Earlier quoted context omitted.
If the proxy changes a new version of a package, when you update it, there's no way to detect it since it fetches through the cache anyways, so a poisoned sum will be added to sumdb, and anyone who isn't fetching their packages through Google's proxy will get told that whatever they're using is trying to trick them.
> anyone who isn't fetching their packages through Google's proxy will get told that whatever they're using is trying to trick them. That is exactly the detection of a poisoned module in the ecosystem. It would break builds, issues would get filed, and a new version would be released (and the malicious party may not be so lucky this time since it’s trust on anyone’s first use).
But I guess it's also fairly easy to test it: just serve a slightly different version to the google's go mirror (by the user agent), and see how long until somebody complains to you about it.