This technically has a TOCTOU where we sync an Epoch's metadata (signifying we
did sync to that point), then check if the Router was deployed, yet at that
very moment the node resets to genesis. By ensuring the Router is deployed, we
avoid this (and don't need to track the deployment block in-contract).
Also uses a JoinSet to sync the 32 blocks in parallel.
The caller is paid a fixed fee per unit of gas spent. That arguably
incentivizes the publisher to raise the gas used by internal calls, yet this
doesn't effect the user UX as they'll have flatly paid the worst-case fee
already. It does pose a risk where callers are arguably incentivized to cause
transaction failures which consume all the gas, not just increased gas, yet:
1) Modern smart contracts don't error by consuming all the gas
2) This is presumably infeasible
3) Even if it was feasible, the gas fees gained presumably exceed the gas fees
spent causing the failure
The benefit to only paying the callers for the gas used, not the gas alotted,
is it allows Serai to build up a buffer. While this should be minor, a few
cents on every transaction at best, if we ever do have any costs slip through
the cracks, it ideally is sufficient to handle those.
The router will now match the top-level transfer so it isn't used as the
justification for the InInstruction it's handling. This allows the theoretical
case where a top-level transfer occurs (to any entity) and an internal call
performs a transfer to Serai.
Also uses a JoinSet for fetching transactions' top-level transfers in the ERC20
crate. This does add a dependency on tokio yet improves performance, and it's
scoped under serai-processor (which is always presumed to be tokio-based).
While we could instead import futures for join_all,
https://github.com/smol-rs/futures-lite/issues/6 summarizes why that wouldn't
be a good idea. While we could prefer async-executor over tokio's JoinSet,
JoinSet doesn't share the same issues as FuturesUnordered. That means our
question is solely if we want the async-executor executor or the tokio
executor, when we've already established the Serai processor is always presumed
to be tokio-based.
