client: implement support for "unix" resolver scheme

[GarrettGutierrez1]

Implemented unix resolver. Fixes #2806 .

[dfawley]

This needs to come from the resolver (with resolver.Address - is this the same as ServerName? @menghanl?). It can be done in a follow-up change but this is not what we want to be doing, ideally.

[menghanl]

We actually don't do that.

The :authority header doesn't come from resolver.Address.ServerName:

grpc-go/internal/transport/http2_client.go

Line 439 in c6cfaba

headerFields = append(headerFields, hpack.HeaderField{Name: ":authority", Value: callHdr.Host})

But the ClientHandshake does:

grpc-go/internal/transport/http2_client.go

Line 225 in c6cfaba

conn, authInfo, err = transportCreds.ClientHandshake(connectCtx, addr.ServerName, conn)

All the authority thing needs a cleanup..

[dfawley]

So this is what I thought and what I was worried about. The resolver needs a way to set the authority for addresses. I think ServerName is the right way to do that -- WDYT? Do we want a different handshaker name from authority header value?

[menghanl]

I think they should be the same. For security reason?

[dfawley]

Agreed; I think we can consider it a bug that we are not using the address's ServerName for the authority header. But this can be done in a follow-up PR.

[dfawley]

LGTM modulo the one minor naming thing.

Please make a follow-up PR that sets resolver.Address.ServerName to "localhost" in the unix resolver and removes the special-casing for "unix" in Dial to set authority.

[menghanl]

We actually don't do that.

The :authority header doesn't come from resolver.Address.ServerName:

grpc-go/internal/transport/http2_client.go

Line 439 in c6cfaba

headerFields = append(headerFields, hpack.HeaderField{Name: ":authority", Value: callHdr.Host})

But the ClientHandshake does:

grpc-go/internal/transport/http2_client.go

Line 225 in c6cfaba

conn, authInfo, err = transportCreds.ClientHandshake(connectCtx, addr.ServerName, conn)

All the authority thing needs a cleanup..