ci: use functools.cache instead of implementing custom cache global

Signed-off-by: Pablo Correa Gómez <pabloyoyoista@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7649>

.ci/lib/common.py

@@ -7,18 +7,12 @@
 import configparser
 import os
 import subprocess
+from functools import cache
 
 
-cache = {}
-
-
+@cache
 def get_pmaports_dir():
-    global cache
-    if "pmaports_dir" in cache:
-        return cache["pmaports_dir"]
-    ret = os.path.realpath(os.path.join(os.path.dirname(__file__) + "/../.."))
-    cache["pmaports_dir"] = ret
-    return ret
+    return os.path.realpath(os.path.join(os.path.dirname(__file__) + "/../.."))
 
 
 def run_git(parameters, check=True, stderr=None):
@@ -51,6 +45,7 @@ def run_pmbootstrap(parameters):
     subprocess.run(cmd, universal_newlines=True, check=True)
 
 
+@cache
 def get_upstream_branch():
     """ Use pmaports.cfg from current branch (e.g. "v20.05_fix-ci") and
         channels.cfg from master to retrieve the upstream branch.
@@ -61,10 +56,6 @@ def get_upstream_branch():
     if target_branch := os.environ.get("CI_MERGE_REQUEST_TARGET_BRANCH_NAME"):
         return target_branch
 
-    global cache
-    if "upstream_branch" in cache:
-        return cache["upstream_branch"]
-
     # Get channel (e.g. "stable") from pmaports.cfg
     # https://postmarketos.org/pmaports.cfg
     pmaports_dir = get_pmaports_dir()
@@ -82,9 +73,7 @@ def get_upstream_branch():
         " This appears to be an old branch, consider recreating your change" \
         " on top of master."
 
-    ret = channels_cfg[channel]["branch_pmaports"]
-    cache["upstream_branch"] = ret
-    return ret
+    return channels_cfg[channel]["branch_pmaports"]
 
 
 def get_base_commit() -> str: