Make sure no nil lists are passed to TF

[VenelinMartinov]

Addresses pulumi/pulumi-aws#3427

We should make sure we send the upstream provider an empty array where MaxItemsOne properties are expected instead of a nil, since that is what TF does.

[iwahbe]

Does terraform distinguish between nil and missing?

[VenelinMartinov]

Great question but I doubt it given the way they use them: https://github.com/hashicorp/terraform-provider-aws/blob/3645bd03060df19f5cf29c132e26f7e40b56230f/internal/service/elbv2/listener.go#L425C1-L427C3

[t0yv0]

In maps yes, in objects no, an object having an optional field F will have it nil if it's missing, there's no other way for it to have it missing.

[VenelinMartinov]

After a discussion with @t0yv0 I dug a bit into the code related to makeTerraformInput trying to trace why we are not converting the missing input into an empty array for TF.

He suggested this is likely related to MaxItemsOne since we don't have a concept of empty array for these properties, while TF does.

Create calls UnmarshalTerraformConfig:

pulumi-terraform-bridge/pkg/tfbridge/provider.go

Line 1000 in 096668c

config, assets, err := UnmarshalTerraformConfig(ctx,

which one call down calls makeTerraformInputs:

pulumi-terraform-bridge/pkg/tfbridge/schema.go

Line 1233 in e877705

inputs, err := cctx.makeTerraformInputs(nil, m, tfs, ps)

Which then calls makeObjectTerraformInputs:

pulumi-terraform-bridge/pkg/tfbridge/schema.go

Line 546 in e877705

return ctx.makeObjectTerraformInputs(olds, news, tfs, ps, false /*rawNames*/)

which only iterates over the news:

pulumi-terraform-bridge/pkg/tfbridge/schema.go

Line 605 in e877705

for key, value := range news {

At this point the forward property is not in the news since it was not specified in the pulumi program.

This means that the code which is meant to fill the empty array in place of the MaxItemsOne nil is never called:

pulumi-terraform-bridge/pkg/tfbridge/schema.go

Lines 357 to 362 in e877705

	if IsMaxItemsOne(tfs, ps) {
	wrap := func(val resource.PropertyValue) resource.PropertyValue {
	if val.IsNull() {
	return resource.NewArrayProperty([]resource.PropertyValue{})
	}

This means the issue is either:

1. news should have contained a nil forward value, it should NOT have been missing.
2. We should call the MaxItemsOne nil handling code for all TF properties, not just the ones in news

The second one seems more likely to me, happy to be corrected here.

[t0yv0]

2 sounds exact to me. The transformation is schema-aware and should be walking object fields even if they're not present in the source PropertyMap but are present in the schema, so it can apply the unflattening treatment as necessary.

[VenelinMartinov]

I still need some tests for this.

[iwahbe]

Looks good to me. LGTM