agents/sysfsgpio: fix misleading Python error when permission denied

If the executor doesn't have the proper permissions to control the GPIO
or export it, a Permission Denied will be raised. However, __del__ is
called regardless and gpio_sysfs_value_fd attribute isn't present if the
__init__ doesn't finish.

Thus, Python prints the following:
"""
Exception ignored in: <function GpioDigitalOutput.__del__ at 0x7f5b017f80e0>
Traceback (most recent call last):
  File "<loaded sysfsgpio>", line 44, in __del__
AttributeError: 'GpioDigitalOutput' object has no attribute 'gpio_sysfs_value_fd'
""" before the raised Exception, possibly misleading the user on their
debugging quest.

Therefore, let's make sure the attribute exists before anything can
throw an Exception in __init__.

os.close() can only be called on a valid file descriptor, which None
isn't, therefore only call os.close() if self.gpio_sysfs_value_fd is not
None.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>

Author: QSchulz

labgrid/util/agents/sysfsgpio.py

@@ -24,6 +24,7 @@ def _assert_gpio_line_is_exported(index):
             raise ValueError("Device not found")
 
     def __init__(self, index):
+        self.gpio_sysfs_value_fd = None
         self._logger = logging.getLogger("Device: ")
         GpioDigitalOutput._assert_gpio_line_is_exported(index)
         gpio_sysfs_path = os.path.join(GpioDigitalOutput._gpio_sysfs_path_prefix,
@@ -41,7 +42,8 @@ def __init__(self, index):
         self.gpio_sysfs_value_fd = os.open(gpio_sysfs_value_path, flags=(os.O_RDWR | os.O_SYNC))
 
     def __del__(self):
-        os.close(self.gpio_sysfs_value_fd)
+        if self.gpio_sysfs_value_fd:
+            os.close(self.gpio_sysfs_value_fd)
         self.gpio_sysfs_value_fd = None
 
     def get(self):