What does 'Failed shutdown of Apache Portable Runtime' log message mean?

After shutting down tomcat, I get this message:

INFO: Failed shutdown of Apache Portable Runtime

Then Tomcat shuts down after all

I haven't found a reasonable explanation googling around, nor in the single duplicate I've found (0 answers)

The most common cause is that a non-daemon thread is still running (Tomcat FAQ). Click here for more information on daemon threads.

To find out which thread is causing problems (on Unix):

• do a kill -3 tomcatProcId (find tomcatProcId using ps)
• look inside $TOMCAT_HOME/logs/catalina.out for the thread dump generated by the kill
• look at all the non-daemon threads (those not marked with "daemon") that are not VM/GC related
• for each one, find out what code spawned it, and why it has not terminated correctly. Some non-daemon threads check frequently to terminate, which is OK.

If you want, you can instruct Tomcat to kill the JVM after shutdown, even if non-daemon threads remain (which might sometimes be a bad thing):

• define the CATALINA_PID environment variable. For example: export CATALINA_PID=/tmp/catalina_pid
• use catalina.sh stop -force to stop Tomcat.

It seems that this is not due to some daemon threads after all, just a poor log message

https://issues.apache.org/bugzilla/show_bug.cgi?id=38652