[Nix-dev] build environment for non-nixos software and different profiles

[Florian Friesdorf]

To sum up what I learned from you over the last days:

There is nix packaged software - no reason to worry about.
There is non-nix packaged software, which should be packaged.

For situations where this is not (yet) feasible and for users who lack
the knowledge I see no reason why they should not be happy on nixos
system.

For non-nixos builds there are two situations. The build system plays
nicely and sticks to environment variables it is given and the software
is broken and has hardcoded paths to /usr.

For broken software a /usr link is needed, for non-broken software it is
not.

From what I understand a nix build would not be affected by a /usr link,
as you can use nixpkgs on any linux system. As a nix developer you don't
want the link to identify broken software.

A nix expression could generate two files when added to a profile:
- hard set build environment variables FOO="..."
- extending build environment variables FOO="... $FOO"

Sourcing the extending files for system, default and user profile would
result in a set of build environment variables that correspond to what a
user's PATH reflects.

The nix expression would when added to the system profile also add
"include" to environment.pathsToLink.

For broken software with hardcoded /usr paths, one profile needs to be
linked there, containing everything necessary. And one would activate
that profile for builds by sourcing the non-extending build environment
variables file of that profile.


To give two real world examples:
- openldap had '/usr/bin/file' in its configure, after patching it was
  satisfied without a /usr link.
- python-ldap in the build system of several of our projects refuses to
  take more than one value for the include path. While this definitely
  should be fixed, I am able to work with a /usr link to a profile
  containing everything necessary.


What do you think?

Where would you place the files with the environment variables?
top-level in the profile as ``build_env_vars_isolated`` and
``build_env_vars``. Somewhere in etc?

Which 

florian
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