apt-get update / fetch.apt_update may never terminate

> The apt-get update helper in charm-helpers goes into a loop if it is unable to run successfully. This is no longer desirable
> behaviour, as in this case the unit should most likely be put into a blocked state.

To be clear, it's not an infinite loop, it's 30 attempts with 10s sleep -> 5 minutes.

The rationale for this loop is for two or more units on the same machine all of which are trying to get the APT lock. IMHO it is not appropriate for a unit that hits this APT lock issue to be blocked. There is no user action required (other than try again).

The retry logic is a bit crude at the moment because there's no way of distinguishing lock failures from any others using just the return code. One could probably tighten it up a bit to e.g. force a C locale, and inspect the error message on stderr.

30 attempts with 10s sleep is a couple of hours (since, at least in the cases I've got, a normal update takes about 1 minute and a failing one quite a bit longer waiting for http timeouts to timeout).

I think the bug here is that there is a fixed number of retries, rather than using the clock to timeout after some reasonable amount of time.

My user action required is 'check the logs, fix the config', eg. specify the correct apt source or add the correct gpg signing key.