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

8b62647680 test: send duplicate blocktxn message in p2p_compactblocks.py (Eugene Siegel)
5e585a0fc4 net: check for empty header before calling FillBlock (Eugene Siegel)

Pull request description:

  This avoids an Assume crash if multiple blocktxn messages are received. The first call to `FillBlock` would make the header empty via `SetNull` and the call right before the second `FillBlock` would crash [here](689a321976/src/net_processing.cpp (L3333)) since `LookupBlockIndex` won't find anything. Fix that by checking for an empty header before the Assume.

ACKs for top commit:
  instagibbs:
    reACK 8b62647680
  fjahr:
    tACK 8b62647680
  achow101:
    ACK 8b62647680
  mzumsande:
    Code Review ACK 8b62647680

Tree-SHA512: d43a6f652161d4f7e6137f207a3e95259fc51509279d20347b1698c91179c39c8fcb75d2668b13a6b220f478a03578573208a415804be1d8843acb057fa1a73a

test/functional/p2p_compactblocks.py

@@ -558,6 +558,42 @@ class CompactBlocksTest(BitcoinTestFramework):
         test_node.send_and_ping(msg_block(block))
         assert_equal(node.getbestblockhash(), block.hash_hex)
 
+    # Multiple blocktxn responses will cause a node to get disconnected.
+    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
+        comp_block = HeaderAndShortIDs()
+        comp_block.initialize_from_block(block, prefill_list=[0], use_witness=True)
+        test_node.send_and_ping(msg_cmpctblock(comp_block.to_p2p()))
+        absolute_indexes = []
+        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])
+
+        # Send a blocktxn that does not succeed in reconstruction, triggering
+        # getdata fallback.
+        msg = msg_blocktxn()
+        msg.block_transactions = BlockTransactions(block.hash_int, [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
+        test_node.wait_for_getdata([block.hash_int], timeout=10)
+        assert test_node.last_message["getdata"].inv[0].type == MSG_BLOCK or \
+               test_node.last_message["getdata"].inv[0].type == MSG_BLOCK | MSG_WITNESS_FLAG
+
+        # Send the same blocktxn and assert the sender gets disconnected.
+        with node.assert_debug_log(['previous compact block reconstruction attempt failed']):
+            test_node.send_without_ping(msg)
+        test_node.wait_for_disconnect()
+
     def test_getblocktxn_handler(self, test_node):
         node = self.nodes[0]
         # bitcoind will not send blocktxn responses for blocks whose height is
@@ -977,6 +1013,12 @@ class CompactBlocksTest(BitcoinTestFramework):
         # The previous test will lead to a disconnection. Reconnect before continuing.
         self.segwit_node = self.nodes[0].add_p2p_connection(TestP2PConn())
 
+        self.log.info("Testing handling of multiple blocktxn responses...")
+        self.test_multiple_blocktxn_response(self.segwit_node)
+
+        # The previous test will lead to a disconnection. Reconnect before continuing.
+        self.segwit_node = self.nodes[0].add_p2p_connection(TestP2PConn())
+
         self.log.info("Testing invalid index in cmpctblock message...")
         self.test_invalid_cmpctblock_message()