Modify getblocktxn handler not to drop requests for old blocks

The current getblocktxn implementation drops and ignores requests for old
blocks, which causes occasional sync_block timeouts during the
p2p-compactblocks.py test as reported in
https://github.com/bitcoin/bitcoin/issues/8842.

The p2p-compactblocks.py test setup creates many new blocks in a short
period of time, which can lead to getblocktxn requests for blocks below the
hardcoded depth limit of 10 blocks. This commit changes the getblocktxn
handler not to ignore these requests, so the peer nodes in the test setup
will reliably be able to sync.

The protocol change is documented in BIP-152 update "Allow block responses
to getblocktxn requests" at https://github.com/bitcoin/bips/pull/469.

The protocol change is not expected to affect nodes running outside the test
environment, because there shouldn't normally be lots of new blocks being
rapidly added that need to be synced.

qa/rpc-tests/p2p-compactblocks.py

@@ -589,8 +589,8 @@ class CompactBlocksTest(BitcoinTestFramework):
         assert_equal(int(node.getbestblockhash(), 16), block.sha256)
 
     def test_getblocktxn_handler(self, node, test_node, version):
-        # bitcoind won't respond for blocks whose height is more than 15 blocks
-        # deep.
+        # bitcoind will not send blocktxn responses for blocks whose height is
+        # more than 10 blocks deep.
         MAX_GETBLOCKTXN_DEPTH = 10
         chain_height = node.getblockcount()
         current_height = chain_height
@@ -623,11 +623,17 @@ class CompactBlocksTest(BitcoinTestFramework):
                 test_node.last_blocktxn = None
             current_height -= 1
 
-        # Next request should be ignored, as we're past the allowed depth.
+        # Next request should send a full block response, as we're past the
+        # allowed depth for a blocktxn response.
         block_hash = node.getblockhash(current_height)
         msg.block_txn_request = BlockTransactionsRequest(int(block_hash, 16), [0])
+        with mininode_lock:
+            test_node.last_block = None
+            test_node.last_blocktxn = None
         test_node.send_and_ping(msg)
         with mininode_lock:
+            test_node.last_block.block.calc_sha256()
+            assert_equal(test_node.last_block.block.sha256, int(block_hash, 16))
             assert_equal(test_node.last_blocktxn, None)
 
     def test_compactblocks_not_at_tip(self, node, test_node):

src/main.cpp

@@ -5450,7 +5450,19 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         }
 
         if (it->second->nHeight < chainActive.Height() - MAX_BLOCKTXN_DEPTH) {
+            // If an older block is requested (should never happen in practice,
+            // but can happen in tests) send a block response instead of a
+            // blocktxn response. Sending a full block response instead of a
+            // small blocktxn response is preferable in the case where a peer
+            // might maliciously send lots of getblocktxn requests to trigger
+            // expensive disk reads, because it will require the peer to
+            // actually receive all the data read from disk over the network.
             LogPrint("net", "Peer %d sent us a getblocktxn for a block > %i deep", pfrom->id, MAX_BLOCKTXN_DEPTH);
+            CInv vInv;
+            vInv.type = State(pfrom->GetId())->fWantsCmpctWitness ? MSG_WITNESS_BLOCK : MSG_BLOCK;
+            vInv.hash = req.blockhash;
+            pfrom->vRecvGetData.push_back(vInv);
+            ProcessGetData(pfrom, chainparams.GetConsensus(), connman);
             return true;
         }