DiffSuppressFunc doesn't work for list-type attributes

[kristian-lesko]

SDK version

{
  "Path": "github.com/hashicorp/terraform-plugin-sdk",
  "Version": "v1.13.0"
}

Relevant provider source code

terraform-plugin-sdk/helper/schema/schema.go

Line 265 in 79b4af5

type SchemaDiffSuppressFunc func(k, old, new string, d *ResourceData) bool

func resourceService() *schema.Resource {
...
                        "test_attribute": &schema.Schema{
                                Type:     schema.TypeList,
                                Optional: true,
                                Elem:     &schema.Schema{Type: schema.TypeString},
                        },
...

Terraform Configuration Files

resource "example_resource" "example_name" {
  test_attribute = ["value1", "value2", "value3"]
}

Expected Behavior

It should be possible to implement a DiffSuppressFunction that would enable ignoring the order on an attribute that is a list of strings.

Actual Behavior

The DiffSuppressFunction schema only expects a string as the old and new attribute values; it doesn't seem possible to use on an attribute whose type is a list of strings.

[bendrucker]

A variation on this: DiffSuppressFunc is also not supported for schema.TypeMap. This prevents a provider from implementing diff suppression on a map, for example:

https://www.terraform.io/docs/providers/kubernetes/r/resource_quota.html
https://www.terraform.io/docs/providers/kubernetes/r/limit_range.html

Both of these resources use maps of resource quantities, mapping from a resource type (e.g. cpu) to a quantity (with an optional unit). CPU is often unit-less (2 = "2 cores") but can also have units (2000m = "2000 millicores").

The provider suppresses diffs where possible (when explicit fields are used) but cannot apply the same logic if the keys are not known in advance:

https://github.com/hashicorp/terraform-provider-kubernetes/blob/64c66f7634de8ca27d79fcda6b7d058ffaec460c/kubernetes/schema_container.go#L97-L121

[CyrusJavan]

Another benefit of making DiffSuppressFunc work with TypeList attributes would be the ability to easily create index-able non-unique element sets. e.g. TypeList attribute with a DiffSuppressFunc that ignores any diff due to an ordering mismatch.

[toumorokoshi]

In my experimentation, adding a DiffSuppressFunc to a TypeList element results in the func running on every element individually, but not the whole element. Although it does have some bugs, such as #743.

The current behavior doesn't allow considering the who list and doing things like shuffling order, but it does allow for examining individual elements and suppressing those diffs.

[diabloneo]

This behavior is really unnatural and undocumented. I had to debug my provider to figure out what's going on under the hood. I figured a simple way to do diff suppress against the list rather than the elements:

func ipListDiffSuppress(key, oldValue, newValue string, d *schema.ResourceData) bool {
	// For a list, the key is path to the element, rather than the list.
	// E.g. "node_groups.2.ips.0"
	lastDotIndex := strings.LastIndex(key, ".")
	if lastDotIndex != -1 {
		key = string(key[:lastDotIndex])
	}

	oldData, newData := d.GetChange(key)
	if oldData == nil || newData == nil {
		return false
	}

	oldIps := common.ConcreteList[string](oldData.([]any)) // from []any to []string
	newIps := common.ConcreteList[string](newData.([]any))
	sort.Strings(oldIps)
	sort.Strings(newIps)

	return reflect.DeepEqual(oldIps, newIps)
}

Bug, as @toumorokoshi said, this will cause the function called on every element in the list. If you want to mitigate the problem, you can use a memory cache whose key is address of the list attribute.

Looking forward to a real DiffSuppressFunc for the TypeList.

[monde]

I ran into a similar situation with another provider and we are using @diabloneo's technique to hack a diff suppress on list of strings that are not maintained in order by the underlying API.

[Ravikc]

Is there any fix planned for this? Or will this only be addressed in the new plugin framework?