Re: Channel dependencies

[Ricardo Wurmus <rekado@elephly.net>]

Hi Chris,

>>  (define (latest-channel-instances store channels)
>>    "Return a list of channel instances corresponding to the latest checkouts of
>> -CHANNELS."
>> -  (map (lambda (channel)
>> -         (format (current-error-port)
>> -                 (G_ "Updating channel '~a' from Git repository at '~a'...~%")
>> -                 (channel-name channel)
>> -                 (channel-url channel))
>> -         (let-values (((checkout commit)
>> -                       (latest-repository-commit store (channel-url channel)
>> -                                                 #:ref (channel-reference
>> -                                                        channel))))
>> -           (channel-instance channel commit checkout)))
>> -       channels))
>> +CHANNELS and the channels on which they depend."
>> +  (append-map (lambda (channel)
>> +                (format (current-error-port)
>> +                        (G_ "Updating channel '~a' from Git repository at '~a'...~%")
>> +                        (channel-name channel)
>> +                        (channel-url channel))
>> +                (let-values (((checkout commit)
>> +                              (latest-repository-commit store (channel-url channel)
>> +                                                        #:ref (channel-reference
>> +                                                               channel))))
>> +                  (let ((instance (channel-instance channel commit checkout)))
>> +                    (cons instance (latest-channel-instances
>> +                                    store
>> +                                    (channel-instance-dependencies instance))))))
>> +              channels))
>
> What happens if the dependency list contains duplicate channels?  This
> might happen if two unrelated channels mutually depend upon a third
> channel.
>
> What happens if the dependency list contains two channels that differ
> only in their branch (or commit), and are the same in every other way?
> This might happen if two unrelated channels mutually depend upon two
> different versions of the same third channel.

I was going to write an email about this just now.  This is indeed a
problem with this naive implementation.

For reproducibility a user may want to pin all channels to a certain
commit, but channels that are dependencies are exempt from that
mechanism as they are added to the list of channels to be instantiated
during the run of “guix pull”.

It should be possible to pin *all* channels, even if they are
dependencies of other channels.  One way is to treat all channels and
their dependencies as a unique set where duplicates are replaced with
the most specific description (e.g. those specifying a commit).  This
way users could fully specify dependencies, which would then be used
instead of the channel’s declared dependency.

This needs some more thought.

--
Ricardo