Unable to set OmitEmptyPsk in PreSharedKeyExtension

[sleeyax]

Not sure if this is considered a bug, but setting OmitEmptyPsk to true in the PreSharedKeyExtension as part of a custom ClientHelloSpec has no effect:

spec := &utls.ClientHelloSpec{
    Extensions: []utls.TLSExtension{
        // ...
        &utls.UtlsPreSharedKeyExtension{
            OmitEmptyPsk: true,
        },
    },
}

The only way to actually enable OmitEmptyPsk is by specifying it in the UConn config:

tlsConfig := &utls.Config{OmitEmptyPsk: true}
conn := utls.UClient(rawConn, tlsConfig, tls.HelloCustom)
// ...

Because it's overwritten in:

utls/u_session_controller.go

Line 291 in 1880d78

s.pskExtension.SetOmitEmptyPsk(s.uconnRef.config.OmitEmptyPsk)

I think this can be fixed/clarified by either making this field private in the extension or by following the configuration in both parts of the code.

Edit: At the time of writing, I'm using the master branch with commit 3546fd9 as the latest change.

[gaukas]

Interesting observation. I think it might worth it if we comment/deprecate on this feature. Apparently we no longer want to remove this exported field since that's already a part of the uTLS API and removing it breaks anyone using an older version and (mistakenly) uses this field in the extension.

[sleeyax]

Ah I see. I can create a PR for this small improvement if you like.

How about adding a deprecation comment to both UtlsPreSharedKeyExtension and FakePreSharedKeyExtension structs like this?

type UtlsPreSharedKeyExtension struct {
	// ...

	// Deprecated: Set OmitEmptyPsk in Config instead.
	OmitEmptyPsk bool
}

[gaukas]

That would be nice! Thanks for doing that.