CaffeineCache puts thread into infinite loop

Under heavy machine load, accessing any Solr CaffeineCache can lead to threads spinning in an endless loop. In our setup we had machines spinning at 70% cpu load without receiving any request for hours.

Background

The problem is caused by delegating the SolrCache#put method to Cache#asMap#put Under heavy machine load with concurrent read and write access to the same key, the ConcurrentHashMap#put method does not terminate an endless loop.

Implementation

This problem can be easy be solved by delegating the CaffeineCache#put method to Cache#asMap#compute which uses a producer pattern. After introducing the fix to our environment we were not able to reproduce this error pattern.

Github Pull Request: https://github.com/apache/solr/pull/1118