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

86edf4a2a502416ba8d6cebbce61030992f7ff6f 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

2 files changed, 8 insertions, 0 deletions

diff --git a/test/functional/feature_pruning.py b/test/functional/feature_pruning.py
index 11a52b9ee2..d400507a66 100755
--- a/test/functional/feature_pruning.py
+++ b/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)
 
diff --git a/test/functional/rpc_blockchain.py b/test/functional/rpc_blockchain.py
index 17e24453e5..7acc59c2c6 100755
--- a/test/functional/rpc_blockchain.py
+++ b/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'])