ikiwiki/doc/bugs/template_creation_error.mdwn

Hi,
I am trying to build a template. The compilation of this template results in a weird exception. I have isolated the cause of the exception to the following point:

If i have this in the template code:

\[[!inline<br/>
pages="\<TMPL_VAR SEL_PAGES\>"<br/>
template=extract-entry<br/>
\]]<br/>

There is no problem at all. I can use the template with the desired result. But if I try to use this (just adding the "show" parameter):

\[[!inline <br/>
pages="\<TMPL_VAR SEL_PAGES>"<br/>
template=extract-entry<br/>
show=\<TMPL_VAR CNTPG><br/>
\]]<br/>

I get this exception on the Git bash console:

<pre>
$ git push
Counting objects: 7, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (4/4), done.
Writing objects: 100% (4/4), 410 bytes, done.
Total 4 (delta 3), reused 0 (delta 0)
remote: From /home/b-odelama-com/source
remote:    eb1421e..5e1bac5  master     -> origin/master
remote: Argument "\x{3c}\x{54}..." isn't numeric in numeric lt (<) at /usr/share/perl5/IkiWiki/Plugin/inline.pm line 231.
remote: Argument "\x{3c}\x{54}..." isn't numeric in numeric lt (<) at /usr/share/perl5/IkiWiki/Plugin/inline.pm line 231.
To ssh://b-odelama-com@odelama-com.branchable.com/
   eb1421e..5e1bac5  master -> master
</pre>

Please, let me know what to do to avoid this kind of error.

> When you add a template page `templates/foo.mdwn` for use
> the [[ikiwiki/directive/template]] directive, two things happen:
>
> 1. `\[[!template id=foo ...]]` becomes available;
> 2. a wiki page `templates/foo` is built, resulting in a HTML file,
>    typically `templates/foo/index.html`
>
> The warnings you're seeing are the second of these: when ikiwiki
> tries to process `templates/foo.mdwn` as an ordinary page, without
> interpreting the `<TMPL_VAR>` directives, `inline` receives invalid
> input.
>
> This is a bit of a design flaw in [[plugins/template]] and
> [[plugins/edittemplate]], I think - ideally it would be possible to
> avoid parts of the page being interpreted when the page is being
> rendered normally rather than being used as a template.
>
> There *is* a trick to avoid parts of the page being interpreted when
> the page is being used as a template, while having them appear
> when it's rendered as a page:
>
>     <TMPL_IF FALSE>
>     <!-- This part only appears when being used as a page.
>          It assumes that you never set FALSE to a true value :-) -->
>     \[[!meta robots="noindex,nofollow"]]
>     This template is used to describe a thing. Parameters:
>     * name: the name of the thing
>     * size: the size of the thing
>     </TMPL_IF>
>
>     The thing is called <TMPL_VAR name> and its size is <TMPL_VAR size>
>
> I suppose you could maybe extend that to something like this:
>
>     <TMPL_IF FALSE>
>     <!-- This part only appears when being used as a page.
>          It assumes that you never set FALSE to a true value :-) -->
>     \[[!meta robots="noindex,nofollow"]]
>     This template is used to describe a thing. Parameters:
>     * name: the name of the thing
>     * size: the size of the thing
>     </TMPL_IF>
>
>     <TMPL_IF FALSE>
>     \[[!if test="included() and !included()" then="""
>     </TMPL_IF>
>     <!-- This part only appears when being used as a template. It also
>          assumes that you never set FALSE to a true value, and it
>          relies on the [[ikiwiki/pagespec]] "included() and !included()"
>          never being true. -->
>     The thing is called <TMPL_VAR name> and its size is <TMPL_VAR size>
>     <TMPL_IF FALSE>
>     """]]
>     </TMPL_IF>
>
> but that's far harder than it ought to be!
>
> Perhaps the right solution would be to change how the template plugin
> works, so that templates are expected to contain a new `definetemplate`
> directive:
>
>     This template is used to describe a thing. Parameters:
>     * name: the name of the thing
>     * size: the size of the thing
>     
>     \[[!definetemplate """
>     The thing is called <TMPL_VAR name> and its size is <TMPL_VAR size>
>     """]]
>
> with templates not containing a `\[[!definetemplate]]` being treated
> as if the whole text of the page was copied into a `\[[!definetemplate]]`,
> for backwards compatibility?
>
> --[[smcv]]

