Kernel: Unblock SignalBlocker if a signal was just unmarked as pending

When updating the signal mask, there is a small frame where we might set
up the receiving process for handing the signal and therefore remove
that signal from the list of pending signals before SignalBlocker has a
chance to block. In turn, this might cause SignalBlocker to never notice
that the signal arrives and it will never unblock once blocked.

Track the currently handled signal separately and include it when
determining if SignalBlocker should be unblocking.

Kernel/Syscalls/sigaction.cpp

@@ -104,6 +104,7 @@ ErrorOr<FlatPtr> Process::sys$sigreturn([[maybe_unused]] RegisterState& register
     auto saved_ax = TRY(copy_typed_from_user<FlatPtr>(stack_ptr));
 
     Thread::current()->m_signal_mask = ucontext.uc_sigmask;
+    Thread::current()->m_currently_handled_signal = 0;
 #if ARCH(X86_64)
     auto sp = registers.rsp;
 #elif ARCH(I386)

Kernel/Thread.cpp

@@ -1052,6 +1052,8 @@ DispatchSignalResult Thread::dispatch_signal(u8 signal)
 
     ScopedAddressSpaceSwitcher switcher(m_process);
 
+    m_currently_handled_signal = signal;
+
     u32 old_signal_mask = m_signal_mask;
     u32 new_signal_mask = m_signal_action_masks[signal].value_or(action.mask);
     if ((action.flags & SA_NODEFER) == SA_NODEFER)

Kernel/Thread.h

@@ -1210,6 +1210,7 @@ class Thread
     u32 m_ticks_in_user { 0 };
     u32 m_ticks_in_kernel { 0 };
     u32 m_pending_signals { 0 };
+    u8 m_currently_handled_signal { 0 };
     u32 m_signal_mask { 0 };
     FlatPtr m_alternative_signal_stack { 0 };
     FlatPtr m_alternative_signal_stack_size { 0 };

Kernel/ThreadBlockers.cpp

@@ -474,7 +474,15 @@ bool Thread::SignalBlocker::check_pending_signals(bool from_add_blocker)
         if (m_did_unblock)
             return false;
 
-        auto matching_pending_signal = bit_scan_forward(thread().pending_signals() & m_pending_set);
+        auto pending_signals = thread().pending_signals() & m_pending_set;
+
+        // Also unblock if we have just "handled" that signal and are in the procecss
+        // of running their signal handler (i.e. we just unmarked the signal as pending).
+        if (thread().m_currently_handled_signal)
+            pending_signals |= (1 << (thread().m_currently_handled_signal - 1)) & m_pending_set;
+
+        auto matching_pending_signal = bit_scan_forward(pending_signals);
+
         if (matching_pending_signal == 0)
             return false;