WICG
Network Partition Key for reports is not specified
Conversion measurement reports have information collected in two different first party network contexts (publisher page, report destination (?), and are potentially reported to yet a third site. To avoid leaking information between different first party network contexts, the reports should therefore be specified to each use a single-use Network Partition Key.
In Chrome-land, these are known as "Transient NetworkIsolationKey" (NIK predates adding Network Partition Key to the fetch spec, so NIKs and NPKs are basically the same thing), but I don't believe the fetch defines them. They're just NPKs for opaque origins, which the fetch spec allows.
To do this, it appears that we need to set the fetch request's client, which is used in https://fetch.spec.whatwg.org/#request-determine-the-network-partition-key.
Right now we set a null client, which appears at first glance to work (i.e. it does not cache the response), but it looks like it reuses a "null"-keyed connection cache (https://fetch.spec.whatwg.org/#concept-connection-obtain) which doesn't seem right.
@MattMenke2 Could you help us understand what's left to do for this in the "create a report request" algorithm?
I'm not sure the fetch standard has a concept of a unique, unshared network partition. I'm not really sure what the next step is here, unfortunately. I know the network partitioning code, I don't know how specs wire it up to the web platform.