Implement Error::source for h2::Error.

[goffrie]

This makes it easier to detect an IO error at the bottom of an error
cause chain (e.g. behind a hyper::Error).

Also, don't implement std::error::Error for the private error types,
to avoid confusion.

[alanhe]

Can we push this forward?

In my case, when a client crashes, the server gets an error similar to this.

// dbg!(&source);

 &source = hyper::Error(
    Body,
    Error {
        kind: Io(
            Kind(
                BrokenPipe,
            ),
        ),
    },
)

I want to match the error kind to record the event, but recursively downcasting the error to std::io::Error couldn't work because h2::Error does not implement std::error::Error#source().

This is what I end up with:

fn match_broken_pipe_error(err: &(dyn std::error::Error + 'static)) -> bool {
    if let Some(e) = err.downcast_ref::<h2::Error>() {
        if let Some(e) = e.get_io() {
            if e.kind() == std::io::ErrorKind::BrokenPipe {
                return true;
            }
        }
    }

    if let Some(source) = err.source() {
        match_broken_pipe_error(source)
    } else {
        false
    }
}

The code is much more awkward.

[kaifastromai]

oof...any update on this? Ran into this myself

[mkatychev]

This is still an issue esp while matching on a stream disconnect in tonic: hyperium/tonic#1579
@seanmonstar what is needed to move this PR forward, happy to start a separate feature branch? This will still affect grpc disonnects since tonic uses h2 0.3 (so it would have to be backported as well).

[goffrie]

I've updated the PR for recent master if anyone would like to review this again :)

[mkatychev]

@seanmonstar could we get some feedback on this PR? This would greatly simplify some tonic error handling cases (especially in disambiguating client disconnections):
https://github.com/hyperium/tonic//blob/f7090c24813041dc62052894dee119520d603d88/examples/src/streaming/server.rs#L23-L29