JSON Discriminators for vim25/types

This patch adds support to GoVmomi for marshaling/unmarshaling
the vim25/types (and other VMODL1 types) to/from JSON using:

* a customized version of the Golang encoding/json package that
  relies upon...
* a new discriminator field added to every every complex object
  marshaled to JSON, the "_typeName" field. this field's value
  must match the value of the type's registered type name from
  the type registry

vim25/types/json_test.go

@@ -0,0 +1,148 @@
+/*
+Copyright (c) 2022-2022 VMware, Inc. All Rights Reserved.
+
+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 types
+
+import (
+	"bytes"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/vmware/govmomi/vim25/json"
+)
+
+const discriminatorName = "_typeName"
+
+func TestJSONMarshalVirtualMachineConfigSpec(t *testing.T) {
+	var w bytes.Buffer
+	enc := json.NewEncoder(&w)
+	enc.SetIndent("", "  ")
+	enc.UseDiscriminator(discriminatorName)
+
+	if err := enc.Encode(VirtualMachineConfigSpec{
+		Name: "Hello, world.",
+		DeviceChange: []BaseVirtualDeviceConfigSpec{
+			&VirtualDeviceConfigSpec{
+				Operation:     VirtualDeviceConfigSpecOperationAdd,
+				FileOperation: VirtualDeviceConfigSpecFileOperationCreate,
+				Device: &VirtualVmxnet3{
+					VirtualVmxnet: VirtualVmxnet{
+						VirtualEthernetCard: VirtualEthernetCard{
+							VirtualDevice: VirtualDevice{
+								Key: 3,
+							},
+							MacAddress: "00:11:22:33:44:55:66:88",
+						},
+					},
+				},
+			},
+		},
+	}); err != nil {
+		t.Fatal(err)
+	}
+	act, exp := w.String(), virtualMachineConfigSpecWithDeviceChangesJSON
+	if act != exp {
+		t.Errorf("act json != exp json\nact=%s\nexp=%s", act, exp)
+	}
+}
+
+func TestJSONUnmarshalVirtualMachineConfigSpec(t *testing.T) {
+	dec := json.NewDecoder(strings.NewReader(virtualMachineConfigSpecWithVAppConfigJSON))
+	dec.UseDiscriminator(discriminatorName, json.DiscriminatorToTypeFunc(TypeFunc()))
+
+	var obj VirtualMachineConfigSpec
+	if err := dec.Decode(&obj); err != nil {
+		t.Fatal(err)
+	}
+
+	var w bytes.Buffer
+	enc := json.NewEncoder(&w)
+	enc.SetIndent("", "  ")
+	enc.UseDiscriminator(discriminatorName)
+
+	if err := enc.Encode(obj); err != nil {
+		t.Fatal(err)
+	}
+
+	act, exp := w.String(), virtualMachineConfigSpecWithVAppConfigJSON
+	if act != exp {
+		t.Errorf("act json != exp json\nact=%s\nexp=%s", act, exp)
+	}
+}
+
+func TestJSONUnmarshalVirtualMachineConfigInfo(t *testing.T) {
+	f, err := os.Open("./testdata/virtualMachineConfigInfo.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	dec := json.NewDecoder(f)
+	dec.UseDiscriminator(discriminatorName, json.DiscriminatorToTypeFunc(TypeFunc()))
+
+	var obj VirtualMachineConfigInfo
+	if err := dec.Decode(&obj); err != nil {
+		t.Fatal(err)
+	}
+
+	enc := json.NewEncoder(os.Stdout)
+	enc.SetIndent("", "  ")
+	enc.UseDiscriminator(discriminatorName)
+
+	if err := enc.Encode(obj); err != nil {
+		t.Fatal(err)
+	}
+}
+
+const virtualMachineConfigSpecWithDeviceChangesJSON = `{
+  "_typeName": "VirtualMachineConfigSpec",
+  "name": "Hello, world.",
+  "deviceChange": [
+    {
+      "_typeName": "VirtualDeviceConfigSpec",
+      "operation": "add",
+      "fileOperation": "create",
+      "device": {
+        "_typeName": "VirtualVmxnet3",
+        "key": 3,
+        "macAddress": "00:11:22:33:44:55:66:88"
+      }
+    }
+  ]
+}
+`
+
+const virtualMachineConfigSpecWithVAppConfigJSON = `{
+  "_typeName": "VirtualMachineConfigSpec",
+  "name": "Hello, world.",
+  "vAppConfig": {
+    "_typeName": "VmConfigSpec",
+    "product": [
+      {
+        "_typeName": "VAppProductSpec",
+        "operation": "add",
+        "info": {
+          "_typeName": "VAppProductInfo",
+          "key": 1,
+          "name": "p1"
+        }
+      }
+    ],
+    "installBootRequired": false
+  }
+}
+`