Eliminate duplicate signature verification of transactions

[akhi3030]

Some time ago we noticed that we are doing unnecessary duplicate signature verification on transactions. In particular, we verify signatures when we receive the tx from the RPC nodes before putting them in the tx pool; and then again when we remove the tx from pool to produce a chunk.

This issue is to track the work to fix this duplicate work.

Our approach will not be to simply disable signature verification when we remove the tx from the pool. This change is risky because a reader might not have a clear picture of when exactly signatures are being verified and we will run the danger of someone accidentally disabling verification on txs before they are inserted in the pool and introducing a huge bug in the protocol.

Instead, we will use the new type idiom to introduce a type ValidatedTransaction that can only be constructed by verifying signatures and the pool will be updated to hold ValidatedTransactions instead. This will provide clear signals to readers that they need to verify signatures before they can insert a tx in the pool and that a tx that they remove from the pool has had its signatures verified. We will also have type safety from the compiler to upload this guarantee.

As this issue is being created after some of the initial work was done, I will recording the various PRs relevant to this work in comments below.

[akhi3030]

Some PRs that have already landed:

• Preliminary works: refactor: expose an internal cloning to the callers #13112; refactor: remove a panic in process_tx_internal by refactoring validate_tx #13000; refactor: some cleanups in process_tx_internal #13003; refactor: split validate_tx into two functions #13009; nit: do not keep copying the hash #13012; refactor: tx_cost does not need to be told if sender_is_receiver #13020; refactor: move tx_cost out of validate_transaction #13027; make TrieUpdate non mut in verify_and_charge_transaction #13029; refactor: expose cloning to callers #13080; refactor: use validated transaction for commiting charging for tx #13088;

• Key PRs: Introduce ValidatedTransaction #13046; Use ValidatedTransactions in the transaction pool #13075;

[akhi3030]

Now we are storing ValidatedTransactions in the tx pool and have eliminated the original duplicate signature verification. However, we have [discovered](#general > Unnecessary signature verification? @ 💬) other potential places where duplicate verification is happening and we will continue to use ValidatedTransaction through the code base to eliminate these.