"unfairness" of juju/mutex

[roger peppe]

On 17 November 2016 at 12:12, Stuart Bishop <stuart.bishop at canonical.com> wrote:
> On 17 November 2016 at 02:34, roger peppe <roger.peppe at canonical.com> wrote:
>>
>> +1 to using blocking flock. Polling is a bad idea with a heavily contended
>> lock.
>>
>> FWIW I still think that mutexing all unit hooks is a bad idea
>> that's really only there to paper over the problem that apt-get
>> doesn't work well concurrently.
>
>
> apt is just the one you commonly trip over. If there was no mutex, then
> charms would need to do their own locking for every single resource they
> need to access that might potentially also be accessed by a subordinate (now
> or in the future), and hope subordinates also use the lock. So I think
> mutexing unit hooks on the same machine is a fantastic idea :) Just
> something innocuous like 'adduser' can collide with a subordinate wanting to
> stick a config file in that user's home directory.

Surely a hook mutex primitve (e.g. "mutex adduser ...") would have been
more appropriate than the sledgehammer approach of mutexing everything
all the time? Sometimes I might want a hook to run for a long time
(or it might unfortunately block on the network) and turning off all
subordinate hooks while that happens doesn't seem right to me.

Anyway, I appreciate that it's too late now. We can't change this assumption
because it'll break all the charms that rely on it.

  cheers,
    rog.