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

From: Mikhail Tsykalov
Subject: [bug#41143] [PATCH 1/2] mapped-devices: Allow target to be list of strings
Date: Fri, 25 Sep 2020 16:36:11 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Icedove/68.12.0

Hi, Ludovic

On 25.09.2020 12:34, Ludovic Courtès wrote:
Hi Mikhail,

Mikhail Tsykalov <> 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
     (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?

I'm sorry if I didn't make myself clear, but it doesn't seem like open/close functions even use any mapped-device-* procedures, they just get passed source and target field directly. What I meant was this change will require changes to luks-device-mapping, raid-device-mapping and all other device mappings that users may have implemented in their local trees/config.

To be fair, after thinking about it for a bit, I think that this issue can be solved by renaming mapped-device-kind and providing compatibility macros similar to %mapped-device. Still question remains about what should we do if a list gets passed to a kind that doesn't expect it, but I think we can just raise an error in macro if that's the case. Does this sound fine to you?


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