As discovered by Todd Veldhuizen, the shell started by nix-shell has
its affinity set to a single CPU.  This is because nix-shell connects
to the Nix daemon, which causes the affinity hack to be applied.  So
we turn this off for Perl programs.

Fixes #161.

This is equivalent to running ‘nix-env -e '*'’ first, except that it
happens in a single transaction.  Thus, ‘nix-env -i pkgs...’ replaces
the profile with the specified set of packages.

The main motivation is to support declarative package management
(similar to environment.systemPackages in NixOS).  That is, if you
have a specification ‘profile.nix’ like this:

  with import <nixpkgs> {};
  [ thunderbird
    geeqie
    ...
  ]

then after any change to ‘profile.nix’, you can run:

  $ nix-env -f profile.nix -ir

to update the profile to match the specification.  (Without the ‘-r’
flag, if you remove a package from ‘profile.nix’, it won't be removed
from the actual profile.)

Suggested by @zefhemel.

That is, you don't need to pass '*' anymore, so

  nix-env -qa

is equivalent to

  nix-env -qa '*'

So if you do "nix-env -qa -A nixos", then other channels won't be
parsed/evaluated at all.

This prevents some duplicate evaluation in nix-env and
nix-instantiate.

Also, when traversing ~/.nix-defexpr, only read regular files with the
extension .nix.  Previously it was reading files like
.../channels/binary-caches/<name>.  The only reason this didn't cause
problems is pure luck (namely, <name> shadows an actual Nix
expression, the binary-caches files happen to be syntactically valid
Nix expressions, and we iterate over the directory contents in just
the right order).

Since we already cache files in normal form (fileEvalCache), caching
parse trees is redundant.

Note that getting rid of this cache doesn't actually save much memory
at the moment, because parse trees are currently not freed / GC'ed.

Fixes #84.

Issue #162.

Reported by Matija Šuklje.

Fixes #163.

This is mostly useful for Hydra to deal with builders that get stuck
in an infinite loop writing data to stdout/stderr.

This reduces the difference between inherited and non-inherited
attribute handling to the choice of which env to use (in recs and lets)
by setting the AttrDef::e to a new ExprVar in the parser rather than
carrying a separate AttrDef::v VarRef member.

As an added bonus, this allows inherited attributes that inherit from a
with to delay forcing evaluation of the with's attributes.

Signed-off-by: Shea Levy <shea@shealevy.com>

On Linux, Nix can build i686 packages even on x86_64 systems.  It's not
enough to recognize this situation by settings.thisSystem, we also have
to consult uname().  E.g. we can be running on a i686 Debian with an
amd64 kernel.  In that situation settings.thisSystem is i686-linux, but
we still need to change personality to i686 to make builds consistent.

These cause an unnecessary runtime dependency :-)

Libpng used to accept this, but no longer does.

So on 64-bit systems, integers are now 64-bit.

Fixes #158.

nar.nix's builder depends on coreutils and nix itself being in $PATH.
Unfortunately, there's no good way to ensure that these packages exist
in the same place on the remote machine: The local machine may have nix
installed in /usr, and the remote machine in /usr/local, but the
generated nar.sh builder will refer to /usr and thus fail on the remote
machine. This ensures that nar.sh is run on the same machine that
instantiates it.

Signed-off-by: Shea Levy <shea@shealevy.com>

http://hydra.nixos.org/build/5663577

http://hydra.nixos.org/build/5662914

On a system with multiple CPUs, running Nix operations through the
daemon is significantly slower than "direct" mode:

$ NIX_REMOTE= nix-instantiate '<nixos>' -A system
real    0m0.974s
user    0m0.875s
sys     0m0.088s

$ NIX_REMOTE=daemon nix-instantiate '<nixos>' -A system
real    0m2.118s
user    0m1.463s
sys     0m0.218s

The main reason seems to be that the client and the worker get moved
to a different CPU after every call to the worker.  This patch adds a
hack to lock them to the same CPU.  With this, the overhead of going
through the daemon is very small:

$ NIX_REMOTE=daemon nix-instantiate '<nixos>' -A system
real    0m1.074s
user    0m0.809s
sys     0m0.098s