Add CheckAptProxy task

Check if the specified APT proxy server can be reached, informing the
user if this can't be done. That will help them to debug the errors that
will be raised by `apt-get` because of the misleading proxy
configuration.

This closes #95.

bootstrapvz/plugins/apt_proxy/__init__.py

@@ -6,6 +6,7 @@ def validate_manifest(data, validator, error):
 
 def resolve_tasks(taskset, manifest):
 	import tasks
+	taskset.add(tasks.CheckAptProxy)
 	taskset.add(tasks.SetAptProxy)
 	if not manifest.plugins['apt_proxy'].get('persistent', False):
 		taskset.add(tasks.RemoveAptProxy)

bootstrapvz/plugins/apt_proxy/tasks.py

@@ -2,12 +2,34 @@ from bootstrapvz.base import Task
 from bootstrapvz.common import phases
 from bootstrapvz.common.tasks import apt
 import os
+import urllib2
+
+
+class CheckAptProxy(Task):
+	description = 'Checking APT proxy server'
+	phase = phases.package_installation
+
+	@classmethod
+	def run(cls, info):
+		proxy_address = info.manifest.plugins['apt_proxy']['address']
+		proxy_port = info.manifest.plugins['apt_proxy']['port']
+		proxy_url = 'http://{address}:{port}'.format(address=proxy_address, port=proxy_port)
+		try:
+			urllib2.urlopen(proxy_url, timeout=5)
+		except Exception as e:
+			# Default response from `apt-cacher-ng`
+			if isinstance(e, urllib2.HTTPError) and e.code == 404 and e.msg == 'Usage Information':
+				pass
+			else:
+				import logging
+				log = logging.getLogger(__name__)
+				log.error('The APT proxy server couldn\'t be reached. `apt-get` commands may fail from now on.')
 
 
 class SetAptProxy(Task):
 	description = 'Setting proxy for APT'
 	phase = phases.package_installation
-	successors = [apt.AptUpdate]
+	successors = [apt.AptUpdate, CheckAptProxy]
 
 	@classmethod
 	def run(cls, info):