>> OK, here is a branch implementing what I said. It adds the `definetemplate`
>> directive to [[plugins/goodstuff]] as its last commit.
>>
>> Templates with the current strange semantics will still work, until
>> IkiWiki breaks compatibility.
>>
>> Possible controversies:
>>
>> * Should the `definetemplate` plugin be core, or in goodstuff, or neither?
>>
>> * Should \[[!definetemplate]] be allowed on any page (with the implementation
>>   of `template("foo")` looking for a `definetemplate` in `templates/foo`,
>>   then a `definetemplate` in `foo`, then fall back to the current logic)?
>>   If not, should \[[!definetemplate]] raise an error when used on a page not
>>   in `templates/`, since it will have no practical effect there?
>>
>> * Is it OK to rely on `definetemplate` being enabled in the basewiki's
>>   templates?
>>
>> * Should the "use definetemplate" wording in the documentation of
>>   template and edittemplate be stronger? Should those plugins automatically
>>   load definetemplate?
>>
>> --[[smcv]]

>>> this looks like a good idea to me.
>>>
>>> * i'd put it in core, and add a transition for the time compatibility gets
>>>   broken, provided the transitioning system will be used in that. templates
>>>   can't be expected to just work as markdown+ikiwiki too.
>>>
>>>   (it being in core would also solve my qualms about `section => "web"` /
>>>   `\[[!tag type/web]]`).
>>>
>>> * if definetemplate gets deemed core, no "use definetemplate!" notes on the
>>>   template/edittemplate pages will be required any more.
>>>
>>> * first i was sceptical of the approach of re-running scan to make sure the
>>>   `my %templates` is filled, but it is indeed a practical solution.
>>>
>>> * the name "`definetemplate`" gives me the first impression that something
>>>   is assigned (as in `#define`), but actually it highlights a region in the
>>>   file. wouldn't "`templatebody`" be a better description of the meaning of
>>>   the directive?
>>>
>>> --[[chrysn]]

>>>> Thanks for your feedback!
>>>> Looking at its description on this wiki, I agree that `type/web` doesn't
>>>> fit, and core does seem better. I like your `templatebody` suggestion,
>>>> too, particularly if templates remain restricted to `/templates`.
>>>> I'll try to come up with better wording for the documentation to say
>>>> "use `templatebody`, like this", with a note about backwards
>>>> compatibility later.
>>>>
>>>> Rationale for `my %templates`: yes it does seem a bit odd, but
>>>> if I used `$pagestate{$tpage}{template}` instead of a `my` variable,
>>>> I'd sometimes _still_ have to force a `scan`, because
>>>> [[plugins/template]] has to expand the template at scan time so that
>>>> it can contain links etc. - so I have to make sure that if the
>>>> template has changed, it has already been scanned (scanning happens
>>>> in random order, so that can't be guaranteed). This means there's
>>>> no benefit in reading it back from the index, so it might as well
>>>> just be in-memory.
>>>>
>>>> I suppose an alternative way to do it would be to remember what was
>>>> passed to `needsbuild`, and only force a `scan` for templates that
>>>> were in that list - which potentially reduces CPU time and I/O a
>>>> little, in exchange for a bigger index. I could do that if Joey
>>>> wants me to, but I think the current approach is simpler,
>>>> so I'll stick with the current approach if it isn't vetoed.
>>>> --[[smcv]]

>>>>> @name: even outside `/templates`, `\[[!templatebody]]` would be
>>>>> interpreted as "when this page is used as a template, this is what its
>>>>> contents should be", and be suitable.
>>>>>
>>>>> @`%templates`: my surprise wasn't to it not being in `%pagestate`, but
>>>>> rather that the `scan` function was used for it at all, rather than plain
>>>>> directive parsing that ignores everything else -- but i agree that it's
>>>>> the right thing to do in this situation.
>>>>>
>>>>> --[[chrysn]]

----

[[!template id=gitbranch author="[[smcv]]" branch=smcv/ready/templatebody
  browse=http://git.pseudorandom.co.uk/smcv/ikiwiki.git/shortlog/refs/heads/ready/templatebody]]
[[!tag patch users/smcv/ready]]
Branch and directive renamed to `ready/templatebody` as chrysn suggested.
It's on-by-default now (or will be if that branch is merged).
Joey, any chance you could review this?

