[dogstatsd] Allow to dynamically use default route as a statsd host

[remh]

When you run in a containerized environment, the dogstatsd server will
likely expose its port on the host network. If you have an application
that runs within a container, it needs to connect to it using the
default route.

[yannmh]

This could be a good candidate for a Python property too.

    @property.setter
    def use_default_route(self, value):
        self._use_default_route = value

       # Invalid the socket 
       self.socket = None

        if self._use_default_route:
            self.host = self._get_default_route()

with the following benefits:

1. We don't need to repeat self.host = self._get_default_route() twice.
2. The host and socket attributes would be systematically updated on use_default_route changes.

[remh]

We don't do it for the host or port though. Should we ?

[yannmh]

Looks good, I simply added a nitpick. Feel free to consider it or not, and merge the PR :)

[aeneaswiener]

On Mac OS X this doesn't degrade gracefully:

>>> from datadog import initialize, statsd
>>> initialize(statsd_use_default_route=True)
>>> statsd.event("Test event", "Test body")
No handlers could be found for logger "dogstatsd"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/env/lib/python2.7/site-packages/datadog/dogstatsd/base.py", line 363, in event
    self._send(string)
  File "/Users/env/lib/python2.7/site-packages/datadog/dogstatsd/base.py", line 299, in _send_to_server
    (self.socket or self.get_socket()).send(packet.encode(self.encoding))
  File "/Users/env/lib/python2.7/site-packages/datadog/dogstatsd/base.py", line 107, in get_socket
    sock.connect((self.host, self.port))
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/socket.py", line 228, in meth
    return getattr(self._sock,name)(*args)
TypeError: coercing to Unicode: need string or buffer, NoneType found

Is this expected / Is there a fix for this?

[yannmh]

That's definitely not expected.

Thanks for reporting @aeneaswiener. We are working on a fix.

[remh]

@aeneaswiener thanks for the feedback.

The main use case for this option is when you run your application in a (docker) container and you have your statsd server listening on the host. So for this use case, OS X support is not necessary.

Can you tell us a bit more about your use case ?

[aeneaswiener]

@remh We run our Python applications locally during development, which includes running a test suite before building and pushing docker images. Of course we could introduce an if statement when initialising dogstatsd to initialise differently depending on the environment, but it would be nicer if the statsd instance would just fail silently if there is no route to dogstatsd.

[remh]

@aeneaswiener not sure that failing silently is the best thing to do here. This is a bad practice that could hide underlying issues.

We could raise a better exception though if we can't find the default router, something like

raise IOError("Unable to find default route. This option is only supported on Linux")

Would that work ?

[aeneaswiener]

That would work, yes. Though I am not sure if IO error is the best one to raise as it leaks an implementation detail of the API. In any case I think the behaviour (exception) should be the same as when an unreachable statsd_host parameter is passed to the datadog.initialize method.