repos: cache properties that run external commands

Some of our properties internally run external VCS commands. As this can
be quite costly, we now cache the result. This is correct, as the repo
properties are referential transparent within a single kas invocation.

As cached_property is not available for python<3.8, we add a dependency
to the retrofit implementation in cached-property.

Signed-off-by: Felix Moessbauer <[email protected]>
Signed-off-by: Jan Kiszka <[email protected]>

kas/repos.py

@@ -36,6 +36,11 @@
 from .libkas import run_cmd_async, run_cmd
 from .kasusererror import KasUserError
 
+if sys.version_info < (3, 8):
+    from cached_property import cached_property
+else:
+    from functools import cached_property
+
 __license__ = 'MIT'
 __copyright__ = 'Copyright (c) Siemens AG, 2017-2018'
 
@@ -121,7 +126,7 @@ def effective_url(self):
                 continue
         return self.url
 
-    @property
+    @cached_property
     def revision(self):
         if self.commit:
             (_, output) = run_cmd(self.get_commit_cmd(),
@@ -144,7 +149,7 @@ def revision(self):
             return branch
         return None
 
-    @property
+    @cached_property
     def dirty(self):
         if not self.url:
             return True

setup.py

@@ -95,7 +95,8 @@
         'distro>=1.0.0,<2',
         'jsonschema>=2.5.0,<5',
         'kconfiglib>=14.1.0,<15',
-        'GitPython>=3.1.0,<4'
+        'GitPython>=3.1.0,<4',
+        "cached-property>=1.0,<2; python_version<'3.8'"
     ],
 
     # At least python 3.6 is needed by now: