* 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

src/libstore/build.cc

@@ -1327,8 +1327,16 @@ HookReply DerivationGoal::tryBuildHook()
     if (!worker.hook)
         worker.hook = boost::shared_ptr<HookInstance>(new HookInstance);
 
-    writeLine(worker.hook->toHook.writeSide, (format("%1% %2% %3%") %
-        (worker.getNrLocalBuilds() < maxBuildJobs ? "1" : "0") % drv.platform % drvPath).str());
+    /* Tell the hook about system features (beyond the system type)
+       required from the build machine.  (The hook could parse the
+       drv file itself, but this is easier.) */
+    Strings features = tokenizeString(drv.env["requiredSystemFeatures"]);
+    foreach (Strings::iterator, i, features) checkStoreName(*i); /* !!! abuse */
+
+    /* Send the request to the hook. */
+    writeLine(worker.hook->toHook.writeSide, (format("%1% %2% %3% %4%")
+        % (worker.getNrLocalBuilds() < maxBuildJobs ? "1" : "0")
+        % drv.platform % drvPath % concatStringsSep(",", features)).str());
 
     /* Read the first line of input, which should be a word indicating
        whether the hook wishes to perform the build. */

src/libutil/util.cc

@@ -966,6 +966,17 @@ Strings tokenizeString(const string & s, const string & separators)
 }
 
 
+string concatStringsSep(const string & sep, const Strings & ss)
+{
+    string s;
+    foreach (Strings::const_iterator, i, ss) {
+        if (s.size() != 0) s += sep;
+        s += *i;
+    }
+    return s;
+}
+
+
 string statusToString(int status)
 {
     if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {

src/libutil/util.hh

@@ -280,6 +280,11 @@ MakeError(Interrupted, BaseError)
 Strings tokenizeString(const string & s, const string & separators = " \t\n\r");
 
 
+/* Concatenate the given strings with a separator between the
+   elements. */
+string concatStringsSep(const string & sep, const Strings & ss);
+
+
 /* Convert the exit status of a child as returned by wait() into an
    error string. */
 string statusToString(int status);