ikiwiki/doc/bugs/conditional_preprocess_during_scan.mdwn

[[!template id=gitbranch branch=GiuseppeBilotta/scanif author="Giuseppe Bilotta"]]

When a directive that should be run during scan preprocessing is inside
an if directive, it doesn't get called because the if preprocessing does
not run during scan.

I've written a simple [[patch]] to fix the issue, currently hosted on the
scanif branch of my repository. The patch also passes the preview option
back to the Ikiwiki::preprocess call, making sure that whatever is being
reprocessed is done so in the same conditions as the original call.

> One problem with this is that it has the same dependency-ordering problems
> as inline-based or pagespec-based trails with my [[plugins/contrib/trail]]
> plugin: `if` takes a pagespec, but pagespecs aren't guaranteed to match
> correctly until everything has been scanned (for instance, `link()` might
> give the wrong results because a page that added or deleted a link hasn't
> been scanned yet). If you have a clever idea for how to fix this, I'd love
> to hear it - being able to specify a [[plugins/contrib/trail]] in terms
> of a sorted pagespec would be useful. --[[smcv]]

>> I have a solution to the dependency-ordering problem in a different
>> branch of my repository, with a post_scan hook mechanism which I use to
>> be able to sort outer inline pages according to the last modification
>> date of their nested inline pages. The way I implemented it currently,
>> though, doesn't use the existing hooks mechanism of ikiwiki (because
>> it's something which I believe to be more efficiently done the way I
>> implemented it) so I don't know how likely it is to be included
>> upstream.

>> For what it's worth, I think that my post_scan hook mechanism would work
>> rather fine with your trail plugin.

>>> We discussed this on IRC, and I think it's actually more complicated
>>> than that: the branch to sort by newest inlined entry wants a
>>> "pagespecs now work" hook, whereas for trail I want a "sorting now
>>> works" hook:
>>>
>>> * scan
>>> * pagespecs now work (post-scan)
>>>   * Giuseppe's version of inline can decide what each inline
>>>     contains, and thus decide where they go in `inline(mtime)`
>>>     order
>>> * pagespecs and sorting now work (pre-render)
>>>   * my trail plugin can decide what each trail contains, and
>>>     also sort them in the right order (which might be
>>>     `inline(mtime)`, so might be undefined until pagespecs work)
>>> * render
>>>
>>> --[[smcv]]

>> However, the case of the if
>> directive is considerably more complicated, because the conditional
>> can introduce a much stronger feedback effect in the pre/post scanning
>> dependency. In fact, it's probably possible to build a couple of pages
>> with vicious conditional dependency circles that would break/unbreak
>> depending on which pass we are in. And I believe this is an intrinsic
>> limitation of the system, which cannot be solved at all.