After working for awhile toward the goals outlined in Child Pays For Parent it has become apparent there was some problems with my initial design. Originally, I was hoping to make a simple copy of the BlockAssembler code. Then remove from it all the complexity surrounding mempool packages, and convert it to a long-lived object which can have transactions added and removed from it.

However, the BlockAssembler needs to have a couple things happend:

  1. It needs to know which block it’s building on top of for providing hashPrevBlock, and ensuring that only valid transactions are being mined.
  2. It needs to be able to be disposed of, or cleared, when a new chainTip is found.

Unfortunately, the mempool is currently a global object. Meaning it’s initialization order is indeterminate with other global objects. Therefor it cannot create a BlockAssembler on startup given the dependency on a chainTip. Additionally, the mempool is not currently notified when a the chainTip is updated. Instead, it has calls such as removeForBlock which reconcile the mempool to the chainstate. but do not provide block information. Additionally, these calls occur multiple times during ActivateBestChain rather than only upon completion.

While we could add new notifications to the mempool to handle this, it is not desirable. The reason being, is that when a new block arrives, we should immediately dispose of the current BlockAssembler and create a new blank one. If there are any transactions remaining in the mempool, these need to be added to the new BlockAssembler. Now, there normally should not be any, but in the case where there is a large mempool due to a spam attack, we want to be able to do this reprocessing out-of-line with block processing for ActivateBestChain.

As such, it seems to be a better idea to:

  1. Introduce a BlockCandidate, which is responsible for maintaining a CBlockHeader (minus the nonce), the coinbase, and a list of transactions. It will ensure that only transactions which would not render the block invalid may be added or removed. This take many of the responsibilities of BlockAssembler.
  2. Introduce a Blockworks which manages the lifetime of all BlockCandidate, shovelling transations from the mempool into the active BlockCandidate, and handling getblocktemplate RPC requests.

This gives us the benefit that all our quality-of-service priotiziation code can be in one place, handled outside of the critical path. Additionally, all the validity checks for the block candidate will be self-contained. The ultimate result will be separation of concerns:

  1. BlockCandiate will be limited to ensuring its own consistency.
  2. Blockworks will handle all transaciton prioritization code, and BlockCandidate lifecycles.
  3. Mempool will be limited to ensuring its own consistency.