mempool::Service supports inserting transactions into the download/verify stream

[dconnolly]

Motivation

Full transactions received via the InboundService need to be verified and if they pass, inserted into mempool::Storage. A nice place to do this is as a mempool::Request type. From there, the Service will be able to insert this UnminedTx into the mempool::Downloads Stream. mempool::Downloads must be updated to handle a method that accepts the full transaction (verify(tx: UnminedTx) instead of download_and_verify(txid: UnminedTxId)?) vs just the transaction id, and skips the downloading, as it's moot. Everything else remains the same, about the stream inserting the verified transaction into the mempool::Storage if it passes.

Designs

https://jamboard.google.com/d/1HUhdqx8afJUqgiBdBcjCw0aAfDod9R9KBJeAH24U1Qg/viewer?f=0
https://docs.google.com/document/d/10PP6wKlnS5gannRph_EQ8V9diR7byddf3W2YFZ564FU/edit#heading=h.1v3bcppsa6wq

Related Work

Related but not resolving: #2708

[conradoplg]

A rough outline of what I think it will look like:

The transaction downloader is created in

zebra/zebrad/src/components/inbound.rs

Lines 194 to 198 in 9c220af

	let tx_downloads = Box::pin(TxDownloads::new(
	Timeout::new(outbound, TRANSACTION_DOWNLOAD_TIMEOUT),
	Timeout::new(tx_verifier, TRANSACTION_VERIFY_TIMEOUT),
	self.state.clone(),
	));

It would be moved to inside Mempool and initialized there. The only issue is passing everything that it requires: the network (from where it gets outbound), state and transaction verifier services. The network is the tricky part since inbound.rs has a lot of logic to handle waiting it to be ready. Maybe we can simplify this by only creating the mempool service when the network service is ready? Not sure.

Another part that I'm not sure of is how to "drive" the downloader. Currently this is done in the poll_ready of the Inbound:

zebra/zebrad/src/components/inbound.rs

Line 237 in 9c220af

while let Poll::Ready(Some(_)) = tx_downloads.as_mut().poll_next(cx) {}

I guess we could just do the same in the Mempool::poll_ready, but I'm not 100% sure it will work the same way.

A small heads up: #2727 adds an GossipedTx enum that can be used for the request to download/verify a transaction regardless if it's just a txid or the whole tx.