[lldb-dev] Process plugin implementation with multiple threads

[Greg Clayton]

> On Nov 14, 2014, at 3:46 PM, Zachary Turner <zturner at google.com> wrote:
> 
> It's possible, but Windows gives us a handle to the thread which has all privileges associated with it (in other words we can use it for just about anything), so it's considered better to use the handle windows gives us. At the very least we'd want to queue up all the incoming event notifications that we get from this loop, and then use it to update the state when someone stops.
> 
> Just to be certain, when you and Greg refer to "when someone has stopped", I assume you are referring to a public stop, i.e. someone has to enter a command to resume the debugger?

No just a private stop. You should _not_ be changing the process state to stopped and running again (via Process::SetPrivateState()) for thread creation or thread death, only for stops where an existing thread stops for some reason (trace (single step), breakpoint, exception, etc). The private state thread will move the thread plans along for private stops, which agains are only actual stops where the process stops due to a existing thread (not a new or dying thread) stopping for a reason. These thread creation events should be suppressed from the private state thread and only handled for your own purposed. You also do not want to be updating the thread list as you are running (the one in Process::GetThreadList()), so you should keep your own ThreadList object within the windows event loop that only your windows event will update. Later when a stop gets propagated to the private state of the process, it will call:

    virtual bool
    UpdateThreadList (ThreadList &old_thread_list, ThreadList &new_thread_list) = 0;

And you can merge your private copy of your ThreadList into new_thread_list as needed. You should be doing something like:

ThreadList newWindowsThreads;
ThreadList deadWindowsThreads;

while (WaitForDebugEvent(&event))   // Block until something happens in the process
{
    if (event is thread creation)
    {
         newWindowsThreads.Append(new thread)
    } 
    else if (event is thread death)
    {
         deadWindowsThreads.Append(...)
    }
    // Windows stopped the process for us automatically.  Do something with the event
    ContinueDebugEvent(&event);  // Resume the process
}


Then in your 

bool
ProcessWindows::UpdateThreadList (ThreadList &old_thread_list, ThreadList &new_thread_list)
{
     // Append any threads from old_thread_list into new_thread_list if they are still alive by checking if any of them are in deadWindowsThreads and only adding them if they aren't in the list
     // Append all threads from newWindowsThreads into new_thread_list
}


> I was wondering why my RefreshStateAfterStop override was never being called, I'm guessing now that it's because I was only updating the private state, and this only gets called on a public stop.

Yes this seems to be for public stops only. ProcessWindows::UpdateThreadList() isn't only for public stops, it is used in private stops as well.

> 
> 
> On Fri Nov 14 2014 at 3:26:47 PM <jingham at apple.com> wrote:
> Is there a way to ask Windows "give me all the threads for this process"?  If so, the path of least resistance would be to ignore the Thread Created events in this loop, and then only generate the thread list on demand when someone has stopped.
> 
> At some point we'll want to do keep alive debugging and then we'll have to handle posting interesting events while the process is still running.  But I don't want to do that piecemeal.
> 
> For now, putting threads in the thread list while the debugger is running from both the private & public state's perspective does no good, and putting them in between a stop & an immediate continue doesn't do anybody any good either.  You're just making your plugin more fragile.
> 
> Jim
> 
> 
> > On Nov 14, 2014, at 3:08 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > The way Windows debug notifications work is like this:
> >
> > while (WaitForDebugEvent(&event))   // Block until something happens in the process
> > {
> >    // Windows stopped the process for us automatically.  Do something with the event
> >
> >    ContinueDebugEvent(&event);  // Resume the process
> > }
> >
> > After WaitForDebugEvent and before I call ContinueDebugEvent, the event might say that a new thread was created.  So at this point I'd like to update the thread list.  The process is actually stopped here, but it shouldn't appear so to the user. So at the beginning I tell LLDB the process is stopped -- SetPrivateState(eStateStopped), and at the end I tell it that it's running again -- SetPrivateState(eStateRunning).
> >
> > However, this code is still running on a different from what LLDB is going to invoke methods on the plugin.  RefreshStateAfterStop, DoResume, etc are all going to be running on a different thread from this loop.  The code is simpler if I can just stick the new thread in the ThreadList (or whatever other state needs updating if it's not thread related) right here from this thread.  Alternatively, when I set the state to eStateStopped presumably that will trigger LLDB to call RefreshStateAfterStop() from the other thread.  I can also modify the state then, it just requiers a little extra work as I have to do some synchronization and marshalling / storage of the event parameters.
> >
> > On Fri Nov 14 2014 at 2:53:35 PM Greg Clayton <gclayton at apple.com> wrote:
> >
> > > On Nov 14, 2014, at 10:15 AM, Zachary Turner <zturner at google.com> wrote:
> > >
> > > Windows doesn't need to do anything special for each thread as it is created. Only need to update the thread list in the Process object as far as we're concerned, but that's it.
> > >
> > > The main thing I'm worried about is that the Process object itself runs a couple of threads in the background (the private state thread, for example), so I was worried about possible race conditions when modifying state asynchronously (i.e. from my debugger thread).
> >
> > What state needs to be updated asynchronously? You shouldn't be touching the thread list (no need to update it, you should be able to update it on demand each time you actually stop right?) at all until you have a valid stop reason and then the process will update the threads itself when and if it needs them.
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