There is one known buglet: `template_syntax.t` asserts that the entire
file is a valid HTML::Template, whereas it would ideally be doing the
same logic as IkiWiki itself. I don't think that's serious. --[[smcv]]

> Looking over this, I notice it adds a hash containing all scanned
> files. This seems to me to be potentially a scalability problem on
> rebuild of a site with many pages. Ikiwiki already keeps a lot
> of info in memory, and this adds to it, for what is a fairly
> minor reason. It seems to me there should be a way to avoid this. --[[Joey]] 

>> Maybe. Are plugins expected to cope with scanning the same
>> page more than once? If so, it's just a tradeoff between
>> "spend more time scanning the template repeatedly" and
>> "spend more memory on avoiding it", and it would be OK to
>> omit that, or reduce it to a set of scanned *templates*
>> (in practice that would mean scanning each template twice
>> in a rebuild). --s
>>> [Commit f7303db5](http://source.ikiwiki.branchable.com/?p=source.git;a=commitdiff;h=f7303db5)
>>> suggests that scanning the same page more than once is problematic,
>>> so that solution is probably not going to work.
>>>
>>> The best idea I've come up with so far is to track whether
>>> we're in the scan or render phase. If we're in the scan
>>> phase, I think we do need to keep track of which pages
>>> we've scanned, so we don't do them again? (Or perhaps that's
>>> unnecessary - commit f7303db5 removed a scan call that's in
>>> the render phase.) If we're in the render phase, we can assume
>>> that all changed pages have been scanned already, so we can
>>> drop the contents of `%scanned` and rely on a single boolean
>>> flag instead.
>>>
>>>> This is not actually good enough for the templatebody
>>>> directive, which does in fact need to scan certain pages
>>>> during the render phase, namely when a page that is rendered
>>>> due to dependencies uses a template that no other page being
>>>> rendered in this pass was using. I've reverted this optimization,
>>>> to fix [[wrong rendering of templatebody]], and applied a more
>>>> limited version which only optimizes rebuilds (the worst case
>>>> of this memory consumption). --[[smcv]]
>>>
>>> `%scanned` is likely to be no larger than `%rendered`, which
>>> we already track, and whose useful lifetime does not overlap
>>> with `%scanned` now. I was tempted to merge them both and call
>>> the result `%done_in_this_phase`, but that would lead to really
>>> confusing situations if a bug led to `render` being called sooner
>>> than it ought to be.
>>>
>>> My ulterior motive here is that I would like to formalize
>>> the existence of different phases of wiki processing - at the
>>> moment there are at least two phases, namely "it's too soon to
>>> match pagespecs reliably" and "everything has been scanned,
>>> you may use pagespecs now", but those phases don't have names,
>>> so [[plugins/write]] doesn't describe them.
>>>
>>> I'm also considering adding warnings
>>> if people try to match a pagespec before scanning has finished,
>>> which can't possibly guarantee the right result, as discussed in
>>> [[conditional_preprocess_during_scan]]. My `wip-too-soon` branch
>>> is a start towards that; the docwiki builds successfully, but
>>> the tests that use IkiWiki internals also need updating to
>>> set `$phase = PHASE_RENDER` before they start preprocessing. --s

>>>> reviewing those modifications, i think this is a good way to go. along
>>>> with warning about pagespecs evaluated in scan phase, i think it should be
>>>> an error to invoke scan in the render phase; that would mean that
>>>> `readtemplate` needs to check whether it's invoked as a scan or not to
>>>> decide whether to scan the template page, but would be generally more
>>>> robust for future plugin writing.
>>>>
>>>>> At the moment templatebody really does need to re-scan templates in
>>>>> the render phase, unfortunately. Not scanning in the render phase
>>>>> seems to be precisely how [[wrong rendering of templatebody]]
>>>>> happened. --s
>>>>
>>>> **addendum**: if the new phase state is used to create warnings/errors
>>>> about improper ikiwiki api use of plugins (which is something i'd
>>>> advocate), that should likewise warn if `add_link` actually adds a link in
>>>> the render phase.  such a warning would have helped spotting the
>>>> link-related [[template evaluation oddities]] earlier. --[[chrysn]]

>>>>> [[Merged|done]] --[[smcv]]