ikiwiki/doc/todo/be_more_selective_about_running_hooks.mdwn

[[!template  id=gitbranch branch=chrismgray/exclusive-hooks author="[[chrismgray]]"]]

Sometimes plugins register a function with `hook`, but they only want
the function called with the content that they know how to deal with.
Normally, this means that they call `pagetype` first thing in the
function, determine if they know how to deal with the content, and
only do anything if they do.  

> So, I can't find any plugins shipped with ikiwiki that actually do that.
> Scan hooks are only ever passed the content of actual wiki pages, and
> so unless a scan hook cares whether a page is written in markdown or
> something else, it has no reason to care what the pagetype is. (Same for
> linkify.) --[[Joey]]

>> My [[org-mode|plugins/contrib/org_mode]] external plugin (which will never
>> really make sense to include with ikiwiki I think) does this.  I
>> think that most plugins defining alternate wiki syntaxes would as
>> well. --[[chrismgray]]

This is a bit wasteful in itself, but for external plugins, it's
really bad.  For functions like `scan` and `linkify`, where the entire
page is sent back and forth over `stdout` and `stdin`, it really slows
things down.  

Thus, I propose that there be a new optional parameter to `hook` that
tells it that the function should only be called for files whose type
is the same as the id of the plugin calling `hook`.  I have called
this parameter `exclusive` in my branch, but this might not be the
best name.

[[!tag patch]]

> It's an interesting idea, but it might be more useful if it was more
> generalized, say, by making it a filter, where the parameter is a regexp.
> 
> --[[KathrynAndersen]]

>> Would it make more sense as a pagespec?  That might be a bit more hard
>> to implement, but would certainly fix the naming issue.
>>
>> --[[chrismgray]]

>>> Considering where it would be called, a pagespec might be overkill. --[[KathrynAndersen]]

>>>> Pagespecs have some overhead themselves. Probably less than shipping
>>>> the page content over stdio.
>>>>
>>>> Rather than putting filtering in the core of ikiwiki, I can think
>>>> of two options. One is to make the main plugin a perl plugin, and
>>>> have it call functions that are provided by another, external plugin.
>>>> This is assuming you're using the other language because something
>>>> is easy to do in it, not to avoid writing perl.
>>>> 
>>>> Or, the external plugin interface could provide a version of `hook()`
>>>> that does not pass the content parameter, but saves a copy that
>>>> the plugin could request with a later rpc call. Assuming that
>>>> it's really the overhead of serializing the page content, that's
>>>> the problem, and not just the general overhead of making rpc calls
>>>> for every page.. --[[Joey]]

>>>>> Since the argument to `hook` is optional, the pagespec is only
>>>>> interpreted if it is given.  So there is no extra overhead
>>>>> (beyond an unused `if` branch) in 99% of the cases.  
>>>>>
>>>>> Rewriting the external plugin's shim using Perl is a good idea,
>>>>> and one that I wish I had thought of earlier.  On the other
>>>>> hand, it doesn't set a great precedent about the usability of
>>>>> external plugins. --[[chrismgray]]