LPS-50484 PortalImpl.getUniqueElementId() may end up in endless loop, because it isn't thread safe

Hi Shuyang,

Yes, you're right, the "synchronized(request)" wasn't working indeed, check that with debugger.

Eventually I added a new lock in addition to PARALLEL_RENDERING_MERGE_LOCK which is unique per request. In ServletRequestUtil.setAttributeIfAbsent() the lock in used only if it's absolutely necessary, so it imposes less sync overhead upon the parallel renderer.

Please let me know if you have a different solution in mind.

Thanks for reviewing!

Cheers,
László