grpc: add optional interface to return channel authority

[easwars]

This is one of the items mentioned in #4717 which did not get done. The below description has been copied over verbatim from #4717. Java and C already support this.

• Determining :authority from the target URI must be the responsibility of the name resolver. See: HTTP2 authority header is not set to resolver.Address.ServerName (?) #4516
  • Add an optional interface, to be implemented by name resolvers or their builders, that contains a single method to return the channel’s authority.
    • This method must not perform any I/O, be non-blocking and the return value must not change over time.
    • This value will be used to populate the :authority pseudo-header, and serverName for TLS handshake by default.
    • gRPC will provide an implementation which conforms to RFC 3986 and returns the path portion of the target, after stripping query parameters. This will be used if the scheme’s name resolver does not provide a custom one.
      • grpcutil.ParseTarget will be enhanced for this purpose.
    • This parsed target will be passed to load balancers in their BuildOptions, with the correct authority set.

[Aditya-Sood]

hi @dfawley @ginayeh, can I pick this up?

[dfawley]

Sure, thanks! Let's name the interface:

package resolver

// AuthorityOverrider is implemented by Builders that wish to override the
// default authority for the ClientConn.  By default, the authority used is
// target.Endpoint().
type AuthorityOverrider interface {
	// OverrideAuthority returns the authority to use for a ClientConn with the
	// given target.  It must not perform I/O or other blocking operaitons.
	OverrideAuthority(target Target) string
}

And I think the only real motivation for this feature is to move this special case into the function to be implemented by the unix resolver's builder:

grpc-go/clientconn.go

Lines 1990 to 1993 in e14d583

	case strings.HasPrefix(target, "unix:") || strings.HasPrefix(target, "unix-abstract:"):
	// TODO: remove when the unix resolver implements optional interface to
	// return channel authority.
	cc.authority = "localhost"

Around here:

grpc-go/internal/resolver/unix/unix.go

Line 60 in e14d583

func (b *builder) Scheme() string {

Which would also be a pretty good test for this functionality in practice.

One thing we need to decide is: do we do the escapeAuthority on the result of this function, to ensure it complies with RFC 3986? I'm of the opinion we should, but I'm not sure what Java and C++ do. They probably just assume it's the responsibility of the name resolver.

[github-actions]

This issue is labeled as requiring an update from the reporter, and no update has been received after 6 days. If no update is provided in the next 7 days, this issue will be automatically closed.

[Aditya-Sood]

WIP, adding comment to prevent issue being closed

[github-actions]

This issue is labeled as requiring an update from the reporter, and no update has been received after 6 days. If no update is provided in the next 7 days, this issue will be automatically closed.

[easwars]

One thing we need to decide is: do we do the escapeAuthority on the result of this function, to ensure it complies with RFC 3986?

Did we make a decision on this one? Thanks.
@dfawley

[Aditya-Sood]

hi @easwars, the PR doesn't include percent-encoding and expects it from the name resolver currently
(point 3 in #6752 (comment))