Cygwin: define fhandler_fifo::fstat

Previously fstat on a FIFO would call fhandler_base::fstat.

The latter is not appropriate if fhandler_fifo::open has already been
called (and O_PATH is not set), for the following reason.  If a FIFO
has been opened as a writer or duplexer, then it has an io_handle that
is a pipe handle rather than a file handle.  fhandler_base::fstat will
use this handle and potentially return incorrect results.  If the FIFO
has been opened as a reader, then it has no io_handle, and a call to
fhandler_base::fstat will lead to a call to fhandler_base::open.
Opening the fhandler a second time can change it in undesired ways;
for example, it can modify the flags and status_flags.

The new fhandler_fifo::fstat avoids these problems by creating an
fhandler_disk_file and calling its fstat method in case
fhandler_fifo::open has already been called and O_PATH is not set.

winsup/cygwin/fhandler.h

@@ -1511,6 +1511,7 @@ public:
   ssize_t __reg3 raw_write (const void *ptr, size_t ulen);
   void fixup_after_fork (HANDLE);
   void fixup_after_exec ();
+  int __reg2 fstat (struct stat *buf);
   int __reg2 fstatvfs (struct statvfs *buf);
   select_record *select_read (select_stuff *);
   select_record *select_write (select_stuff *);

winsup/cygwin/fhandler_fifo.cc

@@ -1494,6 +1494,29 @@ errout:
   len = (size_t) -1;
 }
 
+int __reg2
+fhandler_fifo::fstat (struct stat *buf)
+{
+  if (reader || writer || duplexer)
+    {
+      /* fhandler_fifo::open has been called, and O_PATH is not set.
+	 We don't want to call fhandler_base::fstat.  In the writer
+	 and duplexer cases we have a handle, but it's a pipe handle
+	 rather than a file handle, so it's not suitable for stat.  In
+	 the reader case we don't have a handle, but
+	 fhandler_base::fstat would call fhandler_base::open, which
+	 would modify the flags and status_flags. */
+      fhandler_disk_file fh (pc);
+      fh.get_device () = FH_FS;
+      int res = fh.fstat (buf);
+      buf->st_dev = buf->st_rdev = dev ();
+      buf->st_mode = dev ().mode ();
+      buf->st_size = 0;
+      return res;
+    }
+  return fhandler_base::fstat (buf);
+}
+
 int __reg2
 fhandler_fifo::fstatvfs (struct statvfs *sfs)
 {