severity 36588 minor
tags 36588 + patch
quit

Stefan Kangas <stefan@marxist.se> writes:

> I get an error trying to revert the apropos help buffer on Emacs 27
> (current master).  I could also reproduce this on 26.1.
>
> Steps to reproduce:
> 0. emacs -Q
> 1. M-x apropos RET foo RET
> 2. C-x o
> 3. g yes RET
> 4. Error:
> apply: Symbol’s function definition is void: nil
>
> Backtrace:
> Debugger entered--Lisp error: (void-function nil)
>   nil()
>   apply(nil nil)
>   help-mode-revert-buffer(t nil)
>   revert-buffer(t)
>   funcall-interactively(revert-buffer t)
>   #<subr call-interactively>(revert-buffer nil nil)
>   apply(#<subr call-interactively> revert-buffer (nil nil))
>   call-interactively@ido-cr+-record-current-command(#<subr
> call-interactively> revert-buffer nil nil)
>   apply(call-interactively@ido-cr+-record-current-command #<subr
> call-interactively> (revert-buffer nil nil))
>   call-interactively(revert-buffer nil nil)
>   command-execute(revert-buffer)

This is because Apropos buffers are set up in apropos-print using
with-output-to-temp-buffer, which by default calls help-mode, which sets
revert-buffer-function permanently-locally to help-mode-revert-buffer.

[Aside: Why is revert-buffer-function permanent-local?]

help-mode-revert-buffer expects the command which generated the Help
buffer to have previously called help-setup-xref.  This should be
possible to do in Apropos, but it would be quite ugly as help-setup-xref
needs to be called with the Help buffer current, so Apropos commands
would need to set their own version of help-xref-stack-item before
apropos-print performs the help-setup-xref dance.  Besides, Apropos
buffers are not (currently) really Help buffers.

Instead, I propose emulating a simpler version of help-setup-xref
specific to Apropos: