[PATCH] Cygwin: signal: Fix high load when retrying to process pending signal

[Takashi Yano]

The commit e10f822a2b39 has a problem that CPU load gets high if
pending signal is not processed successfully for a long time.
With this patch, wait_sig() calls Sleep(1), rather than yield(),
if the pending signal has not been processed successfully for a
predetermined time to prevent CPU from high load.

Addresses: https://cygwin.com/pipermail/cygwin/2024-December/256884.html
Fixes: e10f822a2b39 ("Cygwin: signal: Handle queued signal without explicit __SIGFLUSH")
Reported-by: 凯夏 <walkerxk@gmail.com>
Reviewed-by:
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
---
 winsup/cygwin/sigproc.cc | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/sigproc.cc b/winsup/cygwin/sigproc.cc
index 59b4208a6..e01a67ebe 100644
--- a/winsup/cygwin/sigproc.cc
+++ b/winsup/cygwin/sigproc.cc
@@ -1345,6 +1345,12 @@ wait_sig (VOID *)
 
   hntdll = GetModuleHandle ("ntdll.dll");
 
+  /* GetTickCount() here is enough because GetTickCount() - t0 does
+     not overflow until 49 days psss. Even if GetTickCount() overflows,
+     GetTickCount() - t0 returns correct value, since underflow in
+     unsigned wraps correctly. Pending a signal for more thtn 49
+     days would be noncense. */
+  DWORD t0 = GetTickCount ();
   for (;;)
     {
       DWORD nb;
@@ -1354,7 +1360,10 @@ wait_sig (VOID *)
       else if (sigq.start.next
 	       && PeekNamedPipe (my_readsig, NULL, 0, NULL, &nb, NULL) && !nb)
 	{
-	  yield ();
+	  if (GetTickCount () - t0 > 10)
+	    Sleep (1);
+	  else
+	    yield ();
 	  pack.si.si_signo = __SIGFLUSH;
 	}
       else if (!ReadFile (my_readsig, &pack, sizeof (pack), &nb, NULL))
@@ -1364,6 +1373,8 @@ wait_sig (VOID *)
 	  system_printf ("garbled signal pipe data nb %u, sig %d", nb, pack.si.si_signo);
 	  continue;
 	}
+      if (pack.si.si_signo != __SIGFLUSH)
+	t0 = GetTickCount ();
 
       sigq.retry = false;
       /* Don't process signals when we start exiting */
-- 
2.45.1