* When we activate a descriptor in nix-switch, remember its hash.

  This allows us to find out all `live' packages on the system by
  doing

    nix closure $(cat /nix/var/nix/links/*.hash)

  which will print out the activated configurations and all packages
  referenced by them.  We could then garbage collect unused packages
  by deleting the difference between `nix listinst' and the set
  returned by `nix closure ...'.

1 files changed, 7 insertions, 1 deletions

diff --git a/scripts/nix-switch b/scripts/nix-switch
index 74bcef8566f9..aec61cbfe147 100755
--- a/scripts/nix-switch
+++ b/scripts/nix-switch
@@ -20,11 +20,17 @@ chomp $id;
 my $nr = 0;
 while (-e "$linkdir/$id-$nr") { $nr++; }
 my $link = "$linkdir/$id-$nr";
-print "$pkgdir\n";
 
 # 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/$id-$nr.hash";
+open HASH, "> $hashfile" or die "cannot create $hashfile";
+print HASH "$hash\n";
+close HASH;
+
 # 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