[backend] support memory per job constraint

To increase the memory need per used job on the worker. This can be
potentionally racy, because parallel builds may happened on different
times so the need of memory might differ depending on build speed.

docs/api/api/constraints.rng

@@ -116,6 +116,11 @@
                <ref name="size-unit"/>
              </element>
             </optional>
+            <optional>
+             <element name="memoryperjob">
+               <ref name="size-unit"/>
+             </element>
+            </optional>
             <optional>
              <element name="physicalmemory">
                <ref name="size-unit"/>

src/backend/BSDispatcher/Constraints.pm

@@ -111,6 +111,7 @@ sub oracle {
     my $memory = ($worker->{'hardware'}->{'memory'} || 0);
     my $swap = ($worker->{'hardware'}->{'swap'} || 0);
     return 0 if $constraints->{'hardware'}->{'memory'} && getmbsize($constraints->{'hardware'}->{'memory'}) > ( $memory + $swap );
+    return 0 if $constraints->{'hardware'}->{'memoryperjob'} && getmbsize($constraints->{'hardware'}->{'memoryperjob'} * ($worker->{'hardware'}->{'jobs'} || 1)) > ( $memory + $swap );
     return 0 if $constraints->{'hardware'}->{'physicalmemory'} && getmbsize($constraints->{'hardware'}->{'physicalmemory'}) > $memory;
     if ($constraints->{'hardware'}->{'cpu'}) {
       return 0 unless $worker->{'hardware'}->{'cpu'};

src/backend/BSXML.pm

@@ -1758,6 +1758,7 @@ our @constraint = (
 	    'jobs',
 	  [ 'disk' => $size ],
 	  [ 'memory' => $size ],
+	  [ 'memoryperjob' => $size ],
 	  [ 'physicalmemory' => $size ],
       ]
 );

src/backend/testdata/test_dispatcher

@@ -131,6 +131,10 @@ td sandbox24 true  "kernel" i586 medium sandbox_secure  enforce_kvm
 td sandbox25 true  "kernel" i586 large  sandbox         enforce_kvm
 td sandbox26 true  "kernel" i586 large  sandbox_secure  enforce_kvm
 
+# combined constraints for sandbox
+td perjob21 false "chromium" i586 small  kernel
+td perjob22 true "chromium" i586 large  kernel
+
 echo
 
 if [ -n "$FAILS" ]; then