# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   ecr_private_tag_immutability_enabled_check
# 
# Description:
#   This control checks whether a private Amazon ECR repository has tag immutability enabled.
# 
# Reports on:
#   AWS::ECR::Repository
# 
# Evaluates:
#   AWS CloudFormation, AWS CloudFormation hook
# 
# Rule Parameters:
#   None
# 
# Scenarios:
#   Scenario: 1
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document does not contain any ECR repository resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ECR repository resource
#       And: 'ImageTagMutability' has not been provided
#      Then: FAIL
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ECR repository resource
#       And: 'ImageTagMutability' has been provided with a value of 'MUTABLE'
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an ECR repository resource
#       And: 'ImageTagMutability' has been provided with a value of 'IMMUTABLE'
#      Then: PASS

#
# Constants
#
let ECR_REPOSITORY_TYPE = "AWS::ECR::Repository"
let INPUT_DOCUMENT = this

#
# Assignments
#
let ecr_repositories = Resources.*[ Type == %ECR_REPOSITORY_TYPE ]

#
# Primary Rules
#
rule ecr_private_tag_immutability_enabled_check when is_cfn_template(%INPUT_DOCUMENT)
                                                     %ecr_repositories not empty {
    check(%ecr_repositories.Properties)
        <<
        [CT.ECR.PR.3]: Require Amazon ECR private repositories to have tag immutability enabled
        [FIX]: Set 'ImageTagMutability' to 'IMMUTABLE'.
        >>
}

rule ecr_private_tag_immutability_enabled_check when is_cfn_hook(%INPUT_DOCUMENT, %ECR_REPOSITORY_TYPE) {
    check(%INPUT_DOCUMENT.%ECR_REPOSITORY_TYPE.resourceProperties)
        <<
        [CT.ECR.PR.3]: Require Amazon ECR private repositories to have tag immutability enabled
        [FIX]: Set 'ImageTagMutability' to 'IMMUTABLE'.
        >>
}

#
# Parameterized Rules
#
rule check(ecr_repository) {
    %ecr_repository {
        # Scenario 2, 3 and 4
        ImageTagMutability exists
        ImageTagMutability == "IMMUTABLE"
    }
}

#
# Utility Rules
#
rule is_cfn_template(doc) {
    %doc {
        AWSTemplateFormatVersion exists or
        Resources exists
    }
}

rule is_cfn_hook(doc, RESOURCE_TYPE) {
    %doc.%RESOURCE_TYPE.resourceProperties exists
}