Ensure that we do not marshal read-only fields into request body in any level

[ArcturusZhang]

In the current go SDK, we do not have custom marshaler for every struct, and some of them may have read-only fields. Without a custom marshaler, the read-only fields will be marshaled into the request body.

For instance, the VirtualMachineProperties does not have a custom marshaler, despite the VMID field is read-only, it will still be in the request body if assigned.

Sample code of reproduce:

func createVM(authorizer autorest.Authorizer) {
	client := compute.NewVirtualMachinesClient(subsID)
	client.Authorizer = authorizer
	vm := compute.VirtualMachine{
		Location: to.StringPtr("eastus"),
		VirtualMachineProperties: &compute.VirtualMachineProperties{
			VMID: to.StringPtr("TEST"),
		},
	}
	future, err := client.CreateOrUpdate(ctx, "test-rg", "test-vm", vm)
	if err != nil {
		panic(err)
	}
	if err := future.WaitForCompletionRef(ctx, client.Client); err != nil {
		panic(err)
	}
}

Log:

(2020-07-14T16:13:04.0542686+08:00) INFO: REQUEST: PUT https://management.azure.com/subscriptions/**REDACTED**/resourceGroups/test-rg/providers/
Microsoft.Compute/virtualMachines/test-vm?api-version=2019-12-01
Content-Type: application/json; charset=utf-8
User-Agent: Go/go1.13.6 (amd64-windows) go-autorest/v14.0.0 Azure-SDK-For-Go/1.0.0 compute/2019-12-01
Authorization: **REDACTED**
{"location":"eastus","properties":{"vmId":"TEST"}}

[jhendrixMSFT]

It looks like the condition in the template doesn't explicitly check if a model contains R/O properties.

https://github.com/Azure/autorest.go/blob/master/src/Templates/ModelTemplate.cshtml#L142

Can you investigate further?

[ArcturusZhang]

@jhendrixMSFT I will take a look