[PATCH 1/3] Cygwin: signal: Fix deadlock on SIGCONT

[Takashi Yano]

If SIGCONT starts processing while __SIGFLUSHFAST is ongoing,
_main_tls->current_sig will never be cleared because the signal
processing is stopped while waiting for the wake-up event in the
main thread. This leads to a deadlock in the while loop waiting for
current_sig to be cleared. With this patch, the function returns to
wait_sig() if current_sig is set, rather than waiting for it in the
while loop.

Addresses: https://cygwin.com/pipermail/cygwin/2025-February/257473.html
Fixes: 9d2155089e87 ("(sigpacket::process): Call handle_SIGCONT early to deal with SIGCONT.")
Reported-by: Christian Franke <Christian.Franke@t-online.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
---
 winsup/cygwin/exceptions.cc | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/winsup/cygwin/exceptions.cc b/winsup/cygwin/exceptions.cc
index f576c5ff2..c6e82b6c5 100644
--- a/winsup/cygwin/exceptions.cc
+++ b/winsup/cygwin/exceptions.cc
@@ -1425,23 +1425,18 @@ _cygtls::handle_SIGCONT ()
   if (NOTSTATE (myself, PID_STOPPED))
     return;
 
-  myself->stopsig = 0;
-  myself->process_state &= ~PID_STOPPED;
-  /* Carefully tell sig_handle_tty_stop to wake up.
-     Make sure that any pending signal is handled before trying to
-     send a new one.  Then make sure that SIGCONT has been recognized
-     before exiting the loop.  */
-  while (current_sig)  /* Assume that it's ok to just test sig outside of a */
-    yield ();          /* lock since setup_handler does it this way.  */
-
   lock ();
   current_sig = SIGCONT;
   set_signal_arrived (); /* alert sig_handle_tty_stop */
   unlock ();
 
+  /* Make sure that SIGCONT has been recognized before exiting the loop. */
   while (current_sig == SIGCONT)
     yield ();
 
+  myself->stopsig = 0;
+  myself->process_state &= ~PID_STOPPED;
+
   /* Clear pending stop signals */
   sig_clear (SIGSTOP, false);
   sig_clear (SIGTSTP, false);
@@ -1473,7 +1468,17 @@ sigpacket::process ()
   myself->rusage_self.ru_nsignals++;
 
   if (si.si_signo == SIGCONT)
-    _main_tls->handle_SIGCONT ();
+    {
+      /* Carefully tell sig_handle_tty_stop to wake up.
+	 Make sure that any pending signal is handled before trying to
+	 send a new one. */
+      if (_main_tls->current_sig)
+	{
+	  rc = -1;
+	  goto done;
+	}
+      _main_tls->handle_SIGCONT ();
+    }
 
   /* SIGKILL is special.  It always goes through.  */
   if (si.si_signo == SIGKILL)
-- 
2.45.1