With `Version::V1Lazy` and negotiation of a single protocol, a stream initiator optimistically
sends application data right after proposing its protocol. More specifically an application can
write data via `AsyncWrite::poll_write` even though the remote has not yet confirmed the stream protocol.
This saves one round-trip.
``` mermaid
sequenceDiagram
A->>B: "/multistream/1.0.0"
A->>B: "/perf/1.0.0"
A->>B: <some-perf-protocol-data>
B->>A: "/multistream/1.0.0"
B->>A: "/perf/1.0.0"
B->>A: <some-perf-protocol-data>
```
When considering stream closing, i.e. `AsyncWrite::poll_close`, and using stream closing as an
operation in ones protocol, e.g. using stream closing to signal the end of a request, this becomes tricky.
The behavior without this commit was as following:
``` mermaid
sequenceDiagram
A->>B: "/multistream/1.0.0"
A->>B: "/perf/1.0.0"
A->>B: <some-perf-protocol-data>
Note left of A: Call `AsyncWrite::poll_close` which first waits for the<br/>optimistic multistream-select negotiation to finish, before closing the stream,<br/> i.e. setting the FIN bit.
B->>A: "/multistream/1.0.0"
B->>A: "/perf/1.0.0"
Note right of B: Waiting for A to close the stream (i.e. set the `FIN` bit)<br/>before sending the response.
A->>B: FIN
B->>A: <some-perf-protocol-data>
```
The above takes 2 round trips:
1. Send the optimistic multistream-select protocol proposals as well as the initiator protocol
payload and waits for the confirmation of the protocols.
2. Close the stream, i.e. sends the `FIN` bit and waits for the responder protocol payload.
This commit proposes that the stream initiator should not wait for the multistream-select protocol
confirmation when closing the stream, but close the stream within the first round-trip.
``` mermaid
sequenceDiagram
A->>B: "/multistream/1.0.0"
A->>B: "/perf/1.0.0"
A->>B: <some-perf-protocol-data>
A->>B: FIN
B->>A: "/multistream/1.0.0"
B->>A: "/perf/1.0.0"
B->>A: <some-perf-protocol-data>
```
This takes 1 round-trip.
The downside of this commit is, that the stream initiator will no longer notice a negotiation error
when closing the stream. They will only notice it when reading from the stream. E.g. say that B does
not support "/perf/1.0.0", A will only notice on `AsyncRead::poll_read`, not on
`AsyncWrite::poll_close`. This is problematic for protocols where A only sends data, but never
receives data, i.e. never calls `AsyncRead::poll_read`. Though one can argue that such protocol is
flawed in the first place. With a response-less protocol, as even if negotiation succceeds, A
doesn't know whether B received the protocol payload.
Pull-Request: #4019.
Previously, a protocol could be any sequence of bytes as long as it started with `/`. Now, we directly parse a protocol as `String` which enforces it to be valid UTF8.
To notify users of this change, we delete the `ProtocolName` trait. The new requirement is that users need to provide a type that implements `AsRef<str>`.
We also add a `StreamProtocol` newtype in `libp2p-swarm` which provides an easy way for users to ensure their protocol strings are compliant. The newtype enforces that protocol strings start with `/`. `StreamProtocol` also implements `AsRef<str>`, meaning users can directly use it in their upgrades.
`multistream-select` by itself only changes marginally with this patch. The only thing we enforce in the type-system is that protocols must implement `AsRef<str>`.
Resolves: #2831.
Pull-Request: #3746.
As I do frequently, I corrected for the latest clippy warnings. This will make sure the CI won't complain in the future. We could automate this btw and maybe run the nightly version of clippy.
