~1x per hour the diagnostics complains with "No data since last update."

[Ramon Wijnands]

Please complete the following information:

• OS and Version: Ubuntu 20.04.5 LTS
• ROS Version: noetic
• Built from Source or Downloaded from Official Repository: both
• Version: 1.7.0 89faa69

Describe the bug
Randomly, approximatly 1x per hour the velodyne driver gives out a single WARN diagnostics message. The point cloud stream is still good. I have a feeling this is a threading/locking issue with the diagnostics updater.

To Reproduce

1. Run the driver
2. Record a bagfile for 1 hour or more
3. Run the following command to figure out if the bug happens rostopic echo -b file.bag /diagnostics/status[0]/message | grep '\w'
4. In plotjuggler, you can plot Events in window to visually see when the bug occurs.

Expected behavior
The point cloud data stream is good, so I expect the diagnostics to always send out OK.

Additional context
Diagnostics visualized in plotjuggler. I wrote an external diagnostics_updater that listends to the pointcloud data and it never reports a problem. Both diagnostics are visualized below.

This is the full diagnostics message that is produced:

level: 1
name: "velodyne_nodelet_manager_driver: velodyne_packets topic status"
message: "No data since last update."
hardware_id: "Velodyne VLP-16"
values: 
  - 
    key: "Events in window"
    value: "90"
  - 
    key: "Events since startup"
    value: "35120"
  - 
    key: "Duration of window (s)"
    value: "9.077190"
  - 
    key: "Actual frequency (Hz)"
    value: "9.914963"
  - 
    key: "Target frequency (Hz)"
    value: "9.921053"
  - 
    key: "Minimum acceptable frequency (Hz)"
    value: "4.960526"
  - 
    key: "Maximum acceptable frequency (Hz)"
    value: "14.881579"
  - 
    key: "Earliest timestamp delay"
    value: "No data"
  - 
    key: "Latest timestamp delay"
    value: "No data"
  - 
    key: "Earliest acceptable timestamp delay"
    value: "-1.000000"
  - 
    key: "Latest acceptable timestamp delay"
    value: "5.000000"
  - 
    key: "Late diagnostic update count"
    value: "0"
  - 
    key: "Early diagnostic update count"
    value: "0"
  - 
    key: "Zero seen diagnostic update count"
    value: "0"
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