Laser scan subscriber repeatedly consumes first laser scan message

edit

I'm writing a simple mapper in python, that uses rospy to subscribe to odom and laser scan messages, which are used to update an occupancy grid map that is then published for use in rviz. The odometry subscriber and callback work as expected (i.e. when the robot moves in the world, I can see this change in the occupancy grid map when testing in rviz). However, the laser scan subscriber and callback only seem to consume the first laser scan that is published, and then re-consumes this same first laser scan ad infinitum (i.e. no matter where the robot moves in the world, the occupancy grid map always gets updated with the first laser scan that was published).

Since I'm unable to post an rviz screenshot of what the occupancy grid map looks like (the forum complains that I don't have enough points to post an image), I've posted the screenshot to imgur: https://imgur.com/EDmlRha

Thus, what I'm trying to figure out is why my odometry subscriber and callback is working correctly, but my lidar subscriber and callback is not. Any help would be greatly appreciated.

The following code is used to achieve the above explanation:

import rospy
import numpy as np
import matplotlib.pyplot as plt

from sensor_msgs.msg import LaserScan
from nav_msgs.msg import Odometry
from nav_msgs.msg import OccupancyGrid
from geometry_msgs.msg import Quaternion

from mapper import *

def quaternion_to_yaw(q: Quaternion) -> float:
    return np.arctan2(2.0*(q.w*q.z + q.x*q.y), 1.0 - 2.0*(q.y*q.y + q.z*q.z))

class MapperNode:
    def __init__(self, pose: Pose, mapper: Mapper):
        self.pose = pose
        self.mapper = mapper

        self.lidar_sub = rospy.Subscriber("scan", LaserScan, self.lidar_callback)
        self.odom_sub = rospy.Subscriber("odom", Odometry, self.odometry_callback)
        self.map_pub = rospy.Publisher("map", OccupancyGrid, queue_size=1)


    def lidar_callback(self, scan: LaserScan):
        scan_ranges = np.array(scan.ranges)
        self.mapper.update_map(self.pose, scan_ranges, scan.angle_min, scan.angle_increment)        
        self.publish_map()

    def odometry_callback(self, odom: Odometry):
        x = odom.pose.pose.position.x
        y = odom.pose.pose.position.y
        theta = quaternion_to_yaw(odom.pose.pose.orientation)
        self.pose = Pose(x, y, theta)

    def publish_map(self):
        occupancy_grid_map = OccupancyGrid()

        occupancy_grid_map.header.stamp = rospy.Time.now()
        occupancy_grid_map.header.frame_id = "map"
        occupancy_grid_map.info.height = self.mapper.num_rows
        occupancy_grid_map.info.width = self.mapper.num_cols
        occupancy_grid_map.info.resolution = self.mapper.resolution

        occupancy_grid_map.info.origin.position.x = -self.mapper.grid_origin.x*self.mapper.resolution
        occupancy_grid_map.info.origin.position.y = -self.mapper.grid_origin.y*self.mapper.resolution
        occupancy_grid_map.info.origin.position.z = 0.0

        occupancy_grid_map.info.origin.orientation.w = 1.0
        occupancy_grid_map.info.origin.orientation.x = 0.0
        occupancy_grid_map.info.origin.orientation.y = 0.0
        occupancy_grid_map.info.origin.orientation.z = 0.0

        # rviz expects a 1-dimensional int array with probability values ranging from 0 to 100        
        occupancy_grid_map.data = (100*self.mapper.probability_map().ravel()).astype(int, copy=False)

        self.map_pub.publish(occupancy_grid_map)

def main():
    rospy.init_node("mapper_node", anonymous=True)

    pose = Pose(0.0, 0.0, 0.0)

    x_min = -100.0
    x_max ...