<rdar://problem/11052174>

<rdar://problem/11051056>

Found a race condition when sending async packets in the ProcessGDBRemote.

A little background: GDB remote clients can only send one packet at a time. You must send a packet and wait for a response. So when we continue, we obviously can't hold up the calling thread waiting for the process to stop again, so we have an async thread in the ProcessGDBRemote whose only job is to run packets that control the inferior process. When you send a continue packet, the only packet you can send is an interrupt packet (which consists of sending a CTRL+C (or a '\x03' byte)). This then stops the inferior and we can send the async packet, and then resume the target. There was a race condition that often happened during stepping where we are doing a source level single step which consists of many instruction steps and a few runs here and there when we step into a function. So the flow looks like:

inst single step
inst single step
inst single step
inst single step
inst single step
step BP and run
inst single step
inst single step
inst single step

Now if we got an async packet while the program is running we get something like:

send --> continue
send --> interrupt
recv <-- interrupt stop reply packet
send --> async packet
recv <-- async response
send --> continue again and wait for actual stop

Problems arise when this was happening when single stepping a thread where we would get:

send --> step thread 123
send --> interrupt
send --> stop reply for thread 123 (from the step)

Now we _might_ have an extra stop reply packet from the "interrupt" which we weren't checking for and we could end up with:

send --> async packet (like memory read!)
recv <-- async response (which is the interrupt stop reply packet)

Now we have the read memroy reply sitting in our buffer and waiting to be used as the reply for the next packet... 

To further complicate things, the single step should have exited the async thread since the run control is finished, but now it will continue if it was interrupted.

The fixes I checked in to two major things:
- watch for the extra stop reply if we need to
- make sure we exit from the async thread run loop when the previous run control (like the instruction level single step) is finished.

Needless to say this makes very fast stepping in Xcode much more reliable.

llvm-svn: 153629