bug#12081: 24.1; buffer-predicate often not called

[martin rudalics <rudalics@gmx.at>]

 > Adding a buffer predicate test creates a new edge case, which the
 > above code is intended to handle.

It doesn't.  The edge case is already here.

 > With the existing code, it's guaranteed that either (1) we switch to a
 > new buffer while scanning the next buffers or the prev buffers, or (2)
 > we finish those scans with `visible' set to a buffer we can switch to.

No.  Try killing *scratch* with *scratch* the only buffer one can switch
to.

 >  By adding a buffer predicate test to the loops, we break that
 > guarantee; it becomes possible to finish the scan without switching to
 > a new buffer and without setting `visible' to a candidate new buffer.
 > This would break the function unless we provide for that case.

Let's hope not.

 > The question then becomes, how to deal with such a case (when the scan
 > of prev and next buffers does not switch to a new buffer and does not
 > set `visible').  One obvious option is:  Do nothing, and adjust the
 > rest of the function to accomodate that possibility.  Another is to
 > choose *scratch* as the fallback value for `new-buffer', when no other
 > buffer passes all the tests.  The latter seemed to me to be
 > preferable, as it makes replace-buffer-in-windows and
 > replace_buffer_in_windows_safely have similar fallback behavior, and
 > agrees with the fallback behavior of other-buffer as well.

With Emacs 23 I had a scenario (which is much too complicated to repeat
here) to crash Emacs by repeatedly trying to kill all buffers.  That's
why I wrote replace_buffer_in_windows_safely which relies on
other_buffer_safely to reliably recreate *scratch* if needed.  I don't
want to play with Elisp code called by C code recreating the buffer the
C code is just about to delete.  Let's keep other_buffer_safely the only
routine allowed to auto-recreate *scratch* and let's not call routines
like replace_buffer_in_windows_safely and other_buffer_safely from
Elisp.

If you really experience problems after applying the rest of the patch,
we'll find a solution ;-)

martin