gpg: set default in function to allow global patch

Patching globals to modify function behavior is an anti-pattern and
should not be recommended to a user!!

But it seems acceptable in tests (see #542). For this to work, the
global must not be used before being modified, which, AFAIK, is not
possible, if the global is defined and used (e.g. in a function
signature) in the same scope.

This commit changes the relevant function to access the default
value in local scope, so that it global can be imported and
modified before it.

More invasive but probably more correct solutions would be:
- move either GPG_TIMEOUT or is_available_gnupg to a different
  module, or
- call relevant functions in tests with an explicit timeout arg

Signed-off-by: Lukas Puehringer <[email protected]>

securesystemslib/gpg/constants.py

@@ -20,16 +20,19 @@
 import os
 import shlex
 import subprocess  # nosec
-from typing import List
+from typing import List, Optional
 
 log = logging.getLogger(__name__)
 
 GPG_TIMEOUT = 10
 
 
 @functools.lru_cache(maxsize=3)
-def is_available_gnupg(gnupg: str, timeout=GPG_TIMEOUT) -> bool:
+def is_available_gnupg(gnupg: str, timeout: Optional[int] = None) -> bool:
     """Returns whether gnupg points to a gpg binary."""
+    if timeout is None:
+        timeout = GPG_TIMEOUT
+
     gpg_version_cmd = shlex.split(f"{gnupg} --version")
     try:
         subprocess.run(  # nosec