#! /usr/bin/perl -w

use strict;

my $pkglist = $ENV{"NIX_ACTIVATIONS"};
$pkglist or die "NIX_ACTIVATIONS not set";
my $linkdir = $ENV{"NIX_LINKS"};
$linkdir or die "NIX_LINKS not set";
my @dirs = ("bin", "sbin", "lib");

# Figure out a generation number.
my $nr = 1;
while (-e "$linkdir/$nr") { $nr++; }
my $gendir = "$linkdir/$nr";
print "populating $gendir\n";

# Create the subdirectories.
mkdir $gendir;
foreach my $dir (@dirs) {
    mkdir "$gendir/$dir";
}

# For each activated package, create symlinks.

sub createLinks {
    my $srcdir = shift;
    my $dstdir = shift;

    my @srcfiles = glob("$srcdir/*");

    foreach my $srcfile (@srcfiles) {
	my $basename = $srcfile;
	$basename =~ s/^.*\///g; # strip directory
	my $dstfile = "$dstdir/$basename";
	if (-d $srcfile) {
	    # !!! hack for resolving name clashes
	    if (!-e $dstfile) {
		mkdir($dstfile) or 
		    die "error creating directory $dstfile";
	    }
	    -d $dstfile or die "$dstfile is not a directory";
	    createLinks($srcfile, $dstfile);
	} elsif (-l $dstfile) {
	    my $target = readlink($dstfile);
	    die "collission between $srcfile and $target";
	} else {
	    print "linking $dstfile to $srcfile\n";
	    symlink($srcfile, $dstfile) or
		die "error creating link $dstfile";
	}
    }
}


open PKGS, "< $pkglist";

while (<PKGS>) {
    chomp;
    my $hash = $_;
    
    my $pkgdir = `nix getpkg $hash`;
    if ($?) { die "`nix getpkg' failed"; }
    chomp $pkgdir;

    print "merging $pkgdir\n";

    foreach my $dir (@dirs) {
	createLinks("$pkgdir/$dir", "$gendir/$dir");
    }
}

close PKGS;

# Make $gendir 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.

my $current = "$linkdir/current";

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

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