Acquire Lock Before Releasing Python GIL

[Jgunde]

This PR implements a lock in apriltag_pywrap.c to prevent multiple threads which share the same detector object from calling the detect function, apriltag_detector_detect(), at the same time.

Previously, the Python GIL was released during the call to apriltag_detector_detect(). This would cause the program to crash if multiple threads called this function at the same time. By implementing a lock, only a single thread can call it at a time.

The added functionality was tested using the following Python code:

from PIL import Image
import threading

import numpy as np
from apriltag import apriltag


image = Image.open("apriltag.png").convert("L")  # Load apriltag image and convert to grayscale.
image_array = np.array(image)

detector = apriltag(
    family="tag36h11",
    threads=4,
    maxhamming=1,
    decimate=1.0,
    blur=0.0,
    refine_edges=True,
    debug=False
)


def worker(thread_id):
    for i in range(100):
        detector.detect(image_array)
    print(f"Thread {thread_id} complete.")


# Create and start multiple threads
threads = []
for i in range(10):
    thread = threading.Thread(target=worker, args=(i,))
    thread.start()
    threads.append(thread)

# Wait for all threads to finish
for thread in threads:
    thread.join()

[christian-rauch]

Thanks a lot for taking care of the issue.

[christian-rauch]

Unfortunately, the signing key for the ROS apt repo expired yesterday and hasn't been updated in the ros-tooling/setup-ros action yet. The CI will not pass until this has been resolved.

[Jgunde]

I added a few changes:

• Modified comments to improve clarity
• Initialized detections variable with NULL so it doesn't point to a random address
• Removed if statement. Since PyThread_acquire_lock(self->det_lock, 1); is blocking, it almost never fails. The contents of the if block was essentially dead code.

Code was tested in the same manner as above.

[christian-rauch]

The CI seems to work again. Can you rebase and squash your commits together, and then force push again?