parse_user_agent: Assert user agent is not None.

This commit asserts that parse_user_agent never returns None. The
RegEx will match any string, so that `match` is never None. This
brings test coverage of lib/user_agent.py to 100%. Changes were also
made in test/test_decorators.py and views/compatibility.py to reflect
that parse_user_agent cannot return None.

Improves: #7089.
Fixes: #8779.

zerver/lib/user_agent.py

@@ -1,11 +1,10 @@
 import re
-from typing import Optional, Dict
+from typing import Dict
 
 # Warning: If you change this parsing, please test using
 #   zerver/tests/test_decorators.py
 # And extend zerver/fixtures/user_agents_unique with any new test cases
-def parse_user_agent(user_agent: str) -> Optional[Dict[str, str]]:
+def parse_user_agent(user_agent: str) -> Dict[str, str]:
     match = re.match("^(?P<name>[^/ ]*[^0-9/(]*)(/(?P<version>[^/ ]*))?([ /].*)?$", user_agent)
-    if match is None:
-        return None
+    assert match is not None
     return match.groupdict()

zerver/tests/test_decorators.py

@@ -1315,7 +1315,6 @@ class TestUserAgentParsing(ZulipTestCase):
         """Test for our user agent parsing logic, using a large data set."""
         user_agents_parsed = defaultdict(int)  # type: Dict[str, int]
         user_agents_path = os.path.join(settings.DEPLOY_ROOT, "zerver/fixtures/user_agents_unique")
-        parse_errors = []
         for line in open(user_agents_path).readlines():
             line = line.strip()
             match = re.match('^(?P<count>[0-9]+) "(?P<user_agent>.*)"$', line)
@@ -1324,13 +1323,8 @@ class TestUserAgentParsing(ZulipTestCase):
             count = groupdict["count"]
             user_agent = groupdict["user_agent"]
             ret = parse_user_agent(user_agent)
-            self.assertIsNotNone(ret)
-            if ret is None:  # nocoverage
-                parse_errors.append(line)
-                continue
             user_agents_parsed[ret["name"]] += int(count)
 
-        self.assertEqual(len(parse_errors), 0)
 
 class TestIgnoreUnhashableLRUCache(ZulipTestCase):
     def test_cache_hit(self) -> None:

zerver/views/compatibility.py

@@ -7,6 +7,6 @@ from zerver.lib.user_agent import parse_user_agent
 
 def check_compatibility(request: HttpRequest) -> HttpResponse:
     user_agent = parse_user_agent(request.META["HTTP_USER_AGENT"])
-    if user_agent is None or user_agent['name'] == "ZulipInvalid":
+    if user_agent['name'] == "ZulipInvalid":
         return json_error("Client is too old")
     return json_success()