#! /usr/bin/env python3
''' sns_error_notification is intended as a wrapper around the `lambda_handler()`
    function in an AWS serverless design. It catches errors and exceptions and
    posts them to an sns queue, which can then be subscribed to for emails or
    handled by another lambda function. 

    To use:
      1) Set the environment variable 'sns_error_topic' in your lambda
         function to the arn of an SNS topic.
      2) Ensure your lambda function has permission for `sns:Publish` to
         the sns topic.
      3) Import this function via 
         `from sns_error_notification import sns_error_nofication`
      4) Add `@sns_error_notification` directly before your lambda_handler
         definition (`def lambda_handler(event, context):`)

    WARNING: Could potentially generate a lot of email traffic on a high
             volume function. Watch your execution errors metric in 
             CloudWatch before deploying.

    See https://github.com/fdamstra/sns_error_notification
'''
import boto3
import json
import os
import traceback

def sns_error_notification(function_name):
    def wrapper(event, context):
        if os.environ.get('sns_notifications_enabled', 'true') == 'false':
            return function_name(event, context)
        try:
            sns_arn = os.environ.get('sns_error_topic',
              'Please set an environment variable "sns_error" topic')            
            snsclient = boto3.client('sns')
            function_name(event, context)
        except Exception as e:
            # First level error handling, send full details
            subject = f'Execution Error in {context.function_name}'
            estr = str(e)
            tb = traceback.format_exc()
            try:
                event_dump = json.dumps(event, indent=2)
            except:
                event_dump = "ERROR"
            try:
                context_dump = vars(context)
            except:
                context_dump = "ERROR"
            message = (f'Exception:\n{estr}\n\n'
                       f'Traceback:\n{tb}\n\n'
                       f'Event:\n{event_dump}\n\n'
                       f'Context:\n{context_dump}\n'
                      )
            # Only happens if there was an exception
            snsclient.publish(
                TopicArn=sns_arn,
                Subject=subject,
                Message=message
            )
            raise
    return wrapper