trail: Converted all dependencies to presence dependencies.

[ikiwiki] / doc / bugs / trail_excess_dependencies.mdwn

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).

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).

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.

(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]]