Oracle 'deadlock' error on deleteCartItems API call

[stuartpullinger]

The server.log on Diamond's Topcat is reporting an Oracle error when a CartItem is deleted via the API. (I can't see any evidence of this problem on ISIS' Topcat). 2 errors appear in the logs (and have been for over a year):

[2020-05-15T18:48:39.358+0100] [Payara 4.1] [WARNING] [AS-EJB-00056] [javax.enterprise.ejb.container] [tid: _ThreadID=244207 _ThreadName=http-thread-pool::http-listener-2(72)] [timeMillis: 1589564919358] [levelValue: 900] [[
  A system exception occurred during an invocation on EJB UserResource, method: public javax.ws.rs.core.Response org.icatproject.topcat.web.rest.UserResource.deleteCartItems(java.lang.String,java.lang.String,java.lang.String) throws org.icatproject.topcat.exceptions.TopcatException,java.net.MalformedURLException,java.text.ParseException]]

Followed by:

[2020-05-15T18:48:39.358+0100] [Payara 4.1] [WARNING] [] [javax.enterprise.ejb.container] [tid: _ThreadID=244207 _ThreadName=http-thread-pool::http-listener-2(72)] [timeMillis: 1589564919358] [levelValue: 900] [[

javax.ejb.EJBException
        at com.sun.ejb.containers.EJBContainerTransactionManager.processSystemException(EJBContainerTransactionManager.java:750)
...
        at org.icatproject.topcat.web.rest.__EJB31_Generated__UserResource__Intf____Bean__.deleteCartItems(Unknown Source)
...
        at org.icatproject.topcat.filter.CacheControlFilter.doFilter(CacheControlFilter.java:52)
...
Caused by: javax.persistence.PersistenceException: Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.6.4.qualifier): org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: java.sql.SQLException: ORA-00060: deadlock detected while waiting for resource

Error Code: 60
Call: DELETE FROM CARTITEM WHERE (ID = ?)
        bind => [1 parameter bound]
Query: DeleteObjectQuery(org.icatproject.topcat.domain.CartItem@3f1f6116)
...
        at org.icatproject.topcat.web.rest.UserResource.deleteCartItems(UserResource.java:559)
...

I'm not sure yet which bit of the frontend is calling this API endpoint - I haven't been able to recreate the circumstances to log the error. I suspect, though I'm not sure, that it may degrade response times when this error is logged. On the backend, the CartItem Entity does not define a Delete SQL query. The SQL above must be generated by the Java Persistence layer.