[bug#41143] [PATCH 1/2] mapped-devices: Allow target to be list of strings

["Ludovic Courtès" <ludo@gnu.org>]

Hi Mikhail,

Mikhail Tsykalov <tsymsh@gmail.com> skribis:

> On 09.09.2020 23:38, Ludovic Courtès wrote:
>>> diff --git a/gnu/services/base.scm b/gnu/services/base.scm
>>> index 0c154d1c4e..3d09e8220c 100644
>>> --- a/gnu/services/base.scm
>>> +++ b/gnu/services/base.scm
>>> @@ -408,7 +408,10 @@ FILE-SYSTEM."
>>>   (define (mapped-device->shepherd-service-name md)
>>>     "Return the symbol that denotes the shepherd service of MD, a
>>> <mapped-device>."
>>>     (symbol-append 'device-mapping-
>>> -                 (string->symbol (mapped-device-target md))))
>>> +                 (string->symbol (string-join
>>> +                                  (let ((t (mapped-device-target md)))
>>> +                                    (if (list? t) t (list t)))
>>> +                                  "-"))))
>> To avoid duplicating the (if (list? t) …) everywhere, I propose instead
>> the following approach:
>>
>>    1. Rename ‘target’ to ‘targets’ (plural) and likewise for the
>>       accessor, and agree that it always contains a list;
>>
>>    2. Rename ‘mapped-device’ to ‘%mapped-device’ and add a
>>       ‘mapped-device’ backward-compatibility macro that allows for a
>>       ‘target’ (singular) field and automatically turns its value into a
>>       list.  See the ‘origin’ macro in (guix packages) for an example of
>>       how to do that (that macro allows users to specify ‘sha256’ instead
>>       of ‘hash’).
>>
>>    3. Add a deprecated ‘mapped-device-target’ (singular) that returns the
>>       first element returned by ‘mapped-device-targets’.
>
> While this looks like a good idea, doesn't this break code that
> implements mapped-device and assumes that target is a string. Suddenly 
> passing a string to a mapped-device constructor results in a list
> passed to open/close. Also, what functions should do if they expect a
> string but get a list of them? Ignore everything but the first item?
> Implement mandatory check function? Doesn't this change push
> complexity out of mapped-device to implementations of it.

The intent of what I propose above is (1) to not break existing code,
and (2) to avoid duplicating checks and conversions at every call site.

#1 is achieved by providing a deprecated ‘mapped-device-target’
(singular) procedure, for example.

Does that make sense?

Thanks,
Ludo’.