Create page using REST API fails when page containts tasks with @mention

We are working on an experiment where the first user / evaluator creates two spaces and a bunch of content using the Confluence rest api. In a few pages we have tasks with the user @mentioned. These seem to quite often cause our create page to fail with a transaction rollback, and thus causing our experiment to fail.

Here's a few log snippets and analysis:

@4000000054feead01f05278c 2015-03-10 12:59:50,492 ERROR [TP-Processor3] [atlassian.event.internal.AsynchronousAbleEventDispatcher] run There was an exception thrown trying to dispatch event [ConfluenceTaskCreateEvent

Unknown macro: {user=com.atlassian.confluence.user.ConfluenceUserImpl@203f4f[vcosta46,ff8080814c03b7ff014c03b80f960001], task=Task [globalId=0, id=1, contentId=491540, taskStatus=UNCHECKED, title=Vitor Costa [Administrator] https}

] from the invoker [SingleParameterMethodListenerInvoker

Unknown macro: {method=public void com.atlassian.mywork.providers.confluence.ConfluenceTasksEventListener.onTaskCreatedEvent(com.atlassian.confluence.plugins.tasklist.event.ConfluenceTaskCreateEvent) throws java.lang.Exception, listener=com.atlassian.mywork.providers.confluence.ConfluenceTasksEventListener@82bae0}

]
@4000000054feead01f056ddc – referer: https://carglasspt.atlassian.net/wiki/dashboard.action | url: /wiki/rest/api/content | userName: vcosta46
@4000000054feead01f0571c4 java.lang.RuntimeException: java.lang.reflect.UndeclaredThrowableException
@4000000054feead01f0571c4 at com.atlassian.event.internal.SingleParameterMethodListenerInvoker.invoke(SingleParameterMethodListenerInvoker.java:50)
@4000000054feead01f0575ac at com.atlassian.event.internal.AsynchronousAbleEventDispatcher$1$1.run(AsynchronousAbleEventDispatcher.java:48)
@4000000054feead01f057994 at com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService.execute(MoreExecutors.java:253)
@4000000054feead01f057d7c at com.atlassian.event.internal.AsynchronousAbleEventDispatcher.dispatch(AsynchronousAbleEventDispatcher.java:107)
...
@4000000054feead01f0a53c4 at com.sun.proxy.$Proxy2590.createOrUpdate(Unknown Source)
*@4000000054feead01f0a53c4 at com.atlassian.mywork.providers.confluence.ConfluenceTasksEventListener.onTaskCreatedEvent(ConfluenceTasksEventListener.java:65)*
@4000000054feead01f0a5b94 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

...
@4000000054feead01f0c14fc Caused by: org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "u_ao_9412a1_aouser_username"
@4000000054feead01f0c18e4 at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2077)
@4000000054feead01f0c18e4 at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1810)
@4000000054feead01f0c20b4 at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:257)
@4000000054feead01f0c20b4 at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:498)
@4

that causes transaction to rollback

@4000000054feead022dac484 2015-03-10 12:59:50,575 ERROR [TP-Processor3] [rest.api.model.ExceptionConverter] convertServiceException No status code found for exception, converting to internal server error :
@4000000054feead022dacc54 – referer: https://carglasspt.atlassian.net/wiki/dashboard.action | url: /wiki/rest/api/content | userName: vcosta46
@4000000054feead022dad03c org.springframework.transaction.UnexpectedRollbackException: Transaction rolled back because it has been marked as rollback-only
@4000000054feead022dad03c at org.springframework.transaction.support.AbstractPlatformTransactionManager.commit(AbstractPlatformTransactionManager.java:695)
@4000000054feead022dae7ac at sun.reflect.GeneratedMethodAccessor72.invoke(Unknown Source)
@4000000054feead022daeb94 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
@4

From

com.atlassian.mywork.host.dao.ao.AOUserDao.getAO(AOUserDao.java:40)

it looks like the issue is that find AOUser below returns null:

    private AOUser getAO(UserKey userKey)
    {
        AOUser user =  findOnly(select().where("USERNAME = ?", userKey.getStringValue()));
        if (user == null)
        {
            user = ao.create(AOUser.class, new DBParam("USERNAME", userKey.getStringValue()));
            user.setCreated(new Date());
            user.setUpdated(user.getCreated());
        }
        else
        {
            user.setUpdated(new Date());
        }
        return user;
    }

So concurrency / transaction stuff.