Hello,

yesterday I had to debug a really weird problem.  I had a extra-special-file in
my system configuration used to create /etc/guix/channels.scm, but it was not
working, the channels file contained something completely else then I put into
it.  In particular, this sequence in the configuration:

    (extra-special-file "/etc/guix/channels.scm" file/channels.scm)
    (extra-special-file "/etc/guix/xxx" file/channels.scm)
    (extra-special-file "/etc/guix/yyy" file/channels.scm)

Produced this on the file system:

    channels.scm -> /gnu/store/icw828kvva25ns9p0y0np7lp3c7xl0n6-channels.scm
    xxx -> /gnu/store/w3gip7zqrqxbp8kaplq6kia85i5c4fh8-channels.scm
    yyy -> /gnu/store/w3gip7zqrqxbp8kaplq6kia85i5c4fh8-channels.scm

Notice that, despite being created by the exact same code, the store items
differs.

After ~2 hours of digging I realized this is caused by the changes in
883e69cdfd226c8f40b6e3b76ce0740b59857de6.

I see couple of issues here (in no particular order, questions prefixed with Q):

* My configuration file just *silently* stopped working.

That is not great for obvious reasons.

* There is no news entry

I did not realize it stopped working for some time (but it explains few weird
issues I had lately) until I tried to install completely new system where I
actually went to check it works as expected.

* Broken default behavior.

Currently, out of the box, extra-special-file with "/etc/guix/channels.scm" just
does not do anything.  No error, no warning.  That is pretty unexpected.

* There is no terse way to turn it off

Currently my configuration contains this:

      (modify-services %base-services
        (guix-service-type
         config => (guix-configuration
                    (inherit config)
                    (channels #f))))

That is 5 lines instead of single token %base-services just to restore the
original non-broken behavior.  Would be nice to have guix-extension for this
turning it into 2 lines.

* Q: Is there an easy way to fill (channels)?

After I learned that (channels) is a thing, I wanted to use it, but did not find
any simple way how to do so.  My channels are in channels.scm produced by `guix
describe --format=channels', and I do not see any simple way to pass the content
of that file into (channels).  No, it does not accept (local-file).  I tried
just (read)-ing it, but that too does not work.

I could steal the code from scripts/pull.scm, but that cannot be the intended
way, right?  So how should I do this?  What procedure did I failed to find?

* Q: Is the default value right?

Currently it defaults to %default-channels, which causes all this magic.  Would
it not be better to default to #f, so that it would do nothing magical out of
the box and would be backwards compatible?  You could still turn it on by
(Somehow?  See above.) providing desired value.



Have a nice day,
Tomas Volf

--
There are only two hard things in Computer Science:
cache invalidation, naming things and off-by-one errors.