How it's possible to make custom rule works for the DeletionPolicy attribute in cfn-lint?

Question

`cfn-lint 0.86.4`

I have cloudformation template.json, and I want to add custom rule  for "DeletionPolicy" attribute for a specific value which is "Retain" for a specific resource  e.g EC2 or  Route53
```
  "Resources": {
    "EC2Instance": {
      "DeletionPolicy": "Detele",
      "Type": "AWS::EC2::Instance",
      "Properties": {
        "InstanceType": "m4.16xlarge",
        "ImageId": "ami-019a123123123",
        "KeyName": { "Ref": "KeyName" },
        "SecurityGroups": [
          { "Ref": "InstanceSecurityGroup" }
        ]
      }
    }
}
```
and I have `.cfnlintrc`  and `custom_rules.txt` files in the root
```
templates:
  -  myapp/template.json
ignore_templates:
  - codebuild.yaml
include_checks:
  - I
  - E3012 # = type-checking
```

custom_rules.txt file, each rule in a single line:
```
AWS::EC2::Instance InstanceType NOT_EQUALS "m4.16xlarge" WARN "This is an expensive instance type, use t2.micro instead"
AWS::EC2::Instance DeletionPolicy NOT_EQUALS "Delete" ERROR "You should set DeletionPolicy to Retain"
```

I am aware of the custom rule format :
`    [Error Level] [Custom Error Message]`
and I know that "DeletionPolicy" is not a property on the Properties object of a resource.
How is it possible to make the DeletionPolicy custom rule work?
when I run `cfn-lint` I only get warning about the instance type and nothing about the DeletionPolicy value
`W9001 This is an expensive instance type, use t2.micro instead`
Instead I should get :
`W9001 This is an expensive instance type, use t2.micro instead`
`E12345 You should set DeletionPolicy to Retain`

Answer

I don't think this is possible with the one line custom rule format, as it expects to operate on the Properties of the Resource. You could accomplish this using custom rules written in Python and using the -a / --append-rules CLI flag. For example, creating the directory `rules` and placing a file like `DeletionPolicyRetain.py` with the contents:

```
from cfnlint.rules import CloudFormationLintRule, RuleMatch

class DeletionPolicyRetain(CloudFormationLintRule):
    """Errors if EC2 Instance DeletionPolicy is not Retain"""
    id = 'E9001'
    shortdesc = 'Errors if EC2 Instance DeletionPolicy is not Retain'
    description = 'Errors if EC2 Instance DeletionPolicy is not Retain'
    source_url = 'https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-attribute-deletionpolicy.html'
    tags = ['resources', 'deletionpolicy']

def match(self, cfn):
        matches = []

resources = cfn.get_resources([])
        for resourceName, resource in resources.items():
            if resource['Type'] == 'AWS::EC2::Instance' and resource['DeletionPolicy'] != 'Retain':
                matches.append(RuleMatch(['Resources', resourceName], 'Found EC2 instance missing DeletionPolicy of Retain'))

return matches
```

and running the linting command with:

`cfn-lint -a rules`

How it's possible to make custom rule works for the DeletionPolicy attribute in cfn-lint?

Relevant content