common/linux/device.py: don't require sudo if already root user

On generic_linux devices, one might ssh as the root user, in which
case there is no need to use sudo.

In addition, some root filesystems may not have sudo (e.g. minimal
buildroot/busybox).

This patch attempts to detect the root user using 'id -u' and, if it
detects the root user, avoids the use of sudo for running commands as
well.

Cc: Lisa Nguyen <lisa.nguyen@linaro.org>
Signed-off-by: Kevin Hilman <khilman@linaro.org>
(setrofim: modified to only test once)

wlauto/common/linux/device.py

@@ -136,6 +136,7 @@ class BaseLinuxDevice(Device):  # pylint: disable=abstract-method
         self._is_ready = False
         self._just_rebooted = False
         self._is_rooted = None
+        self._is_root_user = False
         self._available_frequencies = {}
         self._available_governors = {}
         self._available_governor_tunables = {}
@@ -757,6 +758,16 @@ class LinuxDevice(BaseLinuxDevice):
     @property
     def is_rooted(self):
         if self._is_rooted is None:
+            # First check if the user is root
+            try:
+                self.execute('test $(id -u) = 0')
+                self._is_root_user = True
+                self._is_rooted = True
+                return self._is_rooted
+            except DeviceError:
+                self._is_root_user = False
+
+            # Otherwise, check if the user has sudo rights
             try:
                 self.execute('ls /', as_root=True)
                 self._is_rooted = True
@@ -852,6 +863,9 @@ class LinuxDevice(BaseLinuxDevice):
                 raise DeviceError('Cannot execute in background with as_root=True unless user is root.')
             return self.shell.background(command)
         else:
+            # If we're already the root user, don't bother with sudo
+            if self._is_root_user:
+                as_root = False
             return self.shell.execute(command, timeout, check_exit_code, as_root, strip_colors)
 
     def kick_off(self, command):