vmupdate: improve waiting for apt-get update lock

"apt-get update" doesn't support waiting for other instances to finish,
it exits immediately instead. With CLI apt-get version, it's possible to
get it waiting externally (by taking the lock manually, and then
make apt-get not try to lock it again). Unfortunately the same cannot
be done for the API version, because there is no way (I can find)
to disable locking in the apt.Cache.update() function.

Workaround for missing feature https://bugs.debian.org/1069167

vmupdate/agent/source/apt/apt_cli.py

@@ -23,6 +23,7 @@
 
 import fcntl
 import os
+import contextlib
 from typing import List
 
 from source.common.package_manager import PackageManager
@@ -38,12 +39,21 @@ def __init__(self, log_handler, log_level,):
         # to prevent a warning: `debconf: unable to initialize frontend: Dialog`
         os.environ['DEBIAN_FRONTEND'] = 'noninteractive'
 
-    def wait_for_lock(self):
+    @contextlib.contextmanager
+    def apt_lock(self):
         """
-        Wait for any other apt-get instance to finish.
+        Contex manager for locking compatible with 'apt-get update' lock.
         """
         with open("/var/lib/apt/lists/lock", "rb+") as f_lock:
             fcntl.lockf(f_lock.fileno(), fcntl.LOCK_EX)
+            yield
+
+    def wait_for_lock(self):
+        """
+        Wait for any other apt-get instance to finish.
+        """
+        with self.apt_lock():
+            pass
 
     def refresh(self, hard_fail: bool) -> ProcessResult:
         """
@@ -52,9 +62,12 @@ def refresh(self, hard_fail: bool) -> ProcessResult:
         :param hard_fail: raise error if some repo is unavailable
         :return: (exit_code, stdout, stderr)
         """
-        self.wait_for_lock()
-        cmd = [self.package_manager, "-q", "update"]
-        result = self.run_cmd(cmd)
+        # apply lock externally to wait for it, until
+        # https://bugs.debian.org/1069167 gets implemented
+        with self.apt_lock():
+            cmd = [self.package_manager,
+                   "-o", "Debug::NoLocking=true", "-q", "update"]
+            result = self.run_cmd(cmd)
         # 'apt-get update' reports error with exit code 100, but updater as a
         # whole reserves it for "no updates"
         if result.code == 100: