If we have a `channel_announcement` which is waiting on a UTXO
lookup before we can process it, and we receive a `channel_update`
or `node_announcement` for the same channel or a node which is a
part of the channel, we have to wait until the lookup completes
until we can decide if we want to accept the new message.
Here, we store the new message in the pending lookup state and
process it asynchronously like the original `channel_announcement`.
If we receive two `channel_announcement`s for the same channel at
the same time, we shouldn't spawn a second UTXO lookup for an
identical message. This likely isn't too rare - if we start syncing
the graph from two peers at the same time, it isn't unlikely that
we'll end up with the same messages around the same time.
In order to avoid this we keep a hash map of all the pending
`channel_announcement` messages by SCID and simply ignore duplicate
message lookups.
For those operating in an async environment, requiring
`ChainAccess::get_utxo` return information about the requested UTXO
synchronously is incredibly painful. Requesting information about a
random UTXO is likely to go over the network, and likely to be a
rather slow request.
Thus, here, we change the return type of `get_utxo` to have both a
synchronous and asynchronous form. The asynchronous form requires
the user construct a `AccessFuture` which they `clone` and pass
back to us. Internally, an `AccessFuture` has an `Arc` to the
`channel_announcement` message which we need to process. When the
user completes their lookup, they call `resolve` on their
`AccessFuture` which we pull the `channel_announcement` from and
then apply to the network graph.
The `chain::Access` trait (and the `chain::AccessError` enum) is a
bit strange - it only really makes sense if users import it via the
`chain` module, otherwise they're left with a trait just called
`Access`. Worse, for bindings users its always just called
`Access`, in part because many downstream languages don't have a
mechanism to import a module and then refer to it.
Further, its stuck dangling in the `chain` top-level mod.rs file,
sitting in a module that doesn't use it at all (it's only used in
`routing::gossip`).
Instead, we give it its full name - `UtxoLookup` (and rename the
error enum `UtxoLookupError`) and put it in the a new
`routing::utxo` module, next to `routing::gossip`.
