Support "unix:///" scheme in Dial

[menghanl]

As defined in https://github.com/grpc/grpc/blob/master/doc/naming.md

[dfawley]

@menghanl can you provide an ETA on or work estimate for this feature request?

[aequitas]

I think I am stumbling on this. I try to get a embedded Etcd server running over unix domain sockets in (https://github.com/purpleidea/mgmt/) but running into this error:

WARNING: 2018/02/20 13:24:57 grpc: addrConn.createTransport failed to connect to {clients.sock:0 0  <nil>}. Err :connection error: desc = "transport: Error while dialing dial tcp: lookup clients.sock: no such host". Reconnecting...

I have boiled it down to:

grpc-go/clientconn.go

Line 438 in 3926816

return dialContext(ctx, "tcp", addr)

Where a TCP dialer is created if none exists yet, even though a correct dialer is passed in via: https://github.com/coreos/etcd/blob/3903385d1b50c26ec0c18e99fc55be33ee0d97e3/clientv3/client.go#L272 afaict, but my go knowledge is yet lacking in this part.

Changing this from "tcp" to "unix" makes it work, but obviously breaks TCP support. I have not yet found a way to properly inspect the opts passed into DialContext in order to make a logical switch here, also target is passed in without scheme by Etcd, but adding it makes no difference.

A similar error message is generated when running Etcd directly using unix sockets:

# etcd --listen-peer-urls unix://etcd:0 --listen-client-urls unix://etcd:1 --advertise-client-urls unix://etcd:1

WARNING: 2018/02/20 13:34:20 grpc: addrConn.resetTransport failed to create client transport: connection error: desc = "transport: Error while dialing dial tcp: lookup etcd on 10.5.8.1:53: no such host"; Reconnecting to {etcd:0 0  <nil>}

Is this due to lack of support for unix domain sockets in grpc? Or is Etcd/mgmt invoking grpc wrongly in this point?

[menghanl]

@aequitas What's the target you are dialing to?
I assume it is "unix://etcd-address" for unix domain socket and "http://etcd-address" for tcp, is that right?

If what I assumed is correct. I think the problem here is that, the etcd dialer you pointed to assumes gRPC doesn't parse the target, so it will still receive host as "unix://etcd-address".
But this is no longer true because gRPC parses the target and passes "etcd-address" empty string without the scheme to it.

Please try this workaround and see if it works: use "passthrough:///unix://etcd-address" as the target you are dialing to.

[aequitas]

@menghanl thanks for the quick response.

Afaict Etcd passes only the 'host:port' part to the DialContext. When giving this url to Etcd: unix://etcd-address:0 only etc-address:0 is passed (etcd requires host:port format even for unix adresses due to input validation, the socket filename created will be host:port in the current directory).

I tried adding the unix:/// and passthrough:///unix:// prefix to host on this line: https://github.com/coreos/etcd/blob/master/clientv3/client.go#L352 and sctx.addr in https://github.com/coreos/etcd/blob/master/embed/serve.go#L195 but it did not completely solve the problem. It has now moved from resetTransport to createTransport:

WARNING: 2018/02/20 20:46:23 grpc: addrConn.createTransport failed to connect to {clients.sock:0 0  <nil>}. Err :connection error: desc = "transport: Error while dialing dial tcp: lookup clients.sock: no such host". Reconnecting...

I missed the difference in these error messages when debugging earlier, and also that there was a second entrypoint in etcd/embed/serve.py so I will pick up my investigation with the information. If you have any more pointers I'm glad to know.

[menghanl]

@aequitas At this stage, I think it would be better to file an issue in etcd repo and ask about this. It's not clear to me what might have caused this problem.
Please leave a comment here if anything is needed from gRPC. Thanks.

[gyuho]

@aequitas Can you file an issue to etcd?

I was planning to debug this as well, while testing etcd with unix sockets.

Thanks.