swarm/src/handler/multi.rs: MultiHandler isn't used anywhere inside rust-libp2p. Maybe get rid of it?

[efarg]

Description

Currently MultiHandler isn't used anywhere inside the project. Is it possible to remove or modify it?

Motivation

• MultiHandler violates the guideline of not implementing UpgradeInfoSend, OutboundUpgradeSend, and InboundUpgradeSend mentioned in swarm/src/upgrade.rs. See comment: core: Remove ProtocolName trait in favor of always using strings #2966 (comment)

• It's a hurdle in solving misc/multistream-select: Treat protocol as String and not [u8] #2831. Try building WIP solution for the said issue at https://github.com/efarg/rust-libp2p/tree/2966
  One of the errors encountered would be:

error[E0271]: expected `std::vec::IntoIter<IndexedProtoName>` to be an iterator that yields `ProtocolName`, but it yields `IndexedProtoName`
   --> swarm/src/handler/multi.rs:458:21
    |
458 |     type InfoIter = std::vec::IntoIter<IndexedProtoName>;
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `ProtocolName`, found struct `IndexedProtoName`

The error above is what brought thomaseizinger's attention to this issue. For further understanding, see comments: #2831 (comment) and #2966 (comment) and the replies by @thomaseizinger on the latter.

This happens due to changing the current definition of UpgradeInfoSend from this

rust-libp2p/swarm/src/upgrade.rs

Lines 31 to 39 in 5b4eab7

	pub trait UpgradeInfoSend: Send + 'static {
	/// Equivalent to [`UpgradeInfo::Info`](upgrade::UpgradeInfo::Info).
	type Info: upgrade::ProtocolName + Clone + Send + 'static;
	/// Equivalent to [`UpgradeInfo::InfoIter`](upgrade::UpgradeInfo::InfoIter).
	type InfoIter: Iterator<Item = Self::Info> + Send + 'static;
	/// Equivalent to [`UpgradeInfo::protocol_info`](upgrade::UpgradeInfo::protocol_info).
	fn protocol_info(&self) -> Self::InfoIter;
	}

to this

pub trait UpgradeInfoSend: Send + 'static {
    /// Equivalent to [`UpgradeInfo::InfoIter`](upgrade::UpgradeInfo::InfoIter).
    type InfoIter: Iterator<Item = upgrade::ProtocolName> + Send + 'static;

    /// Equivalent to [`UpgradeInfo::protocol_info`](upgrade::UpgradeInfo::protocol_info).
    fn protocol_info(&self) -> Self::InfoIter;
}

Are you planning to do it yourself in a pull request?

Yes.

[thomaseizinger]

MultiHandler has been here ever since I started working on rust-libp2p but I personally never had a use for it.

@mxinden Do you know of any users?

[mxinden]

It is used in Substrate.

https://github.com/paritytech/substrate/blob/a92005a5092ebe6a636c96dcae814f9a378f5940/client/network/src/request_responses.rs#L49

I still need to read up on the above. Thanks @efarg for the detailed issue.

[thomaseizinger]

Okay, thanks for the insight. I think I already know how to fix the issue in #2966.

@efarg Did you push all your code? Then I might just push a quick patch on top fixing MultiHandler.

[efarg]

@thomaseizinger yes, the code that would give the multihandler error is all here https://github.com/efarg/rust-libp2p/tree/2966

[thomaseizinger]

I posted a patch onto the PR that fixes the multihandler issue: #2966 (comment)