lint: Add lint rule for scripts importing typing improperly.

This is a common bug that users might be tempated to introduce.

And also fix two instances of this bug that were present in our
codebase, including an important one in our upgrade code path.

puppet/zulip/files/postgresql/pg_backup_and_purge

@@ -10,13 +10,15 @@ import dateutil.parser
 import pytz
 import time
 from datetime import datetime, timedelta
-from typing import Dict, List
+if False:
+    from typing import Dict, List
 
 logging.Formatter.converter = time.gmtime
 logging.basicConfig(format="%(asctime)s %(levelname)s: %(message)s")
 logger = logging.getLogger(__name__)
 
-def run(args: List[str], dry_run: bool=False) -> str:
+def run(args, dry_run=False):
+    # type: (List[str], bool) -> str
     if dry_run:
         print("Would have run: " + " ".join(map(shlex.quote, args)))
         return ""

scripts/lib/upgrade-zulip

@@ -6,7 +6,6 @@ import subprocess
 import logging
 import time
 import configparser
-from typing import List
 
 TARBALL_ARCHIVE_PATH = "/home/zulip/archives"
 os.environ["PYTHONUNBUFFERED"] = "y"
@@ -17,7 +16,7 @@ from scripts.lib.zulip_tools import DEPLOYMENTS_DIR, FAIL, WARNING, ENDC, \
     get_config_file, get_deploy_options
 
 config_file = get_config_file()  # type: configparser.RawConfigParser
-deploy_options = get_deploy_options(config_file)  # type: List[str]
+deploy_options = get_deploy_options(config_file)
 
 assert_running_as_root(strip_lib_from_paths=True)
 

tools/linter_lib/custom_check.py

@@ -479,6 +479,21 @@ def build_custom_checkers(by_lang):
          'description': 'Most scripts are intended to run on systems without sudo.',
          'good_lines': ['subprocess.check_call(["ls"])'],
          'bad_lines': ['subprocess.check_call(["sudo", "ls"])']},
+        {'pattern': '^from typing import',
+         'strip': '\n',
+         'include_only': set([
+             'scripts/',
+             'puppet/',
+         ]),
+         'exclude': set([
+             # Not important, but should fix
+             'scripts/lib/process-mobile-i18n',
+             # Uses setup_path_on_import before importing.
+             'puppet/zulip/files/nagios_plugins/zulip_app_frontend/check_send_receive_time',
+         ]),
+         'description': 'For scripts run as part of installer, cannot rely on typing existing; use `if False` workaround.',
+         'good_lines': ['subprocess.check_call(["ls"])'],
+         'bad_lines': ['subprocess.check_call(["sudo", "ls"])']},
         {'pattern': 'django.utils.translation',
          'include_only': set(['test/']),
          'description': 'Test strings should not be tagged for translation',