backend: Block network access to test suites.

In this commit we basically start to override the request method of
httplib2.Http() to raise an exception whenever it is called i.e.
a trial is made to access the network from test suits.

Fixes: #1472.

docs/testing.md

@@ -79,6 +79,25 @@ if you're working on new database migrations.  To do this, run:
 
 [lxc-sf]: https://github.com/fgrehm/vagrant-lxc/wiki/FAQ#help-my-shared-folders-have-the-wrong-owner
 
+### Internet access inside test suits
+
+Zulip has a policy of requiring its backend tests to not depend upon Internet
+connectivity. We currently enforce this policy by overriding library functions
+which might be used to access internet (eg. `httplib2.Http().request`). Though
+this might not be a full proof blockade to internet access but we intend to
+improve upon this in near time.
+
+- If you are seeing test failures with tracebacks as below, you should
+probably mock the element trying to access the network. You can consult our
+[guide](/docs/testing-with-django.htmls#mocks-and-stubs) on
+mocking if you want to learn how to write mockups.
+
+  ```
+  File "tools/test-backend", line 120, in internet_guard
+    raise Exception("Zulip doesn't allow network access to test suits."
+  Exception: Zulip doesn't allow network access to test suits.
+  You need to mock any network access calls in testsuits.
+  ```
 
 ## Schema and initial data changes
 

tools/test-backend

@@ -2,13 +2,14 @@
 
 from __future__ import print_function
 from __future__ import absolute_import
-from typing import List
+from typing import List, Any
 import glob
 import optparse
 import os
 import sys
 import subprocess
 import ujson
+import httplib2
 
 # check for the venv
 from lib import sanity_check
@@ -110,7 +111,21 @@ def write_failed_tests(failed_tests):
         with open(FAILED_TEST_PATH, 'w') as f:
             ujson.dump(failed_tests, f)
 
+def block_internet():
+    # type: () -> None
+    # We are blocking internet currently by assuming mostly any test would use
+    # httplib2 to access internet.
+    def internet_guard(*args, **kwargs):
+        # type: (*Any, **Any) -> None
+        raise Exception("Zulip doesn't allow network access to test suits. "
+                        "You need to mock any network access calls in test"
+                        "suits. Checkout our docs on testing here for details."
+                        "https://zulip.readthedocs.io/en/latest/testing.html#internet-access-inside-test-suits")
+
+    httplib2.Http.request = internet_guard
+
 if __name__ == "__main__":
+    block_internet()
     TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
     os.chdir(os.path.dirname(TOOLS_DIR))
     sys.path.insert(0, os.path.dirname(TOOLS_DIR))