While using moto server with a test SQS client, I noticed significant CPU usage while the client was long polling. I narrowed this down to the `receive_messages` call of the SQS service sitting in a `while True:` statement with no work to be done, thus looping forever.
To produce this issue, I do:
```
$ python3 -m venv venv
$ . ./venv/bin/activate
(venv) $ pip install moto moto[server] boto3
Collecting moto
Downloading moto-0.4.31-py2.py3-none-any.whl (303kB)
--snip--
(venv) $ moto_server sqs &
[1] 31727
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
(venv) $ python3
Python 3.6.0 (default, Dec 24 2016, 08:01:42)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import boto3
>>> client = boto3.client('sqs', region_name='us-east-1', endpoint_url='http://127.0.0.1:5000');
>>> client.create_queue(QueueName='testing')
127.0.0.1 - - [16/Mar/2017 13:34:20] "POST / HTTP/1.1" 200 -
{'QueueUrl': 'http://sqs.us-east-1.amazonaws.com/123456789012/testing', 'ResponseMetadata': {'RequestId': '7a62c49f-347e-4fc4-9331-6e8e7a96aa73', 'HTTPStatusCode': 200, 'HTTPHeaders': {'content-type': 'text/html; charset=utf-8', 'content-length': '343', 'server': 'Werkzeug/0.12.1 Python/3.6.0', 'date': 'Thu, 16 Mar 2017 20:34:20 GMT'}, 'RetryAttempts': 0}}
>>> client.receive_message(QueueUrl='http://sqs.us-east-1.amazonaws.com/123456789012/testing', MaxNumberOfMessages=10, WaitTimeSeconds=10)
```
At this point the moto server will run at 100% cpu for 10 seconds until the request times out waiting for a message. If multiple clients are continuously reconnected (as in mocking a normal sqs worker setup) the server will sit at 100% cpu indefinitely.
This pull request adds a simple sleep statement to the SQS `receive_messages` call when there are no messages to process. In doing so, the loop will be limited to executing once per 0.001 seconds when the queue is empty. The CPU usage is nearly 0% after this change.
We will continue to store just the unique ID, but since the AWS API
returns /hostedzone/<id>, we should accept attempts to pass that back.
For example, both just the ID as well as /hostedzone/<id> work for
specifying the HostedZoneId of a ResourceRecordSet in CloudFormation. So
we should support that too.
Signed-off-by: Scott Greene <scott.greene@getbraintree.com>
Originally, the code was setting sys.stdout and sys.stderr back to the
original, official forms, but this breaks idioms like mocking stdout to
capture printing output for tests. So instead, we will reset sys.stdout
and sys.stderr to what they were before running the lambda function, so
that in case someone is mocking stdout or stderr, their tests won't
break.
