info, sort attribute sets.

  <derivation outPath=... drvPath=...> attrs </derivation>.  Only emit
  the attributes of any specific derivation only.  This prevents
  exponententially large XML output due to the absense of sharing.

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  (currently 60%).  Large patches aren't very economical.

  from a source directory.  All files for which a predicate function
  returns true are copied to the store.  Typical example is to leave
  out the .svn directory:

    stdenv.mkDerivation {
      ...
      src = builtins.filterSource
        (path: baseNameOf (toString path) != ".svn")
        ./source-dir;
      # as opposed to
      #   src = ./source-dir;
    }

  This is important because the .svn directory influences the hash in
  a rather unpredictable and variable way.

  selectively in/excluded from the dump.

  single derivation specified by the argument.  This is useful when we
  want to have a profile for a single derivation, such as a server
  configuration.  Then we can just say (e.g.)

  $ nix-env -p /.../server-profile -f server.nix --set -A server

  We can't do queries or upgrades on such a profile, but we can do
  rollbacks.  The advantage over -i is that we don't have to worry
  about other packages having been installed in the profile
  previously; --set gets rid of them.

  names of the attributes in an attribute set.

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  matters when running as root, since then we don't use the setuid
  helper (which already used lchown()).
  
* Also check for an obscure security problem on platforms that don't
  have lchown.  Then we can't change the ownership of symlinks, which
  doesn't matter *except* when the containing directory is writable by
  the owner (which is the case with the top-level Nix store directory).

  very carefully, since it can invalidate iterators into the
  `children' map.

* Throw more exceptions as BuildErrors instead of Errors.  This
  matters when --keep-going is turned on.  (A BuildError is caught
  and terminates the goal in question, an Error terminates the
  program.)

  seconds without producing output on stdout or stderr (NIX-65).  This
  timeout can be specified using the `--max-silent-time' option or the
  `build-max-silent-time' configuration setting.  The default is
  infinity (0).

* Fix a tricky race condition: if we kill the build user before the
  child has done its setuid() to the build user uid, then it won't be
  killed, and we'll potentially lock up in pid.wait().  So also send a
  conventional kill to the child.

  in case of failure.

  back to the Nix account.

  since if we're running a build user in non-root mode, we can't.  Let
  the setuid helper do it.

  processes before and after the build.

  ownership, then try again.

  ownership of the build result after the build.

  deleting them using the setuid helper.

  builder under the desired build user.