[newlib-cygwin] Cygwin: serial: select: call ClearCommError prior to calling WaitCommEvent

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=a1f0585454ea60f9e10be1f0cdcc7c0c1ceb2d0e

commit a1f0585454ea60f9e10be1f0cdcc7c0c1ceb2d0e
Author: Corinna Vinschen <corinna@vinschen.de>
Date:   Mon Mar 23 17:23:19 2020 +0100

    Cygwin: serial: select: call ClearCommError prior to calling WaitCommEvent
    
    This (hopefully) eliminates a race condition
    
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/select.cc | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/winsup/cygwin/select.cc b/winsup/cygwin/select.cc
index 07e0d82fb..b933a8ca9 100644
--- a/winsup/cygwin/select.cc
+++ b/winsup/cygwin/select.cc
@@ -1502,6 +1502,9 @@ verify_serial (select_record *me, fd_set *rfds, fd_set *wfds, fd_set *efds)
 select_record *
 fhandler_serial::select_read (select_stuff *ss)
 {
+  COMSTAT st;
+  DWORD io_err;
+
   select_record *s = ss->start.next;
 
   s->startup = no_startup;
@@ -1512,18 +1515,25 @@ fhandler_serial::select_read (select_stuff *ss)
   s->read_ready = false;
   /* This is apparently necessary for the com0com driver.
      See: http://cygwin.com/ml/cygwin/2009-01/msg00667.html */
+  ResetEvent (io_status.hEvent);
   SetCommMask (get_handle_cyg (), 0);
   SetCommMask (get_handle_cyg (), EV_RXCHAR);
-  ResetEvent (io_status.hEvent);
-  if (!WaitCommEvent (get_handle_cyg (), &event, &io_status))
+  if (ClearCommError (get_handle_cyg (), &io_err, &st) && st.cbInQue)
     {
-      if (GetLastError () == ERROR_IO_PENDING)
-	s->h = io_status.hEvent;
-      else
-	select_printf ("WaitCommEvent %E");
+      s->read_ready = true;
+      return s;
     }
-  else
-    s->read_ready = true;
+  if (WaitCommEvent (get_handle_cyg (), &event, &io_status))
+    {
+      s->read_ready = true;
+      return s;
+    }
+  if (GetLastError () == ERROR_IO_PENDING)
+    {
+      s->h = io_status.hEvent;
+      return s;
+    }
+  select_printf ("WaitCommEvent %E");
   return s;
 }