[lldb] Fix infinite recursion in OperatingSystemSwiftTasks::UpdateThreadList

[felipepiovezan]

When querying the old thread list (from a previous stop) for an already created thread, LLDB must not attempt to re-update the list.

rdar://162635997

[felipepiovezan]

@swift-ci test

[jimingham]

Might be better to solve this by adding a ThreadList::Freeze that prevents all updates, and just do that when we copy the ThreadList to the old ThreadList in Process. I don't think we ever intend to update the old thread list, so we shouldn't have to prevent that on a case by case basis.

[felipepiovezan]

Might be better to solve this by adding a ThreadList::Freeze that prevents all updates, and just do that when we copy the ThreadList to the old ThreadList in Process. I don't think we ever intend to update the old thread list, so we shouldn't have to prevent that on a case by case basis.

I think this kind of API adds complexity to the class (it's yet another thing to track internally: "am I frozen?"). IMO the problem here is that we don't use const semantics properly for ThreadList methods. There should be a FindThreadByID method that is const and does not have the "can_update" argument.

[felipepiovezan]

We chatted offline about this, and yes, we need to prevent future misuses of this API. I'm going to merge this first though, as it is causing a lot of crashes.