Fix single thread stepping timeout race condition (#104195)

This PR fixes a potential race condition in
#90930.

This race can happen because the original code set `m_info->m_isAlive =
true` **after** the timer thread is created. So if there is a context
switch happens and timer thread checks `m_info->m_isAlive` before main
thread got a chance to run `m_info->m_isAlive = true`, the timer thread
may treat `ThreadPlanSingleThreadTimeout` as not alive and simply exit
resulting in async interrupt never being sent to resume all threads
(deadlock).

The PR fixes the race by initializing all states **before** worker timer
thread creates.

Co-authored-by: jeffreytan81 <[email protected]>

Author: jeffreytan81

lldb/source/Target/ThreadPlanSingleThreadTimeout.cpp

@@ -29,10 +29,10 @@ ThreadPlanSingleThreadTimeout::ThreadPlanSingleThreadTimeout(
     : ThreadPlan(ThreadPlan::eKindSingleThreadTimeout, "Single thread timeout",
                  thread, eVoteNo, eVoteNoOpinion),
       m_info(info), m_state(State::WaitTimeout) {
-  // TODO: reuse m_timer_thread without recreation.
-  m_timer_thread = std::thread(TimeoutThreadFunc, this);
   m_info->m_isAlive = true;
   m_state = m_info->m_last_state;
+  // TODO: reuse m_timer_thread without recreation.
+  m_timer_thread = std::thread(TimeoutThreadFunc, this);
 }
 
 ThreadPlanSingleThreadTimeout::~ThreadPlanSingleThreadTimeout() {