Don't assume the backing thread shares a Protocol ID

[JDevlieghere]

Summary

When we're dealing with virtual (memory) threads created by the OS
plugins, there's no guarantee that the real thread and the backing
thread share a protocol ID. Instead, we should iterate over the memory
threads to find the virtual thread that is backed by the current real
thread.

rdar://36485830

[JDevlieghere]

(I'll update the diff once I've figured out how to test this)

[clayborg]

source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

1826–1827

I would add a new function to ThreadList:

ThreadSP ThreadList::GetBackingThread(const ThreadSP &real_thread);

Since there is a mutex inside the thread list, we don't want another thread to be able to mutate the thread list while doing this iteration. So this code would become:

ThreadSP memory_thread_sp = m_thread_list.GetBackingThread(thread_sp);
if (memory_thread_sp)
  thread_sp = memory_thread_sp;

[JDevlieghere]

• Address Greg's comment and factor out the method.
• Add a test case.

source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

if (thread_sp) {
 
       if (dispatch_queue_t != LLDB_INVALID_ADDRESS)
         gdb_thread->SetQueueLibdispatchQueueAddress(dispatch_queue_t);
 
       // Make sure we update our thread stop reason just once
       if (!thread_sp->StopInfoIsUpToDate()) {
         thread_sp->SetStopInfo(StopInfoSP());
         // If there's a memory thread backed by this thread, we need to use it
-        // to calcualte StopInfo.
-        ThreadSP memory_thread_sp =
-            m_thread_list.FindThreadByProtocolID(thread_sp->GetProtocolID());
-        if (memory_thread_sp)
-          thread_sp = memory_thread_sp;
+        // to calculate StopInfo.
+        size_t num_threads = m_thread_list.GetSize(false);
+        for (size_t i = 0; i < num_threads; ++i) {
+          ThreadSP mem_thread = m_thread_list.GetThreadAtIndex(i, false);
+          if (mem_thread && mem_thread->GetBackingThread() == thread_sp)
+            thread_sp = mem_thread;
+        }
 
         if (exc_type != 0) {
           const size_t exc_data_size = exc_data.size();
 
           thread_sp->SetStopInfo(
               StopInfoMachException::CreateStopReasonWithMachException(
                   *thread_sp, exc_type, exc_data_size,
                   exc_data_size >= 1 ? exc_data[0] : 0,