0])
+        test_node.send_and_ping(msg_cmpctblock(comp_block.to_p2p()))
+        with p2p_lock:
+            assert "getblocktxn" in test_node.last_message
+            absolute_indexes = test_node.last_message["getblocktxn"].block_txn_request.to_absolute()
+        assert_equal(absolute_indexes, [1, 2])
+
+        # Respond with the transactions in the wrong order so reconstruction
+        # fails (merkle root mismatch), triggering the getdata fallback while
+        # the PartiallyDownloadedBlock is left in flight.
+        msg = msg_blocktxn()
+        msg.block_transactions = BlockTransactions(block.sha256, [block.vtx[2]] + [block.vtx[1]])
+        test_node.send_and_ping(msg)
+
+        # Tip should not have updated
+        assert_equal(int(node.getbestblockhash(), 16), block.hashPrevBlock)
+
+        # We should receive a getdata request for the full block
+        test_node.wait_for_getdata([block.sha256], timeout=10)
+        assert test_node.last_message["getdata"].inv[0].type == MSG_BLOCK
+
+        # Sending the same blocktxn again must get the peer disconnected rather
+        # than re-entering FillBlock on the (now header-less) partial block.
+        with node.assert_debug_log(['previous compact block reconstruction attempt failed']):
+            test_node.send_message(msg)
+        test_node.wait_for_disconnect()
+
     def test_getblocktxn_handler(self, test_node):
         node = self.nodes[0]
         # dashd will not send blocktxn responses for blocks whose height is
@@ -921,5 +960,10 @@ def run_test(self):
         self.log.info("Testing high-bandwidth mode states via getpeerinfo...")
         self.test_highbandwidth_mode_states_via_getpeerinfo()
 
+        self.log.info("Testing handling of multiple blocktxn responses...")
+        # Earlier tests may have left self.test_node disconnected; use a fresh peer.
+        self.test_node = self.nodes[0].add_p2p_connection(TestP2PConn())
+        self.test_multiple_blocktxn_response(self.test_node)
+
 if __name__ == '__main__':
     CompactBlocksTest().main()