Continuous requests to parameter server for tf_prefix with every joint_state message

[nlamprian]

The robot_state_publisher continuously hits the parameter server with 3 requests for every joint_state message and in each fixed joint timer interval.

$ sudo tcpflow -c port 11311 -i lo

127.000.000.001.49382-127.000.001.001.11311: POST / HTTP/1.1
User-Agent: XMLRPC++ 0.7
Host: simtoru-0003:11311
Content-Type: text/xml
Content-length: 236

<?xml version="1.0"?>
<methodCall><methodName>searchParam</methodName>
<params><param><value>/robot_state_publisher</value></param><param><value>tf_prefix</value></param></params></methodCall>

127.000.001.001.11311-127.000.000.001.49382: HTTP/1.1 200 OK
Server: BaseHTTP/0.6 Python/3.8.10
Date: Sat, 08 Oct 2022 22:43:39 GMT
Content-type: text/xml
Content-length: 384

<?xml version='1.0'?>
<methodResponse>
<params>
<param>
<value><array><data>
<value><int>1</int></value>
<value><string>Found [/robot_state_publisher/tf_prefix]</string></value>
<value><string>/robot_state_publisher/tf_prefix</string></value>
</data></array></value>
</param>
</params>
</methodResponse>

127.000.000.001.49382-127.000.001.001.11311: POST / HTTP/1.1
User-Agent: XMLRPC++ 0.7
Host: simtoru-0003:11311
Content-Type: text/xml
Content-length: 296

<?xml version="1.0"?>
<methodCall><methodName>hasParam</methodName>
<params><param><value>/robot_state_publisher</value></param><param><value>/robot_state_publisher/tf_prefix</value></param></params></methodCall>

127.000.001.001.11311-127.000.000.001.49382: HTTP/1.1 200 OK
Server: BaseHTTP/0.6 Python/3.8.10
Date: Sat, 08 Oct 2022 22:43:39 GMT
Content-type: text/xml
Content-length: 307

<?xml version='1.0'?>
<methodResponse>
<params>
<param>
<value><array><data>
<value><int>1</int></value>
<value><string>/robot_state_publisher/tf_prefix</string></value>
<value><boolean>1</boolean></value>
</data></array></value>
</param>
</params>
</methodResponse>

127.000.000.001.49382-127.000.001.001.11311: POST / HTTP/1.1
User-Agent: XMLRPC++ 0.7
Host: simtoru-0003:11311
Content-Type: text/xml
Content-length: 296

<?xml version="1.0"?>
<methodCall><methodName>getParam</methodName>
<params><param><value>/robot_state_publisher</value></param><param><value>/robot_state_publisher/tf_prefix</value></param></params></methodCall>

127.000.001.001.11311-127.000.000.001.49382: HTTP/1.1 200 OK
Server: BaseHTTP/0.6 Python/3.8.10
Date: Sat, 08 Oct 2022 22:43:39 GMT
Content-type: text/xml
Content-length: 317

<?xml version='1.0'?>
<methodResponse>
<params>
<param>
<value><array><data>
<value><int>1</int></value>
<value><string>Parameter [/robot_state_publisher/tf_prefix]</string></value>
<value><string>/</string></value>
</data></array></value>
</param>
</params>
</methodResponse>

This was introduced by #169. The tf_prefix is refreshly retrieved every time. I assume this was unintentional.

One potential solution that would maintain ABI is to move the getTFPrefix calls to static variables in the callbacks.