Bip54: add sequence, timestamp (and version) to GBT

BIP54 aims to ensure that the coinbase transaction fields nLockTime and nSequence are properly set by mining pools, which is a straightforward adjustment but necessitates updates to pool software for forward compatibility. Recognizing the absence of widely used open-source reference software for crafting coinbase transactions, the proposal encourages mining pools to adapt their processes to align with future changes outlined in this BIP. The adjustments can be facilitated through the use of new fields added to the getblocktemplate JSON-RPC call, pending support from node software.

The proposed expansion of BIP54 introduces several key changes to the getblocktemplate function, as specified in BIP22. These include the addition of three specific keys: coinbase_locktime, coinbase_sequence, and coinbase_version. Each key corresponds to a particular field in the coinbase transaction, with coinbase_locktime setting the exact nLockTime value that must be used, coinbase_sequence suggesting a preferred nSequence value (while disallowing 0xffffffff), and coinbase_version indicating a recommended nVersion value for the coinbase transaction. These modifications aim to streamline the process of making coinbase transactions compatible with potential future soft forks by pre-emptively adhering to guidelines that might be introduced.

Furthermore, the document elaborates on the rationale behind these specific requirements, emphasizing the importance of compliance for future-proofing transactions against possible protocol changes. It notes the fixed nature of certain coinbase input fields by consensus, such as the prevout txid and vout index, which simplifies their implementation in mining software. Additionally, while there are no current constraints on the coinbase transaction's nVersion, including it in the getblocktemplate ensures that mining software remains up-to-date without necessitating further modifications in response to future soft forks.

For detailed information and discussion, contributors and interested parties are directed to engage via the provided GitHub link: https://github.com/bitcoin/bips/pull/2097 and to review the current version of BIP54 available at: https://github.com/bitcoin/bips/blob/master/bip-0054.md.