action_utils: Implement a utility to run a command as different user

- To be used to run specific command as another user.

Tests:

- Unit tests.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Joseph Nuthalapati <njoseph@riseup.net>

plinth/action_utils.py

@@ -788,3 +788,13 @@ def move_uploaded_file(source: str | pathlib.Path,
     shutil.move(source, destination)
     shutil.chown(destination, user, group)
     destination.chmod(permissions)
+
+
+def run_as_user(command, username, **kwargs):
+    """Run a command as another user.
+
+    Uses 'runuser' which is similar to 'su'. Creates PAM session unlike
+    setpriv. Sets real/effective uid/gid and resets the environment.
+    """
+    command = ['runuser', '--user', username, '--'] + command
+    return subprocess.run(command, **kwargs)

plinth/tests/test_action_utils.py

@@ -12,7 +12,7 @@ import pytest
 
 from plinth.action_utils import (get_addresses, get_hostname,
                                  is_systemd_running, move_uploaded_file,
-                                 service_action, service_disable,
+                                 run_as_user, service_action, service_disable,
                                  service_enable, service_is_enabled,
                                  service_is_running, service_reload,
                                  service_restart, service_start, service_stop,
@@ -229,3 +229,18 @@ def test_move_uploaded_file(tmp_path, upload_dir):
         assert destination_file.stat().st_mode & 0o777 == 0o600
         assert destination_file.read_text() == 'x-contents-2'
         assert not source.exists()
+
+
+@patch('subprocess.run')
+def test_run_as_user(run):
+    """Test running a command as another user works."""
+    run.return_value = 'test-return-value'
+    return_value = run_as_user(['command', 'arg1', '--foo'],
+                               username='foouser', stdout=subprocess.PIPE,
+                               check=True)
+    assert return_value == 'test-return-value'
+    assert run.mock_calls == [
+        call(
+            ['runuser', '--user', 'foouser', '--', 'command', 'arg1', '--foo'],
+            stdout=subprocess.PIPE, check=True)
+    ]