/v1/iam/oauth/authorize answers 302 to the redirect_uri, and that redirect IS the
code flow — the browser has to receive it. Go's default client follows up to ten,
and this proxy set no policy, so the redirect was consumed here: the process
fetched the app's own callback URL, the browser never saw a location, and the
caller got 500 "iam unreachable" whenever that URL was not reachable FROM THE
SERVER — which is the normal case for a location meant for a browser.
So sign-in could not complete through the proxy at all. Found from a bank whose
PKCE round trip died at exactly that hop.
It also stops the upstream naming a URL this process will then go and fetch.
Verified by making the policy follow again: the test says the proxy chased the
redirect and the browser never sees the location.
Co-authored-by: Hanzo Dev <[email protected]>