[newlib-cygwin] Be truthful about reporting whether readahead is available

[Corinna Vinschen <corinna at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=03ecfb9280ece68cce33c2f2002877d9a000ef1e

commit 03ecfb9280ece68cce33c2f2002877d9a000ef1e
Author: Johannes Schindelin <johannes.schindelin@gmx.de>
Date:   Tue Apr 5 10:52:01 2016 +0200

    Be truthful about reporting whether readahead is available
    
    In 7346568 (Make requested console reports work, 2016-03-16), code was
    introduced to report the current cursor position. It works by using a
    pointer that either points to the next byte in the readahead buffer, or
    to a NUL byte if the buffer is depleted, or the pointer is NULL.
    
    These conditions are heeded in the fhandler_console::read() method, but
    the condition that the pointer can point at the end of the readahead
    buffer was not handled properly in the get_cons_readahead_valid()
    method.
    
    This poses a problem e.g. in Git for Windows (which uses a slightly
    modified MSYS2 runtime which is in turn a slightly modified Cygwin
    runtime) when vim queries the cursor position and immediately goes on to
    read console input, erroneously thinking that the readahead buffer is
    valid when it is already depleted instead. This condition results in an
    apparent freeze that can be helped only by pressing keys repeatedly.
    
    The full Git for Windows bug report is here:
    
    	https://github.com/git-for-windows/git/issues/711
    
    Let's just teach the get_cons_readahead_valid() method to handle a
    depleted readahead buffer correctly.
    
    Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Diff:
---
 winsup/cygwin/fhandler.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/fhandler.h b/winsup/cygwin/fhandler.h
index 4610557..bd1a923 100644
--- a/winsup/cygwin/fhandler.h
+++ b/winsup/cygwin/fhandler.h
@@ -1453,7 +1453,8 @@ private:
   bool focus_aware () {return shared_console_info->con.use_focus;}
   bool get_cons_readahead_valid ()
   {
-    return shared_console_info->con.cons_rapoi != NULL;
+    return shared_console_info->con.cons_rapoi != NULL &&
+      *shared_console_info->con.cons_rapoi;
   }
 
   select_record *select_read (select_stuff *);