fix: phantom attributes on multi-resources (CLOUD-1843)

[jaspervdj-snyk]

We were failing to resolve the
bucket = aws_s3_bucket.nativebucket[0].id attribute in the following snippet:

resource "aws_s3_bucket" "nativebucket" {
    count = 1
    bucket = "test"
}

resource "aws_s3_bucket_versioning" "nativebucket" {
    count = 1
    bucket = aws_s3_bucket.nativebucket[0].id
    versioning_configuration {
      status = "Enabled"
    }
}

This is problematic since it is extremely common to use count = var.create ? 1 : 0 in terraform modules.

This is fixed by a number of changes:

1. We currently reference attributes by LocalName, which is a list of strings. This cannot represent an accessor that also has numbers in it, like aws_s3_bucket.nativebucket[0].id.

   A new accessor is added to take care of this case, including conversion functions to and from LocalName.

2. We take care to keep the trailing part of the accessors around in any conversion functions; so that aws_s3_bucket.nativebucket[0].id gets split into aws_s3_bucket.nativebucket and [0].id.

3. We refactor the phantomAttrs type so it receives a way to tell whether or not a resource is a multi-resource (rather than passing this in through an argument in phantomAttrs.add()).

4. Finally, now that we have all the machinery from 1-3 available, we can adjust phantomAttrs to remove one element from the trailing accessors when applicable.

I added this snippet as a golden test, and included one for the for_each version as well.

[craigfurman]

When I run the golden tests from this branch against the code from main, it neatly illustrates the issue this pr solves:

❯ go test ./pkg/input
--- FAIL: TestGolden (0.05s)
    --- FAIL: TestGolden/golden_test/tf/count-ref-02 (0.00s)
        utils.go:51:
                Error Trace:    /Users/craig/workspace/github.com/snyk/policy-engine/pkg/input/utils.go:51
                                                        /Users/craig/workspace/github.com/snyk/policy-engine/pkg/input/golden_test.go:97
                Error:          Not equal:
                                expected: "{\n  \"format\": \"\",\n  \"format_version\": \"\",\n  \"input_type\": \"tf_hcl\",\n  \"environment_provider\": \"iac\",\n  \"meta\": {\n    \"filepath\": \"golden_test/tf/count-ref-02/main.tf\"\n  },\n  \"resources\": {\n    \"aws_s3_bucket\": {\n      \"aws_s3_bucket.febucket[one]\": {\n        \"id\": \"aws_s3_bucket.febucket[one]\",\n        \"resource_type\": \"aws_s3_bucket\",\n        \"namespace\": \"golden_test/tf/count-ref-02/main.tf\",\n        \"meta\": {},\n        \"attributes\": {\n          \"bucket\": \"test\"\n        }\n      },\n      \"aws_s3_bucket.nativebucket[0]\": {\n        \"id\": \"aws_s3_bucket.nativebucket[0]\",\n        \"resource_type\": \"aws_s3_bucket\",\n        \"namespace\": \"golden_test/tf/count-ref-02/main.tf\",\n        \"meta\": {},\n        \"attributes\": {\n          \"bucket\": \"test\"\n        }\n      }\n    },\n    \"aws_s3_bucket_versioning\": {\n      \"aws_s3_bucket_versioning.febucket[one]\": {\n        \"id\": \"aws_s3_bucket_versioning.febucket[one]\",\n        \"resource_type\": \"aws_s3_bucket_versioning\",\n        \"namespace\": \"golden_test/tf/count-ref-02/main.tf\",\n        \"meta\": {},\n        \"attributes\": {\n          \"bucket\": \"aws_s3_bucket.febucket[one]\",\n          \"versioning_configuration\": [\n            {\n              \"status\": \"Enabled\"\n            }\n          ]\n        }\n      },\n      \"aws_s3_bucket_versioning.nativebucket[0]\": {\n        \"id\": \"aws_s3_bucket_versioning.nativebucket[0]\",\n        \"resource_type\": \"aws_s3_bucket_versioning\",\n        \"namespace\": \"golden_test/tf/count-ref-02/main.tf\",\n        \"meta\": {},\n        \"attributes\": {\n          \"bucket\": \"aws_s3_bucket.nativebucket[0]\",\n          \"versioning_configuration\": [\n            {\n              \"status\": \"Enabled\"\n            }\n          ]\n        }\n      }\n    }\n  },\n  \"scope\": {\n    \"filepath\": \"golden_test/tf/count-ref-02/main.tf\"\n  }\n}"
                                actual  : "{\n  \"format\": \"\",\n  \"format_version\": \"\",\n  \"input_type\": \"tf_hcl\",\n  \"environment_provider\": \"iac\",\n  \"meta\": {\n    \"filepath\": \"golden_test/tf/count-ref-02/main.tf\"\n  },\n  \"resources\": {\n    \"aws_s3_bucket\": {\n      \"aws_s3_bucket.febucket[one]\": {\n        \"id\": \"aws_s3_bucket.febucket[one]\",\n        \"resource_type\": \"aws_s3_bucket\",\n        \"namespace\": \"golden_test/tf/count-ref-02/main.tf\",\n        \"meta\": {},\n        \"attributes\": {\n          \"bucket\": \"test\"\n        }\n      },\n      \"aws_s3_bucket.nativebucket[0]\": {\n        \"id\": \"aws_s3_bucket.nativebucket[0]\",\n        \"resource_type\": \"aws_s3_bucket\",\n        \"namespace\": \"golden_test/tf/count-ref-02/main.tf\",\n        \"meta\": {},\n        \"attributes\": {\n          \"bucket\": \"test\"\n        }\n      }\n    },\n    \"aws_s3_bucket_versioning\": {\n      \"aws_s3_bucket_versioning.febucket[one]\": {\n        \"id\": \"aws_s3_bucket_versioning.febucket[one]\",\n        \"resource_type\": \"aws_s3_bucket_versioning\",\n        \"namespace\": \"golden_test/tf/count-ref-02/main.tf\",\n        \"meta\": {},\n        \"attributes\": {\n          \"bucket\": null,\n          \"versioning_configuration\": [\n            {\n              \"status\": \"Enabled\"\n            }\n          ]\n        }\n      },\n      \"aws_s3_bucket_versioning.nativebucket[0]\": {\n        \"id\": \"aws_s3_bucket_versioning.nativebucket[0]\",\n        \"resource_type\": \"aws_s3_bucket_versioning\",\n        \"namespace\": \"golden_test/tf/count-ref-02/main.tf\",\n        \"meta\": {},\n        \"attributes\": {\n          \"bucket\": null,\n          \"versioning_configuration\": [\n            {\n              \"status\": \"Enabled\"\n            }\n          ]\n        }\n      }\n    }\n  },\n  \"scope\": {\n    \"filepath\": \"golden_test/tf/count-ref-02/main.tf\"\n  }\n}"

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -36,3 +36,3 @@
                                         "attributes": {
                                -          "bucket": "aws_s3_bucket.febucket[one]",
                                +          "bucket": null,
                                           "versioning_configuration": [
                                @@ -50,3 +50,3 @@
                                         "attributes": {
                                -          "bucket": "aws_s3_bucket.nativebucket[0]",
                                +          "bucket": null,
                                           "versioning_configuration": [
                Test:           TestGolden/golden_test/tf/count-ref-02
FAIL
FAIL    github.com/snyk/policy-engine/pkg/input 0.857s
FAIL

Thanks for the detailed explanation too. I don't claim to totally grasp this line-by-line (don't get me wrong, I read it, but glossed over many functions as grey boxes) but I get the gist of what it's doing, and the golden tests prove this works and hasn't broken anything.