identity: add a null value support for user assigned identity

resourcemanager/identity/model.go

@@ -7,12 +7,23 @@ import "encoding/json"
 
 var _ json.Marshaler = UserAssignedIdentityDetails{}
 
+// use a random pointer value
+var nullStringPtr = func(s string) *string { return &s }("")
+
+var NullUserAssignedIdentityDetails = UserAssignedIdentityDetails{
+	ClientId:    nullStringPtr,
+	PrincipalId: nullStringPtr,
+}
+
 type UserAssignedIdentityDetails struct {
 	ClientId    *string `json:"clientId,omitempty"`
 	PrincipalId *string `json:"principalId,omitempty"`
 }
 
 func (u UserAssignedIdentityDetails) MarshalJSON() ([]byte, error) {
 	// none of these properties can be set, so we'll just flatten an empty struct
+	if u == NullUserAssignedIdentityDetails {
+		return []byte("null"), nil
+	}
 	return json.Marshal(map[string]interface{}{})
 }

resourcemanager/identity/model_test.go

@@ -0,0 +1,61 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package identity
+
+import (
+	"reflect"
+	"testing"
+)
+
+func ptr(s string) *string {
+	return &s
+}
+
+func TestUserAssignedIdentityDetails_MarshalJSON(t *testing.T) {
+	tests := []struct {
+		name    string
+		identity  UserAssignedIdentityDetails
+		want    []byte
+		wantErr bool
+	}{
+		{
+			name: "empty identity",
+			identity: UserAssignedIdentityDetails{},
+			want: []byte("{}"),
+		},
+		{
+			name: "nil identity",
+			identity: NullUserAssignedIdentityDetails,
+			want: []byte("null"),
+		},
+		{
+			name: "other value identity",
+			identity: UserAssignedIdentityDetails{
+				ClientId:    ptr("other-client-id"),
+			},
+			want: []byte("{}"),
+		},
+		{
+			name: "other empty identity",
+			identity: UserAssignedIdentityDetails{
+				ClientId:    ptr(""),
+				PrincipalId: ptr(""),
+			},
+			want: []byte("{}"),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t2 *testing.T) {
+			u := tt.identity
+			got, err := u.MarshalJSON()
+			if (err != nil) != tt.wantErr {
+				t2.Errorf("UserAssignedIdentityDetails.MarshalJSON() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t2.Errorf("UserAssignedIdentityDetails.MarshalJSON() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}