Feature request: updateURL processing to support the leaving of negative interest groups via the nascent header feature (#896)

[dmdabbs]

In the “shell IG” pattern a buyer joins an IG with a minimal, updateable attribute set. This centralizes and defers page latency inducing processing (computing and rendering IG content) to updateURL time. Negative targeting adds a wrinkle.

Scenario: a buyer intends to negatively target a positive interest group. The Chrome-suggested approach has the buyer pair the excluded positive IG with a negatively-targetable ‘shadow.’ The buyer joins the browser to “shells” of “EXCLUDED_POSITIVE,” the excluded IG, “EXCLUDED_NEGATIVE,” its negative ‘shadow,’ and “EXCLUDER,” an IG that will negatively target the other group. EXCLUDER’s update will configure EXCLUDED_NEGATIVE in negativeInterestGroups[]. If EXCLUDED_POSITIVE’s update determines IG membership condition doesn’t hold (or the update doesn’t run, times out, &c.), the buyer has a partial, incorrect IG set. The buyer’s ability to leave EXCLUDED_NEGATIVE via response header during EXCLUDED_POSITIVE’s update would ensure a 'primary' can ensure its ‘shadow’ is always appropriately joined.