SubjectDerivedAttributeDefinition Generated no values

[Wessel, Keith]

We're experimenting with the new SAML authn support, trying to proxy requests to our IdP over to ADFS so we can connect the two SSOs together so-to-speak. We followed the docs, but something's not working right.

We're trying to base the subject off of the uid attribute that ADFS is releasing.

<!-- Incoming proxy: map uid to subject from proxied authn -->
<AttributeDefinition id="adfsProxySubjectName" xsi:type="SubjectDerivedAttribute"
    forCanonicalization="true"
    principalAttributeName="uid" />

According to our logs which I now have logging SAML messages, uid is coming through:

<AttributeStatement>
    <Attribute Name="urn:oid:0.9.2342.19200300.100.1.1" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri">
        <AttributeValue>kwessel</AttributeValue>
    </Attribute>
</AttributeStatement>

But just after the processing of the response, I see:
2020-10-13 10:07:28,241 - INFO [net.shibboleth.idp.attribute.resolver.ad.impl.ContextDerivedAttributeDefinition:176] - SubjectDerivedAttributeDefinition adfsProxySubjectName Generated no values, no attribute resolved

The uid attribute is definitely defined elsewhere in attribute-resolver.xml. So, why would my subject derived attribute not be picking it up?

And would I be better off just using a subjectDataConnector?

Thanks,
Keith