bug#40919: 27.0.91; next-error-select-buffer does not always behave as documented

[Trevor Spiteri]

The next-error-select-buffer documentation states that “the selected
buffer becomes the source of locations for the subsequent invocation of
‘next-error’ or ‘previous-error’.” However, it is not the case for the
following:

1. Go in a fresh next-error capable buffer (not *grep*).
2. Grep for something.
3. M-x next-error-select-buffer *grep*
4. M-x next-error

The buffer of 1 (not *grep*) is the source of locations instead of
the expected *grep*.

This is because although next-error-select-buffer sets the variable
next-error-last-buffer, it is not used in this case: When next-error
calls next-error-find-buffer, next-error-buffer has no buffer-local
value yet, so condition 2. in next-error-find-buffer (that
next-error-buffer has no buffer-local value and the current buffer is a
next-error capable buffer) is true, and the function never even checks
next-error-last-buffer.



In GNU Emacs 27.0.91 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.24.13, cairo version 1.16.0)
of 2020-04-23 built on desktop
Repository revision: ba6104d1e8db4e8db2f12acaebf092ef579c6632
Repository branch: emacs-27