apt: use a decorator to ensure apt_pkg is initialized

Instead of performing the check at the module level, we decorate the
functions that use the module without instantiating a Cache object (in
the Cache case, we initialize apt_pkg separately)

Signed-off-by: Renan Rodrigo <[email protected]>

uaclient/apt.py

@@ -8,7 +8,7 @@
 import subprocess
 import sys
 import tempfile
-from functools import lru_cache
+from functools import lru_cache, wraps
 from typing import Dict, Iterable, List, NamedTuple, Optional, Set, Union
 
 import apt_pkg  # type: ignore
@@ -83,12 +83,6 @@
 event = event_logger.get_event_logger()
 LOG = logging.getLogger(util.replace_top_level_logger_name(__name__))
 
-# Most parts of the apt_pkg functionality needs the module to be initialized.
-# This call is checking for the 'Dir' configuration - which needs to be there
-# for apt_pkg to be ready - and if it is empty we initialize.
-if apt_pkg.config.get("Dir") == "":
-    apt_pkg.init()
-
 
 @enum.unique
 class AptProxyScope(enum.Enum):
@@ -100,7 +94,24 @@ class AptProxyScope(enum.Enum):
     "InstalledAptPackage", [("name", str), ("version", str), ("arch", str)]
 )
 
-version_compare = apt_pkg.version_compare
+
+def ensure_apt_pkg_init(f):
+    """Decorator ensuring apt_pkg is initialized."""
+
+    @wraps(f)
+    def new_f(*args, **kwargs):
+        # This call is checking for the 'Dir' configuration - which needs to be
+        # there for apt_pkg to be ready - and if it is empty we initialize.
+        if apt_pkg.config.get("Dir") == "":
+            apt_pkg.init()
+        return f(*args, **kwargs)
+
+    return new_f
+
+
+@ensure_apt_pkg_init
+def version_compare(a: str, b: str):
+    return apt_pkg.version_compare(a, b)
 
 
 def assert_valid_apt_credentials(repo_url, username, password):

uaclient/apt_news.py

@@ -9,6 +9,7 @@
 
 from uaclient import defaults, messages, system, util
 from uaclient.api.u.pro.status.is_attached.v1 import _is_attached
+from uaclient.apt import ensure_apt_pkg_init
 from uaclient.clouds.identity import get_cloud_type
 from uaclient.config import UAConfig
 from uaclient.contract import ContractExpiryStatus, get_contract_expiry_status
@@ -149,6 +150,7 @@ def select_message(
     return None
 
 
+@ensure_apt_pkg_init
 def fetch_aptnews_json(cfg: UAConfig):
     os.makedirs(defaults.UAC_RUN_PATH, exist_ok=True)
     acq = apt_pkg.Acquire()
@@ -214,7 +216,6 @@ def update_apt_news(cfg: UAConfig):
     try:
         news = local_apt_news(cfg)
         if not news:
-            apt_pkg.init()
             news = fetch_and_process_apt_news(cfg)
         if news:
             state_files.apt_news_raw_file.write(news)