[PATCH] Cygwin: signal: Do not handle signal when __SIGFLUSHFAST is sent

[Takashi Yano]

After the commit d243e51ef1d3, zsh sometimes hangs at startup. This
is because SIGCHLD is handled in cygwait() for a wakeup event even
when __SIGFLUSHFAST is sent, despite __SIGFLUSHFAST requiring to
return before handling the signal. With this patch, if the signal
sent is __SIGFLUSHFAST, do not handle the arrived signal and keep
it being asserted after the cygwait() for the wakeup event.

Addresses: https://cygwin.com/pipermail/cygwin/2024-December/256954.html
Fixes: d243e51ef1d3 ("Cygwin: signal: Fix deadlock between main thread and sig thread")
Reported-by: Daisuke Fujimura <booleanlabel@gmail.com>
Reviewed-by:
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
---
 winsup/cygwin/release/3.5.6 |  5 +++++
 winsup/cygwin/sigproc.cc    | 14 ++++++++++----
 2 files changed, 15 insertions(+), 4 deletions(-)
 create mode 100644 winsup/cygwin/release/3.5.6

diff --git a/winsup/cygwin/release/3.5.6 b/winsup/cygwin/release/3.5.6
new file mode 100644
index 000000000..643d58e58
--- /dev/null
+++ b/winsup/cygwin/release/3.5.6
@@ -0,0 +1,5 @@
+Fixes:
+------
+
+- Fix zsh hang at startup.
+  Addresses: https://cygwin.com/pipermail/cygwin/2024-December/256954.html
diff --git a/winsup/cygwin/sigproc.cc b/winsup/cygwin/sigproc.cc
index ba7818a68..d676799cc 100644
--- a/winsup/cygwin/sigproc.cc
+++ b/winsup/cygwin/sigproc.cc
@@ -785,7 +785,16 @@ sig_send (_pinfo *p, siginfo_t& si, _cygtls *tls)
   if (wait_for_completion)
     {
       sigproc_printf ("Waiting for pack.wakeup %p", pack.wakeup);
-      rc = cygwait (pack.wakeup, WSSC);
+      do
+	{
+	  rc = cygwait (pack.wakeup, WSSC, cw_sig_eintr);
+	  if (rc == WAIT_SIGNALED && pack.si.si_signo != __SIGFLUSHFAST)
+	    _my_tls.call_signal_handler ();
+	}
+      while (rc != WAIT_OBJECT_0 && rc != WAIT_TIMEOUT);
+      /* Re-assert signal_arrived which has been cleared in cygwait(). */
+      if (_my_tls.current_sig)
+	_my_tls.set_signal_arrived ();
       ForceCloseHandle (pack.wakeup);
     }
   else
@@ -806,9 +815,6 @@ sig_send (_pinfo *p, siginfo_t& si, _cygtls *tls)
       rc = -1;
     }
 
-  if (wait_for_completion && si.si_signo != __SIGFLUSHFAST)
-    _my_tls.call_signal_handler ();
-
 out:
   if (communing && rc)
     {
-- 
2.45.1