# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   rds_instance_deployed_in_vpc_check
# 
# Description:
#   This control checks whether an Amazon RDS database (DB) instance is deployed in a VPC (that is, an EC2 VPC instance).
# 
# Reports on:
#   AWS::RDS::DBInstance
# 
# 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 RDS DB instance resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an RDS DB instance resource
#       And: 'Engine' is not one of 'mariadb', 'mysql', 'oracle-ee', 'oracle-ee-cdb', 'oracle-se2',
#            'oracle-se2-cdb', 'postgres', 'sqlserver-ee', 'sqlserver-se', 'sqlserver-ex', 'sqlserver-web'
#      Then: SKIP
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an RDS DB instance resource
#       And: 'Engine' is one of 'mariadb', 'mysql', 'oracle-ee', 'oracle-ee-cdb', 'oracle-se2',
#            'oracle-se2-cdb', 'postgres', 'sqlserver-ee', 'sqlserver-se', 'sqlserver-ex', 'sqlserver-web'
#       And: 'DBSubnetGroupName' has not been specified
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an RDS DB instance resource
#       And: 'Engine' is one of 'mariadb', 'mysql', 'oracle-ee', 'oracle-ee-cdb', 'oracle-se2',
#            'oracle-se2-cdb', 'postgres', 'sqlserver-ee', 'sqlserver-se', 'sqlserver-ex', 'sqlserver-web'
#       And: 'DBSubnetGroupName' has been specified but is an empty string
#            or invalid local reference to a DB Subnet Group
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an RDS DB instance resource
#       And: 'Engine' is one of 'mariadb', 'mysql', 'oracle-ee', 'oracle-ee-cdb', 'oracle-se2',
#            'oracle-se2-cdb', 'postgres', 'sqlserver-ee', 'sqlserver-se', 'sqlserver-ex', 'sqlserver-web'
#       And: 'DBSubnetGroupName' has been specified but is a non-empty string
#            or valid local reference to a DB Subnet Group
#      Then: PASS

#
# Constants
#
let RDS_DB_INSTANCE_TYPE = "AWS::RDS::DBInstance"
let SUPPORTED_RDS_INSTANCE_ENGINES = [
    "mariadb", "mysql", "oracle-ee", "oracle-ee-cdb", "oracle-se2",
    "oracle-se2-cdb", "postgres", "sqlserver-ee", "sqlserver-se",
    "sqlserver-ex", "sqlserver-web"
]
let INPUT_DOCUMENT = this

#
# Assignments
#
let rds_db_instances = Resources.*[ Type == %RDS_DB_INSTANCE_TYPE ]

#
# Primary Rules
#
rule rds_instance_deployed_in_vpc_check when is_cfn_template(%INPUT_DOCUMENT)
                                             %rds_db_instances not empty {
    check(%rds_db_instances.Properties)
        <<
        [CT.RDS.PR.11]: Require an Amazon RDS database instance to have a VPC configuration
        [FIX]: Set a 'DBSubnetGroupName'.
        >>
}

rule rds_instance_deployed_in_vpc_check when is_cfn_hook(%INPUT_DOCUMENT, %RDS_DB_INSTANCE_TYPE) {
    check(%INPUT_DOCUMENT.%RDS_DB_INSTANCE_TYPE.resourceProperties)
        <<
        [CT.RDS.PR.11]: Require an Amazon RDS database instance to have a VPC configuration
        [FIX]: Set a 'DBSubnetGroupName'.
        >>
}

#
# Parameterized Rules
#
rule check(rds_db_instance) {
    %rds_db_instance [filter_engine(this)] {
       # Scenario: 3
       DBSubnetGroupName exists

       # Scenario: 4 and 5
       check_db_subnet_group(DBSubnetGroupName)
    }
}

rule filter_engine(db_properties) {
    %db_properties {
        # Scenario: 2
        Engine exists
        Engine is_string
        Engine in %SUPPORTED_RDS_INSTANCE_ENGINES
    }
}

rule check_db_subnet_group(db_subnet_group) {
   %db_subnet_group {
      check_is_string_and_not_empty(this) or
      check_local_references(%INPUT_DOCUMENT, this, "AWS::RDS::DBSubnetGroup")
    }
}

#
# 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
    }
}