nix-super/scripts/nix-switch.in

#! /usr/bin/perl -w

use strict;

my $keep = 0;

if (scalar @ARGV > 0 && $ARGV[0] eq "--keep") {
    shift @ARGV;
    $keep = 1;
}

my $hash = $ARGV[0];
$hash || die "no package hash specified";

my $linkdir = "@localstatedir@/nix/links";

# Build the specified package, and all its dependencies.
system "nix --install $hash";
if ($?) { die "`nix --install' failed"; }

my $pkgdir = `nix --query --list $hash`;
if ($?) { die "`nix --query --list' failed"; }
chomp $pkgdir;

# Figure out a generation number.
my $nr = 0;
while (-e "$linkdir/$nr") { $nr++; }
my $link = "$linkdir/$nr";

# Create a symlink from $link to $pkgdir.
symlink($pkgdir, $link) or die "cannot create $link: $!";

# Also store the hash of $pkgdir.  This is useful for garbage
# collection and the like.
my $hashfile = "$linkdir/$nr.hash";
open HASH, "> $hashfile" or die "cannot create $hashfile";
print HASH "$hash\n";
close HASH;

my $current = "$linkdir/current";

# Read the current generation so that we can delete it (if --keep
# wasn't specified).
my $oldlink = readlink($current);

# Make $link the current generation by pointing $linkdir/current to
# it.  The rename() system call is supposed to be essentially atomic
# on Unix.  That is, if we have links `current -> X' and `new_current
# -> Y', and we rename new_current to current, a process accessing
# current will see X or Y, but never a file-not-found or other error
# condition.  This is sufficient to atomically switch the current link
# tree.

print "switching $current to $link\n";

my $tmplink = "$linkdir/new_current";
symlink($link, $tmplink) or die "cannot create $tmplink";
rename($tmplink, $current) or die "cannot rename $tmplink";

if (!$keep && defined $oldlink) {
    print "deleting old $oldlink\n";
    unlink($oldlink) == 1 || print "cannot delete $oldlink\n";
    unlink("$oldlink.hash") == 1 || print "cannot delete $oldlink.hash\n";
}