dialing against registered schemes is not case-insensitive

[iamricard]

What version of gRPC are you using?

1.45.0

What version of Go are you using (go version)?

go version go1.19.3 darwin/arm64

What operating system (Linux, Windows, …) and version?

Darwin

What did you do?

• Registered custom resolver, with a scheme containing uppercase letters
• Dialed a target with the same scheme
• Registered customer resolver, all lowercase letters
• Dialed a target with the mixed casing scheme

package main

import (
	"fmt"

	"google.golang.org/grpc"
	"google.golang.org/grpc/credentials/insecure"
	"google.golang.org/grpc/resolver"
)

var (
	_ resolver.Builder  = &resolverBuilder{}
	_ resolver.Resolver = &testResolver{}
)

type resolverBuilder struct {
	scheme string
}

type testResolver struct{}

func (r *resolverBuilder) Scheme() string {
	return r.scheme
}

func (r *resolverBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions) (resolver.Resolver, error) {
	panic("built resolver")
}

func (r *testResolver) ResolveNow(opts resolver.ResolveNowOptions) {
}
func (r *testResolver) Close() {
}

func main() {
	resolver.Register(&resolverBuilder{scheme: "CasedScheme"})
	grpc.Dial("CasedScheme:///foo:6400", grpc.WithTransportCredentials(insecure.NewCredentials()))
	fmt.Println("did not panic")

	// will panic
	resolver.Register(&resolverBuilder{scheme: "casedscheme"})
	grpc.Dial("CasedScheme:///foo:6400", grpc.WithTransportCredentials(insecure.NewCredentials()))
}

What did you expect to see?

• Dial should have picked up the custom resolver in both occasions, but didn't when the registered resolver had a scheme with mixed casing
• Register should have overwritten/error'd when registering two resolvers with the same scheme, instead it registered both

It looks like this change was introduced in 1.42, when #4817 landed. Which made Dial more correct by using net/url's parse URL. Before that change was introduced register would preserve the casing so the first example worked, but the second one didn't.

[dfawley]

Thanks for the bug. I'll send a fix in a moment.

[dfawley]

Actually, since the behavior has been this way for over a year, and since Java and C++ both behave this way, I'm just going to simply update the documentation to mention that the case is collapsed to lower-case and all resolver Scheme()s should return only lowercase characters instead.