I've been facing issues with repeatedly adding and deleting the same model into gazebo (for a repetitive training task). The model is that of a simple cylindrical object (a pole) with no joints ... just one link. After a few (~5-10) successful additions/deletions of this model into the scene, subsequent deletemodel() calls start getting stuck for a few minutes each. It hangs even though I can clearly see the model exists in the scene. At this point, the only way to "release" the stuck call is to manually delete the model through the gazebo UI. But, then it keeps getting stuck the same way on all subsequent calls to deletemodel().

Furthermore, if I try to invoke any other service call while the delete_model() call is held up, those calls also sit blocked, seemingly waiting for the previous invocation to return.

In addition, the object I'm adding to the scene sometimes just disappears from the scene as well, though I'm not sure whether the issues share a common cause. Anyway, thought I'd mention that too, just in case. Maybe it's the SDF of the object that isn't correctly setup?

When the delete_model() call hangs, I have consistently seen the following error message on the shell:

[Err] [Joint.cc:712] Joint axis index [0] larger than DOF count [0].

... but I can't quite figure out what it means.

The SDF of the object I'm adding/deleting is below:

<sdf 
    <a="" href="http://
        xmlns:xacro="http://ros.org/wiki/xacro%22" version="1.6">
        xmlns:xacro="http://ros.org/wiki/xacro">
        <model name="pole">
            <link name="pole_link"/>
            <inertial>
                <mass>2.14885</mass>
                <inertia>
                    <ixx>0.0287722</ixx>
                    <ixy>0</ixy>
                    <ixz>0</ixz>
                    <iyy>0.0287722</iyy>
                    <iyz>0</iyz>
                    <izz>0.00024175</izz>
                </inertia>
                <pose frame="">0 0 0 0 -0 0</pose>
            </inertial>
            <pose frame="">0 0 0 0 -0 0</pose>
            <gravity>1</gravity>
            <self_collide>0</self_collide>
            <kinematic>0</kinematic>
            <enable_wind>0</enable_wind>
            <visual name="visual">
                <pose frame="">0 0 0 0 -0 0</pose>
                <geometry>
                    <cylinder>
                        <radius>0.015</radius>
                        <length>0.5</length>
                    </cylinder>
                </geometry>
                <material>
                    <lighting>1</lighting>
                    <script>
                        <uri>file://media/materials/scripts/gazebo.material</uri>
                        <name>Gazebo/Red</name>
                    </script>
                    <shader type="pixel">
                        <normal_map>__default__</normal_map>
                    </shader>
                    <ambient>0.3 0.3 0.3 1</ambient>
                    <diffuse>0.7 0.7 0.7 1</diffuse>
                    <specular>0.01 0.01 0.01 1</specular>
                    <emissive>0 0 0 1</emissive>
                </material>
                <transparency>0</transparency>
                <cast_shadows>1</cast_shadows>
            </visual>
            <collision name="collision">
                <laser_retro>0</laser_retro>
                <max_contacts>10</max_contacts>
                <pose frame="">0 0 0 0 -0 0</pose>
                <geometry>
                    <cylinder>
                        <radius>0.015</radius>
                        <length>0.5</length>
                    </cylinder>
                </geometry>
                <surface>
                    <friction>
                        <ode>
                            <mu>1</mu>
                            <mu2>1</mu2>
                            <fdir1>0 0 0</fdir1>
                            <slip1>0</slip1>
                            <slip2>0</slip2>
                        </ode>
                        <torsional>
                            <coefficient>1</coefficient>
                            <patch_radius>0</patch_radius>
                            <surface_radius>0</surface_radius>
                            <use_patch_radius>1</use_patch_radius>
                            <ode>
                                <slip>0</slip>
                            </ode>
                        </torsional>
                    </friction>
                    <bounce>
                        <restitution_coefficient>0</restitution_coefficient>
                        <threshold>1e+06</threshold>
                    </bounce>
                    <contact>
                        <collide_without_contact>0</collide_without_contact>
                        <collide_without_contact_bitmask>1</collide_without_contact_bitmask>
                        <collide_bitmask>1</collide_bitmask>
                        <ode>
                            <soft_cfm>0</soft_cfm>
                            <soft_erp>0.2</soft_erp>
                            <kp>1e+13</kp>
                            <kd>1</kd>
                            <max_vel>0.01</max_vel>
                            <min_depth>0</min_depth>
                        </ode>
                        <bullet>
                            <split_impulse>1</split_impulse>
                            <split_impulse_penetration_threshold>-0.01</split_impulse_penetration_threshold>
                            <soft_cfm>0</soft_cfm>
                            <soft_erp>0.2</soft_erp>
                            <kp>1e+13</kp>
                            <kd>1</kd>
                        </bullet>
                    </contact>
                </surface>
            </collision>
            <sensor name="pole_imu_sensor" type="imu">
                <always_on>true</always_on>
                <update_rate>15</update_rate>
                <visualize>true</visualize>
                <topic>__default_topic__</topic>
                <plugin filename="libgazebo_ros_imu_sensor.so" name="pole_imu_plugin">
                    <topicname>pole_link_imu_sensor</topicname>
                    <bodyname>pole_link</bodyname>
                    <updateratehz>10.0</updateratehz>
                    <gaussiannoise>0.0</gaussiannoise>
                    <xyzoffset>0 0 0</xyzoffset>
                    <rpyoffset>0 0 0</rpyoffset>
                    <framename>pole_link</framename>
                </plugin>
                <pose>0 0 0 0 0 0</pose>
            </sensor>
            <static>0</static>
            <allow_auto_disable>1</allow_auto_disable>
        </model>
    </sdf>

Environment:

- ROS Melodic (to launch gazebo and other ROS components)
- Gazebo 9
- ROS python client (rospy) to invoke the gazebo services
- Ubuntu 18.04

What am I doing wrong above? Would anyone have some clear instructions for a fix (or reasonable workaround) that a Gazebo newbie could follow?

Thanks in advance.