ZThread::is_worker() incorrectly returns true when executed from a ZRuntimeWorkers context. This function should only return true when executed from a ZWorker context. This issue can lead to data corruption as a ZRuntimeWorker might read and write to data structures that are thread local to a ZWorker.

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (src/hotspot/share/gc/z/zThread.hpp:77), pid=8945, tid=8997
# assert(has_worker_id()) failed: Worker id not initialized
#
# JRE version: Java(TM) SE Runtime Environment (12.0) (fastdebug build 12-internal+0-2018-12-17-1105080.stefan.johansson.jdk12)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 12-internal+0-2018-12-17-1105080.stefan.johansson.jdk12, mixed mode, tiered, z gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x195e65a] ZObjectAllocator::alloc_small_object_from_worker(unsigned long, ZAllocationFlags)+0x37a
#

Current thread (0x00007f6088091a80): Thread "RuntimeWorker#0" [stack: 0x00007f606dde8000,0x00007f606dee8000] [id=8997]

Stack: [0x00007f606dde8000,0x00007f606dee8000], sp=0x00007f606dee6a50, free space=1018k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x195e65a] ZObjectAllocator::alloc_small_object_from_worker(unsigned long, ZAllocationFlags)+0x37a
V [libjvm.so+0x196802e] ZPage::relocate_object_inner(unsigned long, unsigned long)+0x2ae
V [libjvm.so+0x19692e4] ZPage::relocate_object(unsigned long)+0x84
V [libjvm.so+0x193d5db] ZHeap::relocate_object(unsigned long)+0x13b
V [libjvm.so+0x9558fd] AccessInternal::PostRuntimeDispatch<ZBarrierSet::AccessBarrier<1187924ul, ZBarrierSet>, (AccessInternal::BarrierType)2, 1187924ul>::oop_access_barrier(void*)+0x34d
V [libjvm.so+0x94ca82] ClassLoaderData::is_alive() const+0x32
V [libjvm.so+0x95b620] ClassLoaderDataGraph::resize_if_needed()+0x1c0
V [libjvm.so+0x162eac9] ParallelSPCleanupTask::work(unsigned int)+0x169
V [libjvm.so+0x190bb30] GangWorker::loop()+0xe0
V [libjvm.so+0x17bcce5] Thread::call_run()+0x75
V [libjvm.so+0x14a7886] thread_native_entry(Thread*)+0x106