Inconsistent marshalling behavior between value and pointer struct literals

[sudomateo]

I attempted to create an instance using the following value-based struct literal.

						NetworkInterfaces: oxide.InstanceNetworkInterfaceAttachment{
							Params: []oxide.InstanceNetworkInterfaceCreate{
								{
									Description: fmt.Sprintf("Managed by the Oxide Omni infrastructure provider (%s).", ID),
									Name:        oxide.Name(pctx.GetRequestID()),
									VpcName:     oxide.Name(machineClass.VPC),
									SubnetName:  oxide.Name(machineClass.Subnet),
									IpConfig: oxide.PrivateIpStackCreate{
										Value: oxide.PrivateIpStackCreateV4{
											Value: oxide.PrivateIpv4StackCreate{
												Ip: oxide.Ipv4Assignment{
													Type: oxide.Ipv4AssignmentTypeAuto,
												},
											},
										},
									},
								},
							},
							Type: oxide.InstanceNetworkInterfaceAttachmentTypeCreate,
						},

However, the R18 Omicron API responded with the following error.

failed creating oxide instance: POST https://oxide.sys.rack2.eng.oxide.computer/v1/instances?project=matthewsanabria\n----------- RESPONSE -----------\nStatus: 400 \nMessage: unable to parse JSON body: network_interfaces: unknown variant ``, expected one of `v4`, `v6`, `dual_stack` at line 1 column 802\nRequestID: ad964585-a108-48e1-8bd9-7277a1988c3a\n------- RESPONSE HEADERS -------\nX-Request-Id: [ad964585-a108-48e1-8bd9-7277a1988c3a]\nDate: [Tue, 27 Jan 2026 23:11:42 GMT]\nContent-Length: [203]\nContent-Type: [application/json]\n

This is because the serialized form of this was the following where ip_config.type was empty.

{
  "params": [
    {
      "description": "Managed by the Oxide Omni infrastructure provider (oxide).",
      "ip_config": {
        "type": "",
        "value": {
          "ip": {
            "type": "auto"
          }
        }
      },
      "name": "talos-default-control-planes-grwjt5",
      "subnet_name": "default",
      "vpc_name": "default"
    }
  ],
  "type": "create"
}

Instead, I was supposed to use this pointer-based struct literal.

						NetworkInterfaces: oxide.InstanceNetworkInterfaceAttachment{
							Params: []oxide.InstanceNetworkInterfaceCreate{
								{
									Description: fmt.Sprintf("Managed by the Oxide Omni infrastructure provider (%s).", ID),
									Name:        oxide.Name(pctx.GetRequestID()),
									VpcName:     oxide.Name(machineClass.VPC),
									SubnetName:  oxide.Name(machineClass.Subnet),
									IpConfig: oxide.PrivateIpStackCreate{
										Value: &oxide.PrivateIpStackCreateV4{
											Value: oxide.PrivateIpv4StackCreate{
												Ip: oxide.Ipv4Assignment{
													Type: oxide.Ipv4AssignmentTypeAuto,
												},
											},
										},
									},
								},
							},
							Type: oxide.InstanceNetworkInterfaceAttachmentTypeCreate,
						},

Which serializes to the following. Note how ip_config.type is now correctly v4.

{
  "params": [
    {
      "description": "Managed by the Oxide Omni infrastructure provider (oxide).",
      "ip_config": {
        "type": "v4",
        "value": {
          "ip": {
            "type": "auto"
          }
        }
      },
      "name": "talos-default-control-planes-grwjt5",
      "subnet_name": "default",
      "vpc_name": "default"
    }
  ],
  "type": "create"
}

The diff between the value and pointer JSON is likely clearer.

diff /tmp/value.json /tmp/pointer.json
29c29
<             "type": "",
---
>             "type": "v4",

[sudomateo]

This occurs because the MarshalJSON calls the Type method to fill type.

oxide.go/oxide/types.go

Lines 6351 to 6353 in e1ba7ef

	func (v PrivateIpStackCreate) MarshalJSON() ([]byte, error) {
	m := make(map[string]any)
	m["type"] = v.Type()

However, the Type method asserts on pointer types, not value types.

oxide.go/oxide/types.go

Lines 6311 to 6322 in e1ba7ef

	func (v PrivateIpStackCreate) Type() PrivateIpStackCreateType {
	switch v.Value.(type) {
	case *PrivateIpStackCreateV4:
	return PrivateIpStackCreateTypeV4
	case *PrivateIpStackCreateV6:
	return PrivateIpStackCreateTypeV6
	case *PrivateIpStackCreateDualStack:
	return PrivateIpStackCreateTypeDualStack
	default:
	return ""
	}
	}

[sudomateo]

I don't immediately have a solution to this since I found this as I was ending my day and I wanted to quickly file this issue so I don't forget about it tomorrow. Let's discuss this further tomorrow.

[sudomateo]

Looking at this more today the method set for our wrapper struct pattern looks like this.

func (v PrivateIpStack) AsDualStack() (*PrivateIpStackDualStack, bool)
func (v PrivateIpStack) AsV4() (*PrivateIpStackV4, bool)
func (v PrivateIpStack) AsV6() (*PrivateIpStackV6, bool)
func (v PrivateIpStack) MarshalJSON() ([]byte, error)
func (v PrivateIpStack) Type() PrivateIpStackType
func (v *PrivateIpStack) UnmarshalJSON(data []byte) error

All of the methods use value receivers except UnmarshalJSON which needs to use a pointer receiver since it stores data back into the receiver.

From my research it's suggested to implemented both UnmarshalJSON and MarshalJSON using pointer receivers since you're often using var foo T with &foo to call these methods. I'll look around the standard library and popular packages to see what others do here.

For Type, we should continue to use the value receiver and modify its content to assert on both values and pointers. This will allow us to keep our marker methods for variants using value receivers.

I'll submit a pull request for this and we can further discussion things there.

[sudomateo]

@jmcarp let me know what you think about the approach in #369.