jobspec: adding sugar for version constraint

jobspec/parse.go

@@ -242,6 +242,13 @@ func parseConstraints(result *[]*structs.Constraint, obj *hclobj.Object) error {
 			m["hard"] = true
 		}
 
+		// If "version" is provided, set the operand
+		// to "version" and the value to the "RTarget"
+		if constraint, ok := m["version"]; ok {
+			m["Operand"] = "version"
+			m["RTarget"] = constraint
+		}
+
 		// Build the constraint
 		var c structs.Constraint
 		if err := mapstructure.WeakDecode(m, &c); err != nil {

jobspec/parse_test.go

@@ -152,6 +152,26 @@ func TestParse(t *testing.T) {
 			false,
 		},
 
+		{
+			"version-constraint.hcl",
+			&structs.Job{
+				ID:       "foo",
+				Name:     "foo",
+				Priority: 50,
+				Region:   "global",
+				Type:     "service",
+				Constraints: []*structs.Constraint{
+					&structs.Constraint{
+						Hard:    true,
+						LTarget: "$attr.kernel.version",
+						RTarget: "~> 3.2",
+						Operand: "version",
+					},
+				},
+			},
+			false,
+		},
+
 		{
 			"specify-job.hcl",
 			&structs.Job{

jobspec/test-fixtures/version-constraint.hcl

@@ -0,0 +1,6 @@
+job "foo" {
+    constraint {
+        attribute = "$attr.kernel.version"
+        version = "~> 3.2"
+    }
+}