Merge #13451: rpc: expose CBlockIndex::nTx in getblock(header)

86edf4a2a5 expose CBlockIndex::nTx in getblock(header) (Gregory Sanders)

Pull request description:

  Recent publication of a weakness in Bitcoin's merkle tree construction demonstrates many SPV applications vulnerable to an expensive to pull off yet still plausible attack: https://bitslog.wordpress.com/2018/06/09/leaf-node-weakness-in-bitcoin-merkle-tree-design/

  Including the coinbase in the txoutproof seems the most effective fix, however results in a significant efficiency downgrade. Transactors will not even know a priori what the size of their proof will be within a couple orders of magnitude, unless they use the mid-state of SHA2 as detailed in the blog post.

  Some applications, like Elements blockchain platform that take SPV-style proofs have optional access to a bitcoind to verify these proofs of inclusion and check depth in the chain. Returning `CBlockIndex::nTx` would allow an extremely easy and compact way of checking the depth of the tree, with no additional overhead to the codebase, and works with pruned nodes.

  `getblockheader` is arguably not the place for it, but as mentioned before, is a natural workflow for us checking depth of a block in a possibly pruned node.

  We should also ensure that `verifytxoutproof` ends up validating this depth fact as well, but left this for another PR.

Tree-SHA512: af4cf48e704c6088f8da06a477fda1aaa6f8770cee9b876c4465d1075966d6a95831a88817673fe5a0d6bbcdc1ffcbc1892e2be0d838c60fc6958d33eacdcc14

src/rpc/blockchain.cpp

@@ -95,6 +95,7 @@ UniValue blockheaderToJSON(const CBlockIndex* blockindex)
     result.pushKV("bits", strprintf("%08x", blockindex->nBits));
     result.pushKV("difficulty", GetDifficulty(blockindex));
     result.pushKV("chainwork", blockindex->nChainWork.GetHex());
+    result.pushKV("nTx", (uint64_t)blockindex->nTx);
 
     if (blockindex->pprev)
         result.pushKV("previousblockhash", blockindex->pprev->GetBlockHash().GetHex());
@@ -140,6 +141,7 @@ UniValue blockToJSON(const CBlock& block, const CBlockIndex* blockindex, bool tx
     result.pushKV("bits", strprintf("%08x", block.nBits));
     result.pushKV("difficulty", GetDifficulty(blockindex));
     result.pushKV("chainwork", blockindex->nChainWork.GetHex());
+    result.pushKV("nTx", (uint64_t)blockindex->nTx);
 
     if (blockindex->pprev)
         result.pushKV("previousblockhash", blockindex->pprev->GetBlockHash().GetHex());
@@ -694,6 +696,7 @@ static UniValue getblockheader(const JSONRPCRequest& request)
             "  \"bits\" : \"1d00ffff\", (string) The bits\n"
             "  \"difficulty\" : x.xxx,  (numeric) The difficulty\n"
             "  \"chainwork\" : \"0000...1f3\"     (string) Expected number of hashes required to produce the current chain (in hex)\n"
+            "  \"nTx\" : n,             (numeric) The number of transactions in the block.\n"
             "  \"previousblockhash\" : \"hash\",  (string) The hash of the previous block\n"
             "  \"nextblockhash\" : \"hash\",      (string) The hash of the next block\n"
             "}\n"
@@ -782,6 +785,7 @@ static UniValue getblock(const JSONRPCRequest& request)
             "  \"bits\" : \"1d00ffff\", (string) The bits\n"
             "  \"difficulty\" : x.xxx,  (numeric) The difficulty\n"
             "  \"chainwork\" : \"xxxx\",  (string) Expected number of hashes required to produce the chain up to this block (in hex)\n"
+            "  \"nTx\" : n,             (numeric) The number of transactions in the block.\n"
             "  \"previousblockhash\" : \"hash\",  (string) The hash of the previous block\n"
             "  \"nextblockhash\" : \"hash\"       (string) The hash of the next block\n"
             "}\n"

test/functional/feature_pruning.py

@@ -260,10 +260,17 @@ class PruneTest(BitcoinTestFramework):
         # should not prune because chain tip of node 3 (995) < PruneAfterHeight (1000)
         assert_raises_rpc_error(-1, "Blockchain is too short for pruning", node.pruneblockchain, height(500))
 
+        # Save block transaction count before pruning, assert value
+        block1_details = node.getblock(node.getblockhash(1))
+        assert_equal(block1_details["nTx"], len(block1_details["tx"]))
+
         # mine 6 blocks so we are at height 1001 (i.e., above PruneAfterHeight)
         node.generate(6)
         assert_equal(node.getblockchaininfo()["blocks"], 1001)
 
+        # Pruned block should still know the number of transactions
+        assert_equal(node.getblockheader(node.getblockhash(1))["nTx"], block1_details["nTx"])
+
         # negative heights should raise an exception
         assert_raises_rpc_error(-8, "Negative", node.pruneblockchain, -10)
 

test/functional/rpc_blockchain.py

@@ -217,6 +217,7 @@ class BlockchainTest(BitcoinTestFramework):
         assert_equal(header['confirmations'], 1)
         assert_equal(header['previousblockhash'], secondbesthash)
         assert_is_hex_string(header['chainwork'])
+        assert_equal(header['nTx'], 1)
         assert_is_hash_string(header['hash'])
         assert_is_hash_string(header['previousblockhash'])
         assert_is_hash_string(header['merkleroot'])