#! /usr/bin/perl -w

use strict;

my $keep = 0;
my $sourceroot = 0;
my $name = "current";
my $srcid;

my $argnr = 0;
while ($argnr < scalar @ARGV) {
    my $arg = $ARGV[$argnr++];
    if ($arg eq "--keep") { $keep = 1; }
    elsif ($arg eq "--source-root") { $sourceroot = 1; }
    elsif ($arg eq "--name") { $name = $ARGV[$argnr++]; }
    elsif ($arg =~ /^([0-9a-z]{32})$/) { $srcid = $arg; }
    else { die "unknown argument `$arg'" };
}

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

# Build the specified package, and all its dependencies.
my $nfid = `nix --install $srcid`;
if ($?) { die "`nix --install' failed"; }
chomp $nfid;
die unless $nfid =~ /^([0-9a-z]{32})$/;

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

# Figure out a generation number.
opendir(DIR, $linkdir);
my $nr = 0;
foreach my $n (sort(readdir(DIR))) {
    next if (!($n =~ /^\d+$/));
    $nr = $n + 1 if ($n >= $nr);
}
closedir(DIR);

my $link = "$linkdir/$nr";

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

# Store the id of the normal form.  This is useful for garbage
# collection and the like.
my $idfile = "$linkdir/$nr.id";
open ID, "> $idfile" or die "cannot create $idfile";
print ID "$nfid\n";
close ID;

# Optionally store the source id.
if ($sourceroot) {
    $idfile = "$linkdir/$nr-src.id";
    open ID, "> $idfile" or die "cannot create $idfile";
    print ID "$srcid\n";
    close ID;
}

my $current = "$linkdir/$name";

# 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_$name";
symlink($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 or print "cannot delete $oldlink\n";
    unlink("$oldlink.id") == 1 or print "cannot delete $oldlink.id\n";
    unlink("$oldlink-src.id");
}