Test whether buffer/window is ready for start_display, etc.

[Keith David Bershatsky]

In developing implementation of my own feature requests to draw crosshairs 
(#17684) using multiple fake cursors (#22873), I have come across situations 
when Emacs is starting up and restoring buffers (custom version of desktop.el) 
such that several internal functions cause Emacs to crash -- presumably because 
the combination of buffer and window are not ready yet.  I am presently using 
the following five (5) tests to see whether window/buffer are ready for things 
like `start_display`, `move_it_to`, etc.  Is there a better test to see whether 
the buffer/window combination is ready for me to get to work?

  if (w != XWINDOW (selected_window))
    return;

  if (!WINDOW_VALID_P (selected_window))
    return;

  Lisp_Object buf = w->contents;
  CHECK_BUFFER (buf);
  struct buffer *b = XBUFFER (buf);

/* eassert (CHARPOS (pos) >= BEGV && CHARPOS (pos) <= ZV);
   eassert (charpos == BYTE_TO_CHAR (bytepos));
   eassert (BUF_BEG_BYTE (b) <= bytepos && bytepos <= BUF_Z_BYTE (b));
*/

  ptrdiff_t charpos = marker_position (w->start);
  ptrdiff_t bytepos = marker_byte_position (w->start);

  bool barf_crash_one = (charpos >= BEGV && charpos <= ZV) ? false : true;
  if (barf_crash_one)
    return;

  bool barf_crash_two = (charpos == BYTE_TO_CHAR (bytepos)) ? false : true;
  if (barf_crash_two)
    return;

  bool barf_crash_three = (BUF_BEG_BYTE (b) <= bytepos && bytepos <= BUF_Z_BYTE 
(b)) ? false : true;
  if (barf_crash_three)
    return;

Thanks,

Keith