isolator: fixed isolator, added test

`acbuild isolator add ...` was broken, producing the error:

```
isolator add: json: error calling MarshalJSON for type *types.App: invalid isolator
```

This commit fixes this, and adds a test for isolator add to be able to
catch this type of regression in the future.

lib/isolator.go

@@ -16,6 +16,7 @@ package lib
 
 import (
 	"encoding/json"
+	"fmt"
 
 	"github.com/appc/acbuild/util"
 
@@ -64,12 +65,27 @@ func (a *ACBuild) AddIsolator(name string, value []byte) (err error) {
 		if s.App == nil {
 			s.App = newManifestApp()
 		}
+		_, ok := types.ResourceIsolatorNames[*acid]
+		if !ok {
+			_, ok = types.LinuxIsolatorNames[*acid]
+			if !ok {
+				return fmt.Errorf("unknown isolator name: %s", name)
+			}
+		}
+		i := &types.Isolator{
+			Name:     *acid,
+			ValueRaw: &rawMsg,
+		}
+		blob, err := json.Marshal(i)
+		if err != nil {
+			return err
+		}
+		err = i.UnmarshalJSON(blob)
+		if err != nil {
+			return err
+		}
 		removeIsolatorFromMan(*acid)(s)
-		s.App.Isolators = append(s.App.Isolators,
-			types.Isolator{
-				Name:     *acid,
-				ValueRaw: &rawMsg,
-			})
+		s.App.Isolators = append(s.App.Isolators, *i)
 		return nil
 	}
 	return util.ModifyManifest(fn, a.CurrentACIPath)

tests/isolator_test.go

@@ -0,0 +1,74 @@
+// Copyright 2016 The appc Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tests
+
+import (
+	"encoding/json"
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"github.com/appc/spec/schema"
+	"github.com/appc/spec/schema/types"
+)
+
+const (
+	isolatorName  = "os/linux/capabilities-retain-set"
+	isolatorValue = `{ "set": ["CAP_NET_BIND_SERVICE"] }`
+)
+
+func manWithIsolators(isolators types.Isolators) schema.ImageManifest {
+	man := emptyManifestWithApp()
+	man.App.Isolators = isolators
+	return man
+}
+
+func TestAddIsolator(t *testing.T) {
+	workingDir := setUpTest(t)
+	defer cleanUpTest(workingDir)
+
+	isoFile, err := ioutil.TempFile("", "acbuild-test")
+	if err != nil {
+		t.Fatalf("%v\n", err)
+	}
+	_, err = isoFile.Write([]byte(isolatorValue))
+	isoFile.Close()
+	if err != nil {
+		t.Fatalf("%v\n", err)
+	}
+	defer os.RemoveAll(isoFile.Name())
+
+	err = runACBuildNoHist(workingDir, "isolator", "add", isolatorName, isoFile.Name())
+	if err != nil {
+		t.Fatalf("%v\n", err)
+	}
+
+	valueBlob := json.RawMessage(isolatorValue)
+	i := &types.Isolator{
+		Name:     *types.MustACIdentifier(isolatorName),
+		ValueRaw: &valueBlob,
+	}
+	blob, err := json.Marshal(i)
+	if err != nil {
+		t.Fatalf("%v\n", err)
+	}
+	err = i.UnmarshalJSON(blob)
+	if err != nil {
+		t.Fatalf("%v\n", err)
+	}
+
+	checkManifest(t, workingDir, manWithIsolators(types.Isolators{*i}))
+	checkEmptyRootfs(t, workingDir)
+}