Cherry pick 3872 to 202505

scripts/fast-reboot

@@ -1,6 +1,7 @@
 #!/bin/bash -e
 
-REBOOT_USER=$(logname)
+# reboot DBus service, which uses this script, doesn't use a login shell
+REBOOT_USER="$(logname 2> /dev/null || whoami)"
 REBOOT_TIME=$(date)
 REBOOT_CAUSE_FILE="/host/reboot-cause/reboot-cause.txt"
 WARM_DIR=/host/warmboot

tests/fast_reboot_test.py

@@ -0,0 +1,28 @@
+import os
+import pytest
+import shutil
+import subprocess
+import tempfile
+
+
+class TestFastReboot:
+    @pytest.mark.parametrize(
+        "working_script,failing_script", [
+            pytest.param('whoami', 'logname', id='without_logname'),
+            pytest.param('logname', 'whoami', id='without_whoami'),
+        ]
+    )
+    def test_fast_reboot_user(self, working_script, failing_script):
+        test_path = os.path.dirname(os.path.abspath(__file__))
+        fast_reboot = os.path.join(test_path, '..', 'scripts', 'fast-reboot')
+        with tempfile.TemporaryDirectory() as tmp_dir:
+            working_script_path = os.path.join(tmp_dir, working_script)
+            with open(working_script_path, 'w') as f:
+                f.write('echo root')
+                os.chmod(working_script_path, 0o700)
+            shutil.copy('/bin/false', os.path.join(tmp_dir, failing_script))
+            shutil.copy('/bin/true', os.path.join(tmp_dir, 'sonic-cfggen'))
+            env = os.environ.copy()
+            env['PATH'] = f"{tmp_dir}:{env.get('PATH', '')}"
+            res = subprocess.run([fast_reboot, '-h'], env=env)
+        assert res.returncode == 0