# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#    sqs_dlq_check
# 
# Description:
#   This control checks whether an Amazon SQS queue is configured with a dead-letter queue.
# 
# Reports on:
#    AWS::SQS::Queue
# 
# 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 SQS queue resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an SQS queue resource
#       And: 'RedriveAllowPolicy' has been provided on the SQS queue
#       And: 'RedriveAllowPolicy.redrivePermission' is set to 'allowAll' or 'byQueue'
#      Then: SKIP
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an SQS queue resource
#       And: 'RedriveAllowPolicy' has not been provided on the SQS queue or 'RedriveAllowPolicy.redrivePermission'
#            has been provided and is set to a value other than 'allowAll' or 'byQueue'
#       And: 'RedrivePolicy' has not been provided
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an SQS queue resource
#       And: 'RedriveAllowPolicy' has not been provided on the SQS queue or 'RedriveAllowPolicy.redrivePermission'
#            has been provided and is set to a value other than 'allowAll' or 'byQueue'
#       And: 'RedrivePolicy' has been provided
#       And: 'RedrivePolicy.deadLetterTargetArn' has not been provided or has been provided as an empty string or
#            invalid local reference to an SQS queue
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an SQS queue resource
#       And: 'RedriveAllowPolicy' has not been provided on the SQS queue or 'RedriveAllowPolicy.redrivePermission' has
#            been provided and is set to a value other than 'allowAll' or 'byQueue'
#       And: 'RedrivePolicy' has been provided
#       And: 'RedrivePolicy.deadLetterTargetArn' has been provided as a non-empty string or valid local reference to
#             an SQS queue
#      Then: PASS

#
# Constants
#
let SQS_QUEUE_TYPE = "AWS::SQS::Queue"
let INPUT_DOCUMENT = this
let DLQ_REDRIVE_PERMISSION = ["allowAll", "byQueue"]

#
# Assignments
#
let sqs_queues = Resources.*[ Type == %SQS_QUEUE_TYPE ]

#
# Primary Rules
#
rule sqs_dlq_check when is_cfn_template(%INPUT_DOCUMENT)
                        %sqs_queues not empty {
    check(%sqs_queues.Properties)
        <<
        [CT.SQS.PR.1]: Require any Amazon SQS queue to have a dead-letter queue configured
        [FIX]: Create a 'RedrivePolicy' with a 'deadLetterTargetArn' value that's set to the ARN of an Amazon SQS dead-letter queue. For Amazon SQS dead-letter queues, instead provide a redrive configuration in the 'RedriveAllowPolicy' property.
        >>
}

rule sqs_dlq_check when is_cfn_hook(%INPUT_DOCUMENT, %SQS_QUEUE_TYPE) {
    check(%INPUT_DOCUMENT.%SQS_QUEUE_TYPE.resourceProperties)
        <<
        [CT.SQS.PR.1]: Require any Amazon SQS queue to have a dead-letter queue configured
        [FIX]: Create a 'RedrivePolicy' with a 'deadLetterTargetArn' value that's set to the ARN of an Amazon SQS dead-letter queue. For Amazon SQS dead-letter queues, instead provide a redrive configuration in the 'RedriveAllowPolicy' property.
        >>
}

#
# Parameterized Rules
#
rule check(sqs_queues) {
    %sqs_queues [
        # Scenario 2
        RedriveAllowPolicy not exists or
        filter_is_not_dlq(this)
    ] {
        # Scenario 3
        RedrivePolicy exists
        RedrivePolicy is_struct

        # Scenario 4
        RedrivePolicy {
            deadLetterTargetArn exists

            check_is_string_and_not_empty(deadLetterTargetArn) or
            check_local_references(%INPUT_DOCUMENT, deadLetterTargetArn, %SQS_QUEUE_TYPE)
        }
    }
}

rule filter_is_not_dlq(sqs_queue) {
    RedriveAllowPolicy exists
    RedriveAllowPolicy is_struct

    RedriveAllowPolicy {
        redrivePermission exists
        redrivePermission not in %DLQ_REDRIVE_PERMISSION
    }
}

#
# 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
}

rule check_is_string_and_not_empty(value) {
    %value {
        this is_string
        this != /\A\s*\z/
    }
}

rule check_local_references(doc, reference_properties, referenced_resource_type) {
    %reference_properties {
        "Fn::GetAtt" {
            query_for_resource(%doc, this[0], %referenced_resource_type)
                <<Local Stack reference was invalid>>
        } or Ref {
            query_for_resource(%doc, this, %referenced_resource_type)
                <<Local Stack reference was invalid>>
        }
    }
}

rule query_for_resource(doc, resource_key, referenced_resource_type) {
    let referenced_resource = %doc.Resources[ keys == %resource_key ]
    %referenced_resource not empty
    %referenced_resource {
        Type == %referenced_resource_type
    }
}