More robust initialization against a delay of info

[Cryst4L9527]

Hi! I'm a researcher on bug detection and software robustness focusing on robotic applications. It's our new idea to design a language-irrelevant algorithm, so we are trying to check and fix some python-based programs to prove our algorithm. Since your program is one of the best opensource codes in python for the ROS system, we did a test and found some crashes:

maze_solver:
Traceback (most recent call last):
  ...
  File "/root/ROS2-Path-Planning-and-Maze-Solving/path_planning_ws/install/maze_bot/lib/python3.8/site-packages/maze_bot/maze_solver.py", line 221, in maze_solving
    self.bot_localizer.localize_bot(self.sat_view, frame_disp)
  File "/root/ROS2-Path-Planning-and-Maze-Solving/path_planning_ws/install/maze_bot/lib/python3.8/site-packages/maze_bot/bot_localization.py", line 173, in localize_bot
    self.extract_bg(curr_frame.copy())
  File "/root/ROS2-Path-Planning-and-Maze-Solving/path_planning_ws/install/maze_bot/lib/python3.8/site-packages/maze_bot/bot_localization.py", line 92, in extract_bg
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
cv2.error: OpenCV(4.6.0) /io/opencv/modules/imgproc/src/color.simd_helpers.hpp:92: error: (-2:Unspecified error) in function 'cv::impl::{anonymous}::CvtHelper<VScn, VDcn, VDepth, sizePolicy>::CvtHelper(cv::InputArray, cv::OutputArray, int) [with VScn = cv::impl::{anonymous}::Set<3, 4>; VDcn = cv::impl::{anonymous}::Set<1>; VDepth = cv::impl::{anonymous}::Set<0, 2, 5>; cv::impl::{anonymous}::SizePolicy sizePolicy = cv::impl::<unnamed>::NONE; cv::InputArray = const cv::_InputArray&; cv::OutputArray = const cv::_OutputArray&]'
> Invalid number of channels in input image:
>     'VScn::contains(scn)'
> where
>     'scn' is 1

maze_solver:
Traceback (most recent call last):
  ...
  File "/root/ROS2-Path-Planning-and-Maze-Solving/path_planning_ws/install/maze_bot/lib/python3.8/site-packages/maze_bot/maze_solver.py", line 223, in maze_solving
    self.bot_localizer.localize_bot(self.sat_view, frame_disp)
  File "/root/ROS2-Path-Planning-and-Maze-Solving/path_planning_ws/install/maze_bot/lib/python3.8/site-packages/maze_bot/bot_localization.py", line 173, in localize_bot
    self.extract_bg(curr_frame.copy())
  File "/root/ROS2-Path-Planning-and-Maze-Solving/path_planning_ws/install/maze_bot/lib/python3.8/site-packages/maze_bot/bot_localization.py", line 92, in extract_bg
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
cv2.error: OpenCV(4.6.0) /io/opencv/modules/imgproc/src/color.simd_helpers.hpp:94: error: (-2:Unspecified error) in function 'cv::impl::{anonymous}::CvtHelper<VScn, VDcn, VDepth, sizePolicy>::CvtHelper(cv::InputArray, cv::OutputArray, int) [with VScn = cv::impl::{anonymous}::Set<3, 4>; VDcn = cv::impl::{anonymous}::Set<1>; VDepth = cv::impl::{anonymous}::Set<0, 2, 5>; cv::impl::{anonymous}::SizePolicy sizePolicy = cv::impl::<unnamed>::NONE; cv::InputArray = const cv::_InputArray&; cv::OutputArray = const cv::_OutputArray&]'
> Unsupported depth of input image:
>     'VDepth::contains(depth)'
> where
>     'depth' is 6 (CV_64F)

maze_solver:
Traceback (most recent call last):
  File "/root/ROS2-Path-Planning-and-Maze-Solving/path_planning_ws/install/maze_bot/lib/maze_bot/maze_solver", line 11, in <module>
    load_entry_point('maze-bot==0.0.0', 'console_scripts', 'maze_solver')()
  File "/root/ROS2-Path-Planning-and-Maze-Solving/path_planning_ws/install/maze_bot/lib/python3.8/site-packages/maze_bot/maze_solver.py", line 272, in main
    rclpy.spin(node_obj)
  File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/__init__.py", line 191, in spin
    executor.spin_once()
  File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/executors.py", line 714, in spin_once
    raise handler.exception()
  File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/task.py", line 239, in __call__
    self._handler.send(None)
  File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/executors.py", line 429, in handler
    await call_coroutine(entity, arg)
  File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/executors.py", line 343, in _execute_timer
    await await_or_execute(tmr.callback)
  File "/opt/ros/foxy/lib/python3.8/site-packages/rclpy/executors.py", line 118, in await_or_execute
    return callback(*args)
  File "/root/ROS2-Path-Planning-and-Maze-Solving/path_planning_ws/install/maze_bot/lib/python3.8/site-packages/maze_bot/maze_solver.py", line 245, in maze_solving
    bot_view = cv2.resize(self.bot_view, (int(frame_disp.shape[0]/2),int(frame_disp.shape[1]/2)))
AttributeError: 'maze_solver' object has no attribute 'bot_view'

Analysis:
There is a chance some of the messages such as /BotCamera/image_raw may have a delay in the real situation, also even in the simulation of gazebo, which is the reason of the crashes occurred when we tested.

Solution:
Therefore, I just change the logic of maze_solving: check first if a real bot_view and sat_view are received , then continue to do the rest things.

Hoping you can agree this merge.

[Cryst4L9527]

Maybe you can just confirm this? Just a small change of codes for better robustness, it won't change the main functions of this program. Thank you~