* Allow conflicting packages to be kept in a user environment, and

  allow switching between them (NIX-80).

  Example: two versions of Pan:

    $ nix-env -q pan
    pan-0.128
    pan-0.14.2.91

    $ readlink $(which pan)
    /nix/store/l38jrbilw269drpjkx7kinhrxj6fjh59-pan-0.14.2.91/bin/pan

  At most one of them can be active any given time.  Assuming than
  0.14.2.91 is active, you can active 0.128 as follows:

    $ nix-env --set-flag active false pan-0.14.2.91
    $ nix-env --set-flag active true pan-0.128

    $ readlink $(which pan)
    /nix/store/nziqwnlzy7xl385kglxhg75pfl5i936n-pan-0.128/bin/pan

  More flags to follow.

2 files changed, 11 insertions, 3 deletions

diff --git a/corepkgs/buildenv/builder.pl.in b/corepkgs/buildenv/builder.pl.in
index 57fa56903bc1..faebe27cc2a1 100755
--- a/corepkgs/buildenv/builder.pl.in
+++ b/corepkgs/buildenv/builder.pl.in
@@ -108,10 +108,15 @@ sub addPkg {
 
 
 # Symlink to the packages that have been installed explicitly by the user.
-my @args = split ' ', $ENV{"derivations"};
+my @paths = split ' ', $ENV{"derivations"};
+my @active = split ' ', $ENV{"active"};
 
-foreach my $pkgDir (sort @args) {
-    addPkg($pkgDir, 0);
+die if scalar @paths != scalar @active;
+
+for (my $n = 0; $n < scalar @paths; $n++) {
+    my $pkgDir = $paths[$n];
+    my $isActive = $active[$n];
+    addPkg($pkgDir, 0) if $isActive ne "false";
 }
 
 
diff --git a/corepkgs/buildenv/default.nix b/corepkgs/buildenv/default.nix
index 9bc704d8d511..a5452db5ea89 100644
--- a/corepkgs/buildenv/default.nix
+++ b/corepkgs/buildenv/default.nix
@@ -6,4 +6,7 @@ derivation {
   builder = ./builder.pl;
   derivations = derivations;
   manifest = manifest;
+
+  # !!! grmbl, need structured data for passing this in a clean way.
+  active = map (x: if x ? meta && x.meta ? active then x.meta.active else "true") derivations;
 }