I've just modified the trail plugin to use only presence, and not
content dependencies. Using content dependencies, particularly to the page
that defines the trail, meant that every time that page changed, *every*
page in the trail gets rebuilt. This leads to users setting up sites that
have horrible performance, if the trail is defined in, for example, the top
page of a blog.

Unfortunatly, this change to presence dependencies has
introduced a bug. Now when an existing trail is removed, the pages in the
trail don't get rebuilt to remove the trail (both html display and state).

> Actually, this particular case is usually OK. Suppose a trail `untrail`
> contains `untrail/a` (as is the case in the regression
> test I'm writing), and you build the wiki, then edit `untrail` to no
> longer be a trail, and refresh. `untrail` has changed, so it is
> rendered. Assuming that the template of either `untrail` or another
> changed page happens to contain the `TRAILS` variable (which is not
> guaranteed, but is highly likely), `I::P::t::prerender`
> is invoked. It notices that `untrail/a` was previously a trail
> member and is no longer, and rebuilds it with the diagnostic
> "building untrail/a, its previous or next page has changed".
> 
> Strictly speaking, I should change `I::P::t::build_affected`
> so it calls `prerender`, so we're guaranteed to have done the
> recalculation. Fixed in my branch. --[[smcv]]

I think that to fix this bug, the plugin should use a hook to 
force rebuilding of all the pages that were in the trail, when
the trail is removed (or changed).

> The case of "the trail is changed" is still broken:
> if the order of items changes, or the trail is removed,
> then the logic above means it's OK, but if you
> change the `\[[!meta title]]` of the trail, or anything else
> used in the prev/up/next bar, the items won't show that
> change. Fixed in my branch. --[[smcv]]

There's a difficulty in doing that: The needsbuild hook runs before the scan
hook, so before it has a chance to see if the trail directive is still there.
It'd need some changes to ikiwiki's hooks.

> That's what `build_affected` is for, and trail already used it. --s

(An improvement in this area would probably simplify other plugins, which
currently abuse the needsbuild hook to unset state, to handle the case
where the directive that resulted in that state is removed.)

I apologise for introducing a known bug, but the dependency mess was too
bad to leave as-is. And I have very little time (and regrettably, even less
power) to deal with it right now. :( --[[Joey]] 

[[!template id=gitbranch branch=smcv/ready/trail author="[[Simon_McVittie|smcv]]"]]
[[!tag patch]]

> I believe my `ready/trail` branch fixes this. There are regression tests.
>
> Here is an analysis of how the trail pages interdepend.
>
> * If *trail* contains a page *member* which does exist, *member* depends
>   on *trail*. This is so that if the trail directive is deleted from
>   *trail*, or if *trail*'s "friendly" title or trail settings are changed,
>   the trail navigation bar in *member* will pick up that change. This is
>   now only a presence dependency, which isn't enough to make those happen
>   correctly. [Edited to add: actually, the title is the only thing that
>   can affect *member* without affecting the order of members.]
>
> * If *trail* contains consecutive pages *m1* and *m2* in that order,
>   *m1* and *m2* depend on each other. This is so that if one's
>   "friendly" title changes, the other is rebuilt. This is now only
>   a presence dependency, which isn't enough to make those happen
>   correctly. In my branch, I explicitly track the "friendly" title
>   for every page that's edited and is involved in a trail somehow.
>
> * If *trail* has *member* in its `pagenames` but there is no page called
>   *member*, then *trail* must be rebuilt if *member* is created. This
>   was always a presence dependency, and is fine.
>
> In addition, the `trail` plugin remembers the maps
> { trail => next item in that trail } and { trail => previous item in
> that trail } for each page. If either changes, the page gets rebuilt
> by `build_affected`, with almost the same logic as is used to update
> pages that link to a changed page. My branch extends this to track the
> "friendly title" of each page involved in a trail, either by being
> the trail itself or a member (or both).
>
> I think it's true to say that the trail always depends on every member,
> even if it doesn't display them. This might mean that we can use
> "render the trail page" as an opportunity to work out whether any of
> its members are also going to need re-rendering?
> [Edited to add: actually, I didn't need this to be true, but I made the
> regression test check it anyway.]
>
> --[[smcv]]

>>> Thanks **very** much! [[done]] --[[Joey]]