[[!comment format=mdwn
 username="http://smcv.pseudorandom.co.uk/"
 nickname="smcv"
 subject="comment 1"
 date="2013-07-01T09:13:51Z"
 content="""
(If you want to branch from my version of album, please add my git repo
as a remote and merge or cherry-pick the album4 branch: pasting from my
gitweb seems to have given you some incorrect UTF-8.)

The problem you have here is that for this plugin, the correct order
for IkiWiki to do things is quite subtle. Am I right in thinking that
the feature you want goes something like this?

> To add a set of tags to every \"viewer\" page in the album, you can add
> the tags parameter to the album:
>
>     \[[!album tags=\"holiday hawaii\"]]
>
> The individual viewers will all be tagged \"holiday\" and \"hawaii\",
> for instance. These tags cannot be removed by editing the viewers.

`preprocess_albumimage` runs twice: once in the scan stage, and once in
the render stage. In the render stage, it's too late to add tags, because
tags are a special form of [[ikiwiki/wikilinks]], and wikilinks have to
be added during the scan stage to work correctly.

The part of `preprocess_albumimage` after the line
`return unless defined wantarray;` only runs in the render stage, which
is too late. You'd need to set up the tags further up: just after the
calls to `IkiWiki::Plugin::meta::preprocess` would be a good place.

I would also suggest checking for
`IkiWiki::Plugin::tag->can('preprocess_tag')`,
like I do for meta - if you do that, you won't need to force the tag plugin
to be loaded.

Unfortunately, I'm still not sure that this is fully correct. Pages
are scanned in a random order. If the `\[[!album]]` is scanned before
a \"viewer\" page, then everything is fine: the tags are present when
the \"viewer\" is scanned. However, if the \"viewer\" is scanned first,
then it will get the tags that the `\[[!album]]` had in the *previous*
IkiWiki run (if any), which are still in the index, because the
`\[[!album]]` hasn't been re-scanned yet...

Are you sure this form of the feature is what you want? You'll end up with
a *lot* of pages with those tags. If it's what you want, it might be
clearer how it works if you changed the syntax to something like this,
perhaps?

>
>     \[[!album tag_all=\"holiday hawaii\"]]

Another possible syntax would be to have the feature be more like this:

> If you use the `tag_default` parameter to the `\[[!album]]` directive,
> each \"viewer\" page created for images will have those tags by
> default. Changing the `\[[!album]]` will not affect any \"viewer\"
> pages that have already been created, and editing the \"viewer\"
> can add or remove those default tags.
>
>     \[[!album tag_default=\"holiday hawaii\"]]

which I think removes the ordering problems? If you go this route,
you'd want to either add e.g. `[[!tag holiday hawaii]]`
to the generated viewer page in `create_viewer`, or add a `tag`
parameter to `\[[!albumimage]]` that's a shortcut for the
tag directive, in the same way that author is a shortcut for
`[[!meta author]]`).

The purpose of the \"shortcut\" parameters in `\[[!albumimage]]`,
like title, author and date, is that I eventually want to add
a specialized CGI interface to this plugin so you can edit
all the images of an album in one go; when I add that,
it'll probably only be able to process something as
machine-readable as `\[[!albumimage]]`.
"""]]