* Experimental feature: allow a derivation to tell the build hook that

it requires a certain feature on the build machine, e.g.

    requiredSystemFeatures = [ "kvm" ];

  We need this in Hydra to make sure that builds that require KVM
  support are forwarded to machines that have KVM support.  Probably
  this should also be enforced for local builds.

scripts/build-remote.pl.in

@@ -36,6 +36,8 @@ sub sendReply {
     print STDERR "# $reply\n";
 }
 
+sub all { $_ || return 0 for @_; 1 }
+
 
 # Initialisation.
 my $loadIncreased = 0;
@@ -64,13 +66,14 @@ if (defined $conf && -e $conf) {
         chomp;
         s/\#.*$//g;
         next if /^\s*$/;
-        /^\s*(\S+)\s+(\S+)\s+(\S+)\s+(\d+)(\s+([0-9\.]+))?\s*$/ or die;
+        my @tokens = split /\s/, $_;
         push @machines,
-            { hostName => $1
-            , systemTypes => [split(/,/, $2)]
-            , sshKeys => $3
-            , maxJobs => $4
-            , speedFactor => 1.0 * ($6 || 1)
+            { hostName => $tokens[0]
+            , systemTypes => [ split(/,/, $tokens[1]) ]
+            , sshKeys => $tokens[2]
+            , maxJobs => int($tokens[3])
+            , speedFactor => 1.0 * (defined $tokens[4] ? int($tokens[4]) : 1)
+            , features => [ split(/,/, $tokens[5] || "") ]
             , enabled => 1
             };
     }
@@ -85,7 +88,8 @@ my ($drvPath, $hostName, $slotLock);
 REQ: while (1) {
     $_ = <STDIN> || exit 0;
     my ($amWilling, $neededSystem);
-    ($amWilling, $neededSystem, $drvPath) = split;
+    ($amWilling, $neededSystem, $drvPath, $requiredFeatures) = split;
+    my @requiredFeatures = split /,/, $requiredFeatures;
 
     my $canBuildLocally = $amWilling && ($localSystem eq $neededSystem);
 
@@ -103,12 +107,15 @@ REQ: while (1) {
     
     while (1) {
         # Find all machine that can execute this build, i.e., that
-        # support builds for the given platform and are not at their
-        # job limit.
+        # support builds for the given platform and features, and are
+        # not at their job limit.
         my $rightType = 0;
         my @available = ();
         LOOP: foreach my $cur (@machines) {
-            if ($cur->{enabled} && grep { $neededSystem eq $_ } @{$cur->{systemTypes}}) {
+            if ($cur->{enabled}
+                && (grep { $neededSystem eq $_ } @{$cur->{systemTypes}})
+                && all(map { my $f = $_; 0 != grep { $f eq $_ } @{$cur->{features}} } @requiredFeatures))
+            {
                 $rightType = 1;
 
                 # We have a machine of the right type.  Determine the load on