CDN circuit breaker doesn't return CDN urls for snapd

[Michael Nelson]

While testing the circuit breaker on staging, I noticed that while some responses contain the direct CDN url, those returned to snapd did not. The difference seemed to be whether the fields qparam is used to filter the fields returned [1].

Checking the code, indeed it currently requires 3 field (download_url, anon_download_url and _download_url_rewritable) or it will not rewrite the urls. This wasn't an issue when the circuit breaker was tested, but since then snapd has updated to explicitly name which fields it requires and the index code doesn't yet account for this when rewriting cdn urls.

[1] https://pastebin.canonical.com/185353/

[Michael Nelson (michael.nelson)]

QA on staging: https://pastebin.canonical.com/185626/