Created attachment 37624 [details]
Test servlet

Under certain IO error conditions, tomcat will fail to alert that an asynchronous request is completed and destroyed.
Both the AsyncListener's onComplete method and ServletRequestListener's requestDestroyed method are not called.
This may cause a webapp to believe that a request is still valid while tomcat has already recycled the objects.

I was able to reproduce this issue using a WriteListener (non-blocking IO servlet API 3.1) with the httpnio connector on the latest tomcat release (9.0.41).
This appears to be caused when an IOException is raised while attempting to write to the ServletOutputChannel during the WriteListener's onWritePossible callback.

It also seems possible to trigger this issue by manually throwing an exception in the onWritePossible callback.

The following exception can also be seen in the logs:
java.lang.IllegalStateException: Calling [asyncPostProcess()] is not valid for a request with Async state [ERROR]

I've attached a test servlet that demonstrates this issue.
To simulate IO errors, an http request is aborted before the full response body is read. The servlet then records various async callbacks it receives and it can be seen that some callbacks are not always called, i.e. the AsyncListener's onComplete callback and ServletRequestListener's requestDestroyed callback.
The servlet compares synchronous requests, asynchronous requests, and asynchronous requests with non-blocking IO.