/run/user/$UID as fallback if XDG_RUNTIME_DIR is not set

XDG mentions `/run/user/$UID` as the value for `XDG_RUNTIME_DIR`:
https://www.freedesktop.org/software/systemd/man/latest/pam_systemd.html
https://serverfault.com/questions/388840/good-default-for-xdg-runtime-di
r/727994#727994

Archlinux, Debian, RedHat, Ubuntu, etc all use `/run/user/$UID`
because they follow XDG:
https://wiki.archlinux.org/title/XDG_Base_Directory

Signed-off-by: Hans-Christoph Steiner <[email protected]>

podman/api/path_utils.py

@@ -7,11 +7,19 @@
 
 
 def get_runtime_dir() -> str:
-    """Returns the runtime directory for the current user"""
+    """Returns the runtime directory for the current user
+
+    The value in XDG_RUNTIME_DIR is preferred, but that is not always set, for
+    example, on headless servers. /run/user/$UID is defined in the XDG documentation.
+
+    """
     try:
         return os.environ['XDG_RUNTIME_DIR']
     except KeyError:
         user = getpass.getuser()
+        run_user = f'/run/user/{user}'
+        if os.path.isdir(run_user):
+            return run_user
         fallback = f'/tmp/podmanpy-runtime-dir-fallback-{user}'
 
         try:

podman/tests/unit/test_path_utils.py

@@ -0,0 +1,28 @@
+import datetime
+import os
+import unittest
+import tempfile
+from unittest import mock
+
+from podman import api
+
+
+class PathUtilsTestCase(unittest.TestCase):
+    def setUp(self):
+        self.xdg_runtime_dir = os.getenv('XDG_RUNTIME_DIR')
+        print('XDG_RUNTIME_DIR', self.xdg_runtime_dir)
+
+    @mock.patch.dict(os.environ, clear=True)
+    def test_get_runtime_dir_env_var_set(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            os.environ['XDG_RUNTIME_DIR'] = str(tmpdir)
+            self.assertEqual(str(tmpdir), api.path_utils.get_runtime_dir())
+
+    @unittest.skipUnless(os.getenv('XDG_RUNTIME_DIR'), 'XDG_RUNTIME_DIR must be set')
+    @mock.patch.dict(os.environ, clear=True)
+    def test_get_runtime_dir_env_var_not_set(self):
+        self.assertNotEqual(self.xdg_runtime_dir, api.path_utils.get_runtime_dir())
+
+
+if __name__ == '__main__':
+    unittest.main()