From 082f2513c721e942d0fd563c4dc9117eee3513ab Mon Sep 17 00:00:00 2001
From: Corinna Vinschen <corinna@vinschen.de>
Date: Wed, 25 Mar 2020 12:21:59 +0100
Subject: [PATCH] Cygwin: serial: read: if VMIN > 0, wait for VMIN chars in
 inbound queue

Per termios, read waits for MIN chars even if the number of requested
bytes is less.  This requires to add WaitCommEvent to wait non-busily
for MIN chars prior to calling ReadFile, so, reintroduce it.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
---
 winsup/cygwin/fhandler_serial.cc | 38 ++++++++++++++++++++++----------
 1 file changed, 26 insertions(+), 12 deletions(-)

diff --git a/winsup/cygwin/fhandler_serial.cc b/winsup/cygwin/fhandler_serial.cc
index b6d1e4f4c..1750a9f97 100644
--- a/winsup/cygwin/fhandler_serial.cc
+++ b/winsup/cygwin/fhandler_serial.cc
@@ -34,19 +34,20 @@ void __reg3
 fhandler_serial::raw_read (void *ptr, size_t& ulen)
 {
   OVERLAPPED ov = { 0 };
-  DWORD io_err;
+  DWORD io_err, event;
   COMSTAT st;
   DWORD bytes_to_read, read_bytes;
   ssize_t tot = 0;
+  bool wait_for_vmin, ret;
 
   if (ulen > SSIZE_MAX)
     ulen = SSIZE_MAX;
   if (ulen == 0)
     return;
 
-  /* If VMIN > 0 in blocking mode, we have to read at least VMIN chars,
-     otherwise we're in polling mode and there's no minimum chars. */
-  ssize_t minchars = (!is_nonblocking () && vmin_) ? MIN (vmin_, ulen) : 0;
+  /* If VMIN > 0 in blocking mode, we have to wait for at least VMIN chars.
+     Otherwise we're in polling mode and there's no minimum chars. */
+  ssize_t minchars = is_nonblocking () ? 0 : vmin_;
 
   debug_printf ("ulen %ld, vmin_ %u, vtime_ %u", ulen, vmin_, vtime_);
 
@@ -54,6 +55,8 @@ fhandler_serial::raw_read (void *ptr, size_t& ulen)
 
   do
     {
+      wait_for_vmin = false;
+
       /* First check if chars are already in the inbound queue. */
       if (!ClearCommError (get_handle (), &io_err, &st))
 	goto err;
@@ -82,14 +85,22 @@ fhandler_serial::raw_read (void *ptr, size_t& ulen)
 	     and don't wait. */
 	  if (st.cbInQue && st.cbInQue >= minchars)
 	    bytes_to_read = MIN (st.cbInQue, bytes_to_read);
+	  /* Otherwise, if VMIN > 0, VTIME == 0, we have to wait until
+	     VMIN bytes are available in the inbound queue. */
+	  else if (minchars && !vtime_)
+	    wait_for_vmin = true;
 	}
 
       ResetEvent (ov.hEvent);
-      if (!ReadFile (get_handle (), ptr, bytes_to_read, &read_bytes, &ov))
+      if (wait_for_vmin)
+	ret = WaitCommEvent (get_handle (), &event, &ov);
+      else
+	ret = ReadFile (get_handle (), ptr, bytes_to_read, &read_bytes, &ov);
+      if (!ret)
 	{
 	  if (GetLastError () != ERROR_IO_PENDING)
 	    goto err;
-	  if (is_nonblocking ())
+	  if (!wait_for_vmin && is_nonblocking ())
 	    {
 	      CancelIo (get_handle ());
 	      if (!GetOverlappedResult (get_handle (), &ov, &read_bytes,
@@ -134,12 +145,15 @@ fhandler_serial::raw_read (void *ptr, size_t& ulen)
 		}
 	    }
 	}
-      tot += read_bytes;
-      ptr = (void *) ((caddr_t) ptr + read_bytes);
-      ulen -= read_bytes;
-      minchars -= read_bytes;
-      debug_printf ("vtime_ %u, vmin_ %u, read_bytes %u, tot %D",
-		    vtime_, vmin_, read_bytes, tot);
+      if (!wait_for_vmin)
+	{
+	  tot += read_bytes;
+	  ptr = (void *) ((caddr_t) ptr + read_bytes);
+	  ulen -= read_bytes;
+	  minchars -= read_bytes;
+	  debug_printf ("vtime_ %u, vmin_ %u, read_bytes %u, tot %D",
+			vtime_, vmin_, read_bytes, tot);
+	}
       continue;
 
     err: