How to get block height from CTransactionRef object in Bitcoin C++ code?

Question

I'm working with the Bitcoin core code. I have a CTransactionRef object and I'm trying to determine what the block height of the block containing the transaction is. What would be the most efficient way to determine the height given the starting point of a CTransactionRef?

Answer 1

The block height for a transaction may be recorded in two different places: the txindex, or in the wallet. Both of these have functions that can take a transaction hash and return information about the transaction, including the transaction itself, the block hash it is confirmed in, etc. However these are not always available as they are optional features, nor are they available in all parts of the codebase. It depends on what you are actually trying to do.

Without a txindex or a wallet, it's not possible to get the block height as this data is not recorded anywhere, and generally does not need to be.

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If you are in the wallet, i.e. have a CWallet available, then you can do something like:

// Assuming wallet is a CWallet& that already exists
// Assuming tx is a CTransactionRef that already exists
const CWalletTx* wtx = wallet.GetWalletTx(tx->GetHash());
if (wtx && (auto conf = wtx->state<TxStateConfirmed>())) {
    uint256 block_hash = conf->confirmed_block_hash;
    int block_height;
    chain().findBlock(block_hash, FoundBlock().height(block_height));
}

The height will be placed into block_height.

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If the txindex is available, you could do something like:

// Assuming tx is a CTranasctionRef that already exists
// Assuming chainman is a CChainMan that already exists
CTransactionRef dummy;
uint256 block_hash;
g_txindex->FindTx(tx->GetHash(), block_hash, dummy);
int block_height = chainman.m_blockman.LookupBlockIndex(block_hash)->nHeight;

The FindTx function will lookup the transaction in the txindex given by the hash in the first parameter. The block hash will be placed into the second parameter, and the transaction that was found into the third parameter.