Re: Channel dependencies

[Ludovic Courtès]

Hello,

Ricardo Wurmus <address@hidden> skribis:

>>>  (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.

We should error out in this case because at run time, there can be only
one Guile module with a given name.  Thus, conflicting dependencies
cannot be honored.  (Not like Node.js, for better or worse.  ;-))

> 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.

Of course it’s easier to pin all channels when they are directly
expressed in ~/.config/guix/channels.scm.

Say, I need channel A, which depends on B.

What we could do is that if B is already in ~/.config/guix/channels.scm,
then we take this particular B; if it’s not there, we take the latest
version.

In terms of the dependency code, it means that dependencies found in
.guix-channels file are added if and only if they are not already
present in the user-provided channel list.

Does that make sense?

Thanks!

Ludo’.