mempool: Deprecate the priority mempool

[thanethomson]

In light of ABCI++ landing, we need to strongly consider deprecating and removing the priority mempool as the same functionality can be provided by implementing an app-side mempool with ABCI++.

Definition of Done

When we've either decided that we still need the priority mempool (in which case this issue can just be closed), or we've clearly deprecated it and scheduled it for removal in a specific future release.

Follow-up here, if we decide to remove it, would be to create a separate issue for tracking its removal.

[cmwaters]

Even without an app-side mempool, applications don't need a priority mempool. The proposer would simply request all the txs in prepare proposal, sort through them accordingly and return what it wants in a block.

It seems from talking with others that it's pretty unanimous that abci++ supersedes priority mempool. We don't need to rush this. I would go about adding a deprecation notice in v0.38 and removing it in v0.39. This would also involve modifying CheckTxResponse.

[thanethomson]

It seems reasonable to deprecate priority mempool support in favor of what's possible with ABCI++ Prepare/Process Proposal.

We could mark it as deprecated in v0.37 (deprecated due to ABCI++) and remove it in v0.38? Othewise we'll need to support it (and all the weirdness it could cause in interaction with ABCI++ uses) through the v0.38 release cycle.

[rootulp]

Even without an app-side mempool, applications don't need a priority mempool. The proposer would simply request all the txs in prepare proposal, sort through them accordingly and return what it wants in a block.

Is there an attack vector where a malicious user could spam the mempool with many low fee paying transactions? Without a prioritized mempool, I assume the proposer would need to request all txs in prepare proposal and be forced to sort through all these low fee paying transactions.

Is there a benefit to the prioritization occurring in the mempool rather than in prepare proposal?

[adizere]

Relevant -- SDK progress on using a no-op mempool: cosmos/cosmos-sdk#14288