[Nix-dev] 5 somewhat related questions

[Klaas van Schelven]

Hello Nixians,

I've installed NixOs a number of days ago. So far I really like it!

I've read the documentation I could find, but I'm left with a number of
questions; not so much "how to do X" but rather of a slightly more
philosphofical (or cultural, a.k.a. "best practices") nature. I hope
someone can enlighten me on the "Nix Way".

1. In NixOs /etc/nixos/configuration.nix is the single configuration file
that determines the state of the system as a whole. How does this file
relate to the existence of the nix-env command, either executed as root or
by a single non-privileged user? In particular, I would assume that any
nix-env is undone by the time the system is rebuilt from the configuration
file. Assuming this is the case: should the usage of nix-env not be
actively discouraged in NixOs? (perhaps it is, and I simply did not find
the reference)

2. (Context: Assuming for a moment there _is_ in fact a use-case for
nix-env; e.g. the scenario where you're not running NixOs, but are using
Nix on top of another distribution). nix-env uses an "imperative style" of
manipulating your environment, i.e. using a sequence of commands in a
particular order. I understand that after each succesful manipulation the
_resulting_ environment becomes available as a separate generation. As far
as I understand there's even a "half-product", the so called "derivation"
that is available per generation, although I did not study those yet. My
question is, however, whether the original commands that led to these
constructions can somehow be retrieved. The reason for this question is the
observation that the sequence of nix-env (and potentially other similar)
commands can be seen as a transactional log that could simply be replayed
to reconstruct the resulting generations (assuming that the commands fully
express the information needed to construct the associated environments;
this assumption might not actually hold in practice. Question 2b: does the
assumption hold?).

3. In the scenario where I use the single configuration file
/etc/nixos/configuration.nix but I'm also subscribed to a channel, the
state of this channel may influence the outcome of nixos-rebuild (This is
by design, it allows us to stay up to date with e.g. security updates). The
consecutive states of the channel, as seen by my system when rebuilding,
are valuable pieces of information in their own right when I want to debug
a problem. Take the following example:

t=0, my system is good.
t=1, I want to install some extra package, I modify configuration.nix, and
run nixos-rebuild
t=2, system broken.

I understand that I always have the ability to roll back the system _as a
whole_, even using Grub if needed. This is awesome of course. The question
is: do I also have the ability to debug the parts that lead to that whole?
In particular: the precise state of the channel[s] on each rebuild? And
preferably also: the state of /etc/nixos/configuration.nix on each build?

4. Is a "single declarative file per user" (e.g. for dotfiles, but
potentially also to make it possible to declare which user-specific
packages are installed) available? I understand there some options exist,
but how do they relate? Is there convergence on a "one way to do it"?

5. In the commit linked below, the nix file for VTE 2.91 adds the following
2 propagatedBuildInputs: pcre2 & gnutls. As far as I understand this might
be not good practice. The reason I've added them is because pkg-config,
when run in the build context of xfce.terminal, cannot otherwise find the
package vte-2.91 because of a dependency error. Ignoring for a moment the
rationale of the commit itself (I've been convinced that adding this
particular version of xfce4-terminal to the repo by itself is not a good
idea) can someone tell me what the proper way to handle this particular
situation would be?

https://github.com/NixOS/nixpkgs/pull/26742/commits/5e566d3c8a078f6cd6304e7cf0b409a8260ee71c#diff-52903c4477fc53869e7e92148494cbe5R17

Hope this isn't too overwhelming a wall of text - feel free to answer only
partly if you see fit.

Thanks in advance,
Klaas

-- 
Klaas van Schelven
+31 6 811 599 10
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.science.uu.nl/pipermail/nix-dev/attachments/20170622/8a5b7981/attachment.html>