* DevNotes: Add entry cgf-000008.

* fhandler_tty.cc (bytes_available): Simplify by returning the number of bytes
available in the message unless that is zero.

winsup/cygwin/ChangeLog

@@ -1,3 +1,9 @@
+2012-05-15  Christopher Faylor  <me.cygwin2012@cgf.cx>
+
+	* DevNotes: Add entry cgf-000008.
+	* fhandler_tty.cc (bytes_available): Simplify by returning the number
+	of bytes available in the message unless that is zero.
+
 2012-05-14  Christopher Faylor  <me.cygwin2012@cgf.cx>
 
 	* child_info.h (CURR_CHILD_INFO_MAGIC): Update.

winsup/cygwin/DevNotes

@@ -1,3 +1,35 @@
+2012-05-14  cgf-000008
+
+<1.7.16>
+- Fix hang when zero bytes are written to a pty using
+  Windows WriteFile or equivalent.  Fixes:
+  http://cygwin.com/ml/cygwin/2012-05/msg00323.html
+</1.7.16>
+
+cgf-000002, as usual, fixed one thing while breaking another.  See
+Larry's predicament in: http://goo.gl/oGEr2 .
+
+The problem is that zero byte writes to the pty pipe caused the dread
+end-of-the-world-as-we-know-it problem reported on the mailing list
+where ReadFile reads zero bytes even though there is still more to read
+on the pipe.  This is because that change caused a 'record' to be read
+and a record can be zero bytes.
+
+I was never really keen about using a throwaway buffer just to get a
+count of the number of characters available to be read in the pty pipe.
+On closer reading of the documentation for PeekNamedPipe it seemed like
+the sixth argument to PeekNamedPipe should return what I needed without
+using a buffer.  And, amazingly, it did, except that the problem still
+remained - a zero byte message still screwed things up.
+
+So, we now detect the case where there is zero bytes available as a
+message but there are bytes available in the pipe.  In that scenario,
+return the bytes available in the pipe rather than the message length of
+zero.  This could conceivably cause problems with pty pipe handling in
+this scenario but since the only way this scenario could possibly happen
+is when someone is writing zero bytes using WriteFile to a pty pipe, I'm
+ok with that.
+
 2012-05-14  cgf-000007
 
 <1.7.16>

winsup/cygwin/fhandler_tty.cc

@@ -53,17 +53,20 @@ fhandler_pty_slave::get_unit ()
 bool
 bytes_available (DWORD& n, HANDLE h)
 {
-  char buf[INP_BUFFER_SIZE];
-  /* Apparently need to pass in a dummy buffer to read a real "record" from
-     the pipe.  So buf is used and then discarded just so we can see how many
-     bytes will be read by the next ReadFile().  */
-  bool succeeded = PeekNamedPipe (h, buf, sizeof (buf), &n, NULL, NULL);
-  if (!succeeded)
+  DWORD navail, nleft;
+  navail = nleft = 0;
+  bool succeeded = PeekNamedPipe (h, NULL, 0, NULL, &navail, &nleft);
+  if (succeeded)
+    /* nleft should always be the right choice unless something has written 0
+       bytes to the pipe.  In that pathological case we return the actual number
+       of bytes available in the pipe. See cgf-000008 for more details.  */
+    n = nleft ?: navail;
+  else
     {
       termios_printf ("PeekNamedPipe(%p) failed, %E", h);
       n = 0;
     }
-  debug_only_printf ("%u bytes available", n);
+  debug_only_printf ("n %u, nleft %u, navail %u");
   return succeeded;
 }
 

winsup/cygwin/release/1.7.16

@@ -7,10 +7,14 @@ Bug fixes:
 ----------
 
 - Fix pipe creation problem which manifested as a problem creating a
-fifo.  Fixes: http://cygwin.com/ml/cygwin/2012-05/msg00253.html
+  fifo.  Fixes: http://cygwin.com/ml/cygwin/2012-05/msg00253.html
 
 - Fix hang when calling pthread_testcancel in a canceled thread.
   Fixes some of: http://cygwin.com/ml/cygwin/2012-05/msg00186.html
 
 - Fix invocation of strace from a cygwin process.  Fixes:
   http://cygwin.com/ml/cygwin/2012-05/msg00292.html
+
+- Fix hang when zero bytes are written to a pty using
+  Windows WriteFile or equivalent.  Fixes:
+  http://cygwin.com/ml/cygwin/2012-05/msg00323.html