add latching field to header

[jobafr]

When recording the output of rosrust based nodes using rosbag, the connection header inside the rosbag file contains an empty latching= field, because this field is missing from the original connection header sent by rosrust's publisher.

While upstream rosbag supports parsing this (and interprets it the same as latching=0),

• The information that a publisher was latching at runtime is lost
• rosbag-rs (rust parser for rosbag files) outright refuses to parse the connection header, rendering all messages sent by the publisher inaccessible.

rosbag-rs issues:

• Allow missing latching field in connection newpavlov/rosbag-rs#2
• InvalidRecord error when latching header has zero bytes newpavlov/rosbag-rs#4

Code to reproduce the behavior:

rosrust::rosmsg_include!(std_msgs / String,);

fn main() {
    rosrust::init("rosrust_test");

    let publisher_unlatched = rosrust::publish("chatter_unlatched", 1).unwrap();
    for i in 0..=10 {
        publisher_unlatched
            .send(std_msgs::String {
                data: format!("hello unlatched {}", i),
            })
            .unwrap();
        std::thread::sleep(std::time::Duration::from_millis(200));
    }

    let mut publisher_latched = rosrust::publish("chatter_latched", 1).unwrap();
    publisher_latched.set_latching(true);
    for i in 0..=10 {
        publisher_latched
            .send(std_msgs::String {
                data: format!("hello latched {}", i),
            })
            .unwrap();
        std::thread::sleep(std::time::Duration::from_millis(200));
    }
}

Without the patch, the bagfiles end up with an empty latching= header. With the patch, it's either latching=0 or latching=1, depending on whether the publisher was latched or not.

[jobafr]

Update: Although rosbag-rs has since merged a PR that makes their parser work with the malformed header, I think this should still be merged here. The information whether a publisher is latched can be useful for all sorts of tools, and can for example be accessed by subscribers through the rospy API (sub.impl.connections[0].is_latched).