Automatically verify transactions in blocks when the same transaction is already in the mempool

[teor2345]

Motivation

Miners need RPCs and verification to be fast, to improve profits.

When Zebra has already verified a transaction in the mempool, we know it will be valid in a block. (The mempool checks are stricter than the block transaction checks.)

So we can save time by automatically approving any block transaction verify requests that match a transaction already in the mempool.

Security

We will need to test that the mempool checks are stricter than the block transaction checks. Otherwise, we could approve invalid blocks.

[daira]

This only applies to non-contextual checks, right? Otherwise you have the following condition:

• UTXO $u$ is not spent as of the current tip, and so a transaction $tx$ that spends $u$ can be accepted.
• A block $B$ is mined that spends $u$ in a different transaction $tx'$, that the zebra node never accepted to the mempool.
• Now $tx$ cannot be mined.

The same can happen for nullifiers, and for any other contextual check (e.g. mempool transactions can expire, etc.) It's fine as long as mining $B$ kicks $tx$ out of the mempool and there is no possible race condition.

[daira]

Ah, it sounds as though that was considered in #5376 (comment) .