[Nix-dev] substitute hell ?

[Sergey Mironov]

Thanks for quick answer



2012/6/3 Eelco Dolstra <eelco.dolstra at logicblox.com>:
> Hi,
>
> On 02/06/12 16:25, Sergey Mironov wrote:
>
>> We know that vlc package depends on a52dec. We have a substitution for vlc:
>>
>> $ sudo nix-env -qas  'vlc' --system --out-path
>> --S  vlc-2.0.1  x86_64-linux
>> /nix/store/x332qx57qbak8rbmg4hhbjr5m7afn2df-vlc-2.0.1
>> --S  vlc-2.0.1  i686-linux
>> /nix/store/vwvz7hh1yc1bvjn6avylbzlgib8d2b60-vlc-2.0.1
>> --S  vlc-2.0.1  i686-linux
>> /nix/store/vwvz7hh1yc1bvjn6avylbzlgib8d2b60-vlc-2.0.1
>> (hm.. why are there two of them. ok, let it be so)
>
> I only have one.  Maybe you have multiple channels (e.g. nixos-unstable and
> nixpkgs-unstable)?  If you want to find out where they're coming from, use the
> -P flag to see the corresponding attribute name.

O, thanks, with -P flag it says

$ sudo nix-env -qas  'vlc' --system --out-path -P
--S  latest.0        vlc-2.0.1  x86_64-linux
/nix/store/x332qx57qbak8rbmg4hhbjr5m7afn2df-vlc-2.0.1
--S  latest.1        vlc-2.0.1  i686-linux
/nix/store/vwvz7hh1yc1bvjn6avylbzlgib8d2b60-vlc-2.0.1
--S  nixos.pkgs.vlc  vlc-2.0.1  i686-linux
/nix/store/vwvz7hh1yc1bvjn6avylbzlgib8d2b60-vlc-2.0.1

where "latest" is the name of vlc's channel I've added, but then
removed (!). Currently, my user's ~/.nix-channel is empty and root's
~/.nix-channel contains only reference to nixos-latest. Probably there
is another cache elsewhere, maybe nix-env has one? (oh, I've found the
place: ~/.nix-defexp/channels/latest -> vlc's manifest. May it be a
result of unlucky Ctrl+C ?) Anyway,  probably It is not a major
problem.

>> Threre is no substitute for a52dec! (And for some reason I can't find
>> a52dec job on a hydra website). How did hydra compile vlc without
>> building a52dec? If it actually did compile a52dec, then what is wrong
>> with it's substitution?
>
> This is because a52dec is only a build-time dependency and not a runtime
> dependency.  (This might suggest that vlc doesn't actually do anything with
> a52dec, in which case it should be removed as a dependency...)  So Hydra does
> build it, but it doesn't end up in the channel because the channel only contains
> runtime dependencies.
>
> (In the past the Nixpkgs channel also contained build-time dependencies.)
>

The actual reason I started investigation is the behavior of
nixos-rebuild. It wants to build  derivations for vlc and a52dec
locally. By the way, a52dec is plainly listed in vlc's buildInputs.
How can nix determine that it is a built-time dep without building vlc
and scanning it's binary?

In general, Is it possible to ask nix, why does it want to build a52dec?

[~/proj/nix] $ sudo nixos-rebuild -I . dry-run
building Nix...
building the system configuration...
these derivations will be built:
  /nix/store/7g09vsdbp72zhbd0pkada0klynmax153-etc.drv
  /nix/store/8ha1qnpy6havqb468xzzhy3nr6gr47xl-upstart-xserver.conf.drv
  /nix/store/9zqw7kri0n7i138rzv25dc0zqa3fcqyy-nixos-0.1pre-svn.drv
  /nix/store/h2y1c5gaa5x1h34qx3jyf42n8knkzjwb-system-crontab.drv
  /nix/store/lvghnv1ifyx3jw10icvy2nvf4hmj5z2z-system-path.drv
  /nix/store/n2fwa36yjl0acrvglhh79mb615g84h44-slim.cfg.drv
  /nix/store/pnwplhaa56nqjshsy9karl692kdigdwz-vlc-2.0.1.drv
  /nix/store/x90bxbllmwz41q7mvs5lbiy3apxab900-dbus-conf.drv
  /nix/store/xjcp6j81yjclkrsndk0kn459mpm8sygq-xsession.drv
(a52dec is already locally compiled and doesn't mentioned here)

PS
My custom dev tree is basically a current svn-head + some new
packages. All packages are installed via nixos-config, nix-env -q '*'
is empty for all users.