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bug#20489: 25.0.50; next-error-find-buffer chooses non-current buffer wi

From: Dmitry Gutov
Subject: bug#20489: 25.0.50; next-error-find-buffer chooses non-current buffer without good reason
Date: Wed, 28 Feb 2018 04:13:54 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:59.0) Gecko/20100101 Thunderbird/59.0

On 2/27/18 11:16 PM, Juri Linkov wrote:
On that subject: why did we make next-error-last-buffer always buffer-local?

next-error-last-buffer is buffer-local to keep the reference to the
parent buffer used to navigate to the current buffer, so the next call of
next-buffer will use the same parent buffer to continue navigation from it.

Thank you for the reminder. Looking at the previous discussion, I wasn't fond of this design back then, and still think it's largely unnecessary complexity (it introduces implicit state, and in the situations that it can be useful in, the user will have to remember which windows came from which navigation).

Anyway, I've fixed the current problem (see below), so this is a matter of opinion. If you still consider this feature to be important, I think ideally we'd abstract it away behind a new -function variable as well. This way, someone would also be able to implement window-local navigation relationship instead of buffer-local (you've mentioned this option before).

This works well in most cases except the case of xref buffers.
Below is the explanation for code from next-error with added remarks:

(defun next-error (&optional arg reset)
   (let ((buffer (next-error-find-buffer)))
     (when buffer
       (with-current-buffer buffer

Here the current buffer is *xref*

         (funcall next-error-function (prefix-numeric-value arg) reset)

next-error-function should navigate from *xref* to another buffer
and change the current-buffer to the navigated buffer, e.g. ChangeLog.1.

If that's something next-error-function must do, let's document it better. Right now it only says "Function to use to find the next error in the current buffer" (how does one "find" an error?) and describes input arguments, but not the return value (which is luckily unused) or which buffer must be current, or which window selected at the end.

This works fine in most cases, for example when next-error-function is
compilation-next-error-function, but fails when next-error-function is
xref--next-error-function that switches to ChangeLog.1, but doesn't set
the value current-buffer to ChangeLog.1.

         (setq next-error-last-buffer buffer)

In normal cases this sets buffer-local next-error-last-buffer in the
navigated buffer, e.g. ChangeLog.1 that should be the current buffer.
But since xref--next-error-function doesn't set the right current buffer
to ChangeLog.1, this sets buffer-local next-error-last-buffer in the wrong
buffer, i.e. in *xref*.  IOW, the value returned from (current-buffer)
is wrong here after xref--next-error-function call.

I see.

So the question is: in xref--next-error-function can we use code similar
to what is used in compilation-next-error-function that works without problems?

You probably mean compilation-goto-locus (it contains the navigation part), but even so, it's not easy to tell which part you mean (the function does more than one would expect).

In short, adapting that code is kind of difficult, but hopefully I found and fixed the problem in xref directly in 11c58c4fc495ea4f7bff52ca077fd3e4382aa900.

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