This reverts commit ddc58e78.

https://hydra.nixos.org/eval/1435322

The overhead of sandbox builds is a problem on NixOS (since building a
NixOS configuration involves a lot of small derivations) but not for
typical non-NixOS use cases. So outside of NixOS we can enable it.

Issue #179.

Alos add a command "nix ping-store" to make it easier to see if Nix
can connect to a remote builder (e.g. 'nix ping-store --store
ssh://mac').

This reverts commit 4ea97075.

It causes an infinite loop in Nixpkgs evaluation,
e.g. "nix-instantiate -A hello" hung.

PR #1886.

maybe a left-over from nix-store -r ?

The assertion is broken because there is no one-to-one mapping from
length of a base64 string to the length of the output.

E.g.

"1q69lz7Empb06nzfkj651413n9icx0njmyr3xzq1j9q=" results in a 32-byte output.
"1q69lz7Empb06nzfkj651413n9icx0njmyr3xzq1j9qy" results in a 33-byte output.

To reproduce, evaluate:

builtins.derivationStrict {
    name = "0";
    builder = "0";
    system = "0";
    outputHashAlgo = "sha256";
    outputHash = "1q69lz7Empb06nzfkj651413n9icx0njmyr3xzq1j9qy";
}

Found by afl-fuzz.

Otherwise, running e.g.

nix-instantiate --eval -E --strict 'builtins.replaceStrings [""] ["X"] "abc"'

would just hang in an infinite loop.

Found by afl-fuzz.

E.g. nix-instantiate --eval -E 'abort "\r"' hangs.

Found by afl-fuzz.

Avoids ~180,000 string temporaries created when evaluating a headless
NixOS system.

https://hydra.nixos.org/eval/1434547#tabs-now-fail

This is already handled by the installer.

Also add some examples to nix --help.

Happily the failing tests should prevent anyone from using such a Nix
in situations where they expect sandboxing to be on,
which would otherwise be a risk.

Don't silently succeed seccomp setup when !HAVE_SECCOMP.

Running Nix with build users without seccomp on Linux is dangerous,
and administrators should very explicitly opt-in to it.

configure.ac: check if lzma has MT support, fix deb build/etc.

This is needed for new arches where libseccomp support doesn't exist
yet.

Fixes #1878.

drvName is already assigned to the same value right at the start of the
function.

Boehm guarantees that memory returned by GC_malloc() is zeroed, so take
advantage of that.

Instead of having lexicographicOrder() create a temporary sorted array
of Attr*:s and copying attr names from that, copy the attr names
first and then sort that.

Avoids some malloc() traffic.

Set MANPATH in case man path isn’t set correctly.

Previously, this would fail at startup for non-NixOS installs:

nix-env --help

The fix for this is to just use "nixManDir" as the value for MANPATH
when spawning "man".

To test this, I’m using the following:

$ nix-build release.nix -A build
$ MANPATH= ./result/bin/nix-env --help

Fixes #1627