Consider Adding Attribute State Copying Plan Modifiers

[bflad]

Module version

v0.4.0

Use-cases

Due to the proposed behavior changes of #175, it is likely that an upcoming version of the framework will begin marking all Computed-only attributes as unknown in plans. The side effect is that provider developers may explicitly know that the attribute's state value should be copied to the plan as it is expected to not change during update (and therefore be hidden from plan output).

Attempted Solutions

Provider developers are able to manually create plan modifiers which copy the prior state value into the plan.

Proposal

This is likely to be a common use case for providers to improve plan outputs. The framework should likely provide two helper plan modifiers for this case:

• Always copy known state value to plan for the attribute (attribute is never expected to change after creation)
• Conditionally copy known state value to plan for the attribute (attribute may change due to other updates)

These likely would live in the tfsdk package, similar to RequiresReplace and RequiresReplaceIf

References

• Can't update Computed fields #175

[bflad]

I was helping a colleague write one of these for their provider today and here is an example implementation:

type ComputedValueDoesNotChangeModifier struct {}

func (m ComputedValueDoesNotChangeModifier) Description(ctx context.Context) string {
	return "Value does not change after creation."
}

func (m ComputedValueDoesNotChangeModifier) MarkdownDescription(ctx context.Context) string {
	return m.Description()
}

func (m ComputedValueDoesNotChangeModifier) Modify(ctx context.Context, req tfsdk.ModifyAttributePlanRequest, resp *tfsdk.ModifyAttributePlanResponse) {
	// Do nothing for Create. It will automatically be marked as unknown.
	if req.AttributeState == nil || req.AttributeState.IsNull() {
		return
	}

	resp.AttributePlan = req.AttributeState
}

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