actions: Add method to handle privileged JSON request to a server

Tests:

- When privileged daemon receives a non-JSON request, a proper error structure
  is returned with SyntaxError.

- When privileged daemon receives a request without 'module', 'action', 'args'
  or 'kwargs' parameters, a proper error structure is returned with TypeError.

- When privileged daemon receives a request for invalid 'module' or 'action', a
  proper error structure is returned with SyntaxError.

- When an exception is thrown in a privileged method, the error is properly
  returned in error structure and caller is shown all the proper details.

- Valid return values are sent when a privileged call is made.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>

- Refactor validation of fields in the JSON object.
- Throw distinct errors for missing field and wrong type.
Signed-off-by: Joseph Nuthalapati <njoseph@riseup.net>

Reviewed-by: Joseph Nuthalapati <njoseph@riseup.net>

plinth/actions.py

@@ -359,6 +359,55 @@ def privileged_main():
         sys.exit(1)
 
 
+def privileged_handle_json_request(request_string: str) -> str:
+    """Parse arguments for the program spawned as a privileged action."""
+
+    def _parse_request() -> dict:
+        """Return a JSON parsed and validated request."""
+        try:
+            request = json.loads(request_string)
+        except json.JSONDecodeError:
+            raise SyntaxError('Invalid JSON in request')
+
+        required_parameters = [('module', str), ('action', str),
+                               ('args', list), ('kwargs', dict)]
+
+        for parameter, expected_type in required_parameters:
+            if parameter not in request:
+                raise TypeError(f'Missing required parameter "{parameter}"')
+            if not isinstance(request[parameter], expected_type):
+                raise TypeError(
+                    f'Parameter "{parameter}" must be of type {expected_type.__name__}'
+                )
+
+        return request
+
+    try:
+        request = _parse_request()
+        logger.info('Received request for %s..%s(..)', request['module'],
+                    request['action'])
+        arguments = {'args': request['args'], 'kwargs': request['kwargs']}
+        return_value = _privileged_call(request['module'], request['action'],
+                                        arguments)
+    except (PermissionError, SyntaxError, TypeError, Exception) as exception:
+        if isinstance(exception, (PermissionError, SyntaxError, TypeError)):
+            logger.error(exception.args[0])
+        else:
+            logger.exception(exception)
+
+        return_value = {
+            'result': 'exception',
+            'exception': {
+                'module': type(exception).__module__,
+                'name': type(exception).__name__,
+                'args': exception.args,
+                'traceback': traceback.format_tb(exception.__traceback__)
+            }
+        }
+
+    return json.dumps(return_value, cls=JSONEncoder)
+
+
 def _privileged_call(module_name, action_name, arguments):
     """Import the module and run action as superuser"""
     if '.' in module_name: