bufnet not supported by NewClient() ?

[Martin Desrumaux]

With the v1.63.0 release, I tried to migrate code that uses the, now deprected, DialContext function to NewClient().

However, when using NewClient with the bufnet protocol (https://pkg.go.dev/google.golang.org/grpc/test/bufconn), my test fails.

Here's the change:

-       conn, err := grpc.DialContext(
-               ctx, "bufnet",
+       conn, err := grpc.NewClient(
+               "bufnet",
                grpc.WithContextDialer(func(ctx context.Context, _ string) (net.Conn, error) {
                        return lis.DialContext(ctx)
                }),

The test fails with:

rpc error: code = Unavailable desc = dns: A record lookup error: lookup bufnet on 127.0.0.53:53: server misbehaving

How can I use bufnet with the NewClient function?

[Stéphane Loeuillet]

Default resolver is now dns with NewClient, was passthrough
I suppose you'll have to force passthrough resolver

With WithResolvers ?

[Stéphane Loeuillet]

From DialContext 1.63.0 doc :

One subtle difference between NewClient and Dial and DialContext is that the former uses "dns" as the default name resolver, while the latter use "passthrough" for backward compatibility. This distinction should not matter to most users, but could matter to legacy users that specify a custom dialer and expect it to receive the target string directly.

[Martin Desrumaux]

Thanks for the feedback @SLoeuillet!

I've gave a check to WithResolvers() (which is experimental). It expects a resolver.Builder.

Ideally, I'd pass a passthrough's Builder in order to get the original behavior.
But the passthrough package is only available behind an internal/ directory, so I can't import it:

grpc-go/resolver/passthrough/passthrough.go

Lines 24 to 26 in c68f456

 package passthrough 
  
 import _ "google.golang.org/grpc/internal/resolver/passthrough" // import for side effects after package was moved 

Initially, I also checked if I could set the defaultScheme to passthrough (that's what grpc.DialContext does now).
But the field and the setter are are not exported either.

grpc-go/dialoptions.go

Lines 662 to 668 in c68f456

 // withDefaultScheme is used to allow Dial to use "passthrough" as the default 
 // name resolver, while NewClient uses "dns" otherwise. 
 func withDefaultScheme(s string) DialOption { 
 	return newFuncDialOption(func(o *dialOptions) { 
 		o.defaultScheme = s 
 	}) 
 } 

The only workaround I think of would be to copy the passthrough implementation, so I can pass it to WithResolvers() but that seems counter-intuitive.

As of now, in order to upgrade grpc-go to v1.63.0, I think the best way is to keep using the deprecated grpc.DialContext() function.

Is it intended by the grpc-go developer team? Is there an alternative?

Thanks!

[Doug Fawley]

You can specify the passthrough resolver by using "passthrough:///bufnet" as your target instead (and actually buffnet is arbitrary; you could put anything there instead, or probably omit it entirely).

Or you can globally make the default resolver "passthrough" by using https://pkg.go.dev/google.golang.org/grpc/resolver#SetDefaultScheme -- I do not recommend this approach, though, as "passthrough" has some negative properties and really only exists for backward compatibility reasons.

You can also feel free to keep using Dial; it is not going anywhere. We don't recommend its use, but we will continue to support it for all 1.x releases (and no 2.x release is being considered).

Ideally we will eventually add support for an "in-memory" transport for your exact use case. However, that does not currently exist. That would probably require something like #906 which we just don't have the resources to prioritize.

[Martin Desrumaux]

Oh awesome! It works like a charm!
Thanks a lot!