p2p: reject empty getblocktxn requests

A getblocktxn msg is only needed when at least one tx is
missing from a compact block. If no txs are missing, the
block can be reconstructed without sending the request.

This avoids reading the requested block from disk
unnecessarily and also alerts the peer operator about
their node's buggy behavior.

test/functional/p2p_compactblocks.py

@@ -843,6 +843,17 @@ class CompactBlocksTest(BitcoinTestFramework):
         msg = msg_cmpctblock(comp_block.to_p2p())
         test_node.send_await_disconnect(msg)
 
+    def test_empty_getblocktxn_disconnects(self):
+        self.log.info("Testing empty getblocktxn disconnects the peer...")
+        node = self.nodes[0]
+        block_hash = int(node.getbestblockhash(), 16)
+        peer = node.add_p2p_connection(P2PInterface())
+        msg = msg_getblocktxn()
+        msg.block_txn_request = BlockTransactionsRequest(blockhash=block_hash, indexes=[])
+        with node.assert_debug_log(['getblocktxn received with no transaction indexes']):
+            peer.send_without_ping(msg)
+            peer.wait_for_disconnect()
+
     # peer generates a block and sends it to node, which makes the peer a
     # candidate for high-bandwidth 'to' (up to 3 peers according to BIP 152)
     def make_peer_hb_to_candidate(self, node, peer):
@@ -1097,6 +1108,7 @@ class CompactBlocksTest(BitcoinTestFramework):
 
         self.log.info("Testing handling of invalid compact blocks...")
         self.test_invalid_tx_in_compactblock(self.segwit_node)
+        self.test_empty_getblocktxn_disconnects()
 
         # The previous test will lead to a disconnection. Reconnect before continuing.
         self.segwit_node = self.nodes[0].add_p2p_connection(TestP2PConn())