1 files changed, 61 insertions, 0 deletions
diff --git a/scripts/nix-switch.in b/scripts/nix-switch.in
new file mode 100755
index 000000000000..55305418c7f6
--- /dev/null
+++ b/scripts/nix-switch.in
@@ -0,0 +1,61 @@
+#! /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.
+my $pkgdir = `nix -qph $hash`;
+if ($?) { die "`nix -qph' 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";
+}