Kernel: Restore the previous thread state on SIGCONT after SIGSTOP

When stopping a thread with the SIGSTOP signal, we now store the thread
state in Thread::m_stop_state. That state is then restored on SIGCONT.
This fixes an issue where previously-blocked threads would unblock
upon resume. Now they simply resume in the Blocked state, and it's up
to the regular unblocking mechanism to unblock them.

Fixes SerenityOS#1326.

Kernel/Thread.cpp

@@ -495,13 +495,19 @@ ShouldUnblockThread Thread::dispatch_signal(u8 signal)
  m_pending_signals &= ~(1 << (signal - 1));
 
  if (signal == SIGSTOP) {
- m_stop_signal = SIGSTOP;
- set_state(Stopped);
+ if (!is_stopped()) {
+ m_stop_signal = SIGSTOP;
+ m_stop_state = m_state;
+ set_state(State::Stopped);
+ }
  return ShouldUnblockThread::No;
  }
 
- if (signal == SIGCONT && state() == Stopped)
- set_state(Runnable);
+ if (signal == SIGCONT && is_stopped()) {
+ ASSERT(m_stop_state != State::Invalid);
+ set_state(m_stop_state);
+ m_stop_state = State::Invalid;
+ }
 
  auto handler_vaddr = action.handler_or_sigaction;
  if (handler_vaddr.is_null()) {

Kernel/Thread.h

@@ -486,6 +486,7 @@ class Thread {
  u32 m_priority_boost { 0 };
 
  u8 m_stop_signal { 0 };
+ State m_stop_state { Invalid };
 
  bool m_dump_backtrace_on_finalization { false };
  bool m_should_die { false };