Web resource cache doesn't recover properly when it's temporary files are deleted

Issue Summary

Jira uses files to cache web resources compiled in runtime (less->css, etc).
These files are stored in $JIRA_HOME/tmp/webresources.

When one of these files is deleted/disappears during application uptime, cache file is not being restored and web resource manager triggers compilation for each request to that resource.

That faulty situation results in a very high CPU load/usage.

Environment

Steps to Reproduce

1. Remove one/many files from $JIRA_HOME/tmp/webresources
2. Observe load average and CPU usage

Expected Results

• missing cache file is properly detected,
• recompilation happens only once,
• cache file is being restored
• no significant spike on load average / CPU usage is observed (apart from that single compilation event)

Actual Results

• resource files are recompiled for each request
• significant load average / CPU usage is observed

Workaround

Restarting an instance is currently the only known workaround for the issue