expr: Get rid of MatchEmpty.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

scheduler/filter/constraint.go

@@ -34,8 +34,13 @@ func (f *ConstraintFilter) Filter(config *dockerclient.ContainerConfig, nodes []
 					if constraint.Match(label) {
 						candidates = append(candidates, node)
 					}
-				} else if constraint.MatchEmpty() {
+				} else {
-					candidates = append(candidates, node)
+					// The node doesn't have this particular label.
+					if constraint.operator == NOTEQ {
+						// Special case: If the operator is != and the node doesn't
+						// have the label at all, consider it as a candidate.
+						candidates = append(candidates, node)
+					}
 				}
 			}
 		}

scheduler/filter/expr.go

@@ -72,10 +72,6 @@ func parseExprs(key string, env []string) ([]expr, error) {
 	return exprs, nil
 }
 
-func (e *expr) MatchEmpty() bool {
-	return e.operator == NOTEQ
-}
-
 func (e *expr) Match(whats ...string) bool {
 	var (
 		pattern string