# tag::fencing-start[]
import boto3
import jmespath
import json

from base64 import b64decode
from urllib.parse import unquote


def handle_site_offline(labels):
    a_client = boto3.client('globalaccelerator', region_name='us-west-2')

    acceleratorDNS = labels['accelerator']
    accelerator = jmespath.search(f"Accelerators[?DnsName=='{acceleratorDNS}']", a_client.list_accelerators())
    if not accelerator:
        print(f"Ignoring SiteOffline alert as accelerator with DnsName '{acceleratorDNS}' not found")
        return

    accelerator_arn = accelerator[0]['AcceleratorArn']
    listener_arn = a_client.list_listeners(AcceleratorArn=accelerator_arn)['Listeners'][0]['ListenerArn']

    endpoint_group = a_client.list_endpoint_groups(ListenerArn=listener_arn)['EndpointGroups'][0]
    endpoints = endpoint_group['EndpointDescriptions']

    # Only update accelerator endpoints if two entries exist
    if len(endpoints) > 1:
        # If the reporter endpoint is not healthy then do nothing for now
        # A Lambda will eventually be triggered by the other offline site for this reporter
        reporter = labels['reporter']
        reporter_endpoint = [e for e in endpoints if endpoint_belongs_to_site(e, reporter)][0]
        if reporter_endpoint['HealthState'] == 'UNHEALTHY':
            print(f"Ignoring SiteOffline alert as reporter '{reporter}' endpoint is marked UNHEALTHY")
            return

        offline_site = labels['site']
        endpoints = [e for e in endpoints if not endpoint_belongs_to_site(e, offline_site)]
        del reporter_endpoint['HealthState']
        a_client.update_endpoint_group(
            EndpointGroupArn=endpoint_group['EndpointGroupArn'],
            EndpointConfigurations=endpoints
        )
        print(f"Removed site={offline_site} from Accelerator EndpointGroup")
    else:
        print("Ignoring SiteOffline alert only one Endpoint defined in the EndpointGroup")


def endpoint_belongs_to_site(endpoint, site):
    lb_arn = endpoint['EndpointId']
    region = lb_arn.split(':')[3]
    client = boto3.client('elbv2', region_name=region)
    tags = client.describe_tags(ResourceArns=[lb_arn])['TagDescriptions'][0]['Tags']
    for tag in tags:
        if tag['Key'] == 'site':
            return tag['Value'] == site
    return false


def get_secret(secret_name, region_name):
    session = boto3.session.Session()
    client = session.client(
        service_name='secretsmanager',
        region_name=region_name
    )
    return client.get_secret_value(SecretId=secret_name)['SecretString']


def decode_basic_auth_header(encoded_str):
    split = encoded_str.strip().split(' ')
    if len(split) == 2:
        if split[0].strip().lower() == 'basic':
            try:
                username, password = b64decode(split[1]).decode().split(':', 1)
            except:
                raise DecodeError
        else:
            raise DecodeError
    else:
        raise DecodeError

    return unquote(username), unquote(password)


def handler(event, context):
    print(json.dumps(event))

    authorization = event['headers'].get('authorization')
    if authorization is None:
        print("'Authorization' header missing from request")
        return {
            "statusCode": 401
        }

# end::fencing-start[]
    expected_user = 'keycloak'
    secret_name = 'keycloak-master-password'
    secret_region = 'eu-central-1'
# tag::fencing-end[]
    expectedPass = get_secret(secret_name, secret_region)
    username, password = decode_basic_auth_header(authorization)
    if username != expected_user and password != expectedPass:
        print('Invalid username/password combination')
        return {
            "statusCode": 403
        }

    body = event.get('body')
    if body is None:
        raise Exception('Empty request body')

    body = json.loads(body)
    print(json.dumps(body))
    for alert in body['alerts']:
        labels = alert['labels']
        if labels['alertname'] == 'SiteOffline':
            handle_site_offline(labels)

    return {
        "statusCode": 204
    }
# end::fencing-end[]