ikiwiki/doc/todo/pagespec_aliases.mdwn

[[!tag reviewed]]
[[!template id=gitbranch branch=jon/pagespec_alias author="[[Jon]]"]]
[[!tag patch wishlist]]I quite often find myself repeating a boiler-plate
[[ikiwiki/pagespec]] chunk, e.g.

    and !*.png and !*.jpg...

it would be quite nice if I could conveniently bundle them together into a
pagespec "alias", and instead write

    and !image()...

I wrote the following plugin to achieve this:

    <snip old patch; see git branch outlined above>

I need to reflect on this a bit more before I send a pull request.  In
particular I imagine the strict/warnings stuff will make you puke.  Also, I'm
not sure whether I should name-grab 'alias' since [[todo/alias_directive]] is
an existing wishlist item.

> I think it would make sense to have "pagespec" in the name somehow.

>> Good idea, how about `pagespecalias`? — [[Jon]]

> No, the strict/warnings does not make me puke. Have you read my perl
> code? :-P
> 
> Note that your XXX is right. It would be a security hole to not validate
> `$key`, as anyone with websetup access could cause it to run arbitrary
> perl code.
> 
> Well, except that websetup doesn't currently support configuring hashes
> like used here. Which is a pity, but has led me to try to avoid using
> such hashes in the setup file.

> > If I removed the `getsetup` subroutine, it would not be exposed via
> > website, is that right?  I suppose it doesn't hurt to validate key, even if
> > this risk was not there.  Is the use of a hash here a blocker for adoption?
> > — [[Jon]]

> Have you considered not defining the pagespec aliases in the setup file, but
> instead as directives on pages in the wiki? Using pagestate could store
> up the aliases that have been defined. It could however, be hard to get
> the dependencies right; any page that uses a pagespec containing 
> an alias `foo` would need to somehow depend on the page where the alias
> was defined. --[[Joey]] 

> > I haven't thought the dependency issue through beyond "that might be hard".
> > Personally, I don't like defining stuff like this in pages, but I appreciate
> > some do.  There could be some complex scenarios where some pages rely on a
> > pagespec alias defined on others; and could have their meanings changed by
> > changing the definition.  A user might have permission to edit a page with a
> > definition on it but not on the pages that use it, and similar subtle permission
> > bugs.  I'm also not sure what the failure mode is if someone redefines an alias,
> > and whether there'd be an unpredictable precedence problem.
> > How about both methods? — [[Jon]]

Here's an example setup chunk:

     pagespec_aliases:
       image: "*.png or *.jpg or *.jpeg or *.gif or *.ico"
       helper: "*.css or *.js"
       boring: "image() or helper()"

The above demonstrates self-referential dynamic pagespec aliases.  It doesn't work,
however, to add ' or internal()' to `boring`, for some reason.

-- [[Jon]]

> Probably needs to be `or internal(*)` --[[Joey]] 

> > Ah yes, could be, thanks. — [[Jon]]

> another useful pagespec alias for large maps:

       basewiki: "sandbox or templates or templates/* or ikiwiki or ikiwiki/* or shortcuts or recentchanges or wikiicons/*"

> -- [[Jon]]

>> Useful indeed! --[[Joey]] 


>>> I've tweaked my patch in light of your above feedback:  The plugin has been
>>> renamed, and I now validate keys.  I've also added documentation and tests
>>> to the branch.  I haven't read rubykat's code properly yet, and don't have
>>> access at the time of writing (I'm on a beach in Greece ☺), but I expect it
>>> would be possible to extend what I've got here to support defining the
>>> aliases in a PageSpec, once the dependency stuff has been reasoned out
>>> properly.
>>>
>>> I'd like to solve the issue of this not being web-configurable by
>>> implementing support for more nested datatypes in [[plugins/websetup]]. —
>>> [[Jon]]

>>>> Well, it's a difficult problem. websetup builds a form using
>>>> CGI::FormBuilder, which makes it easy to build the simple UI we have
>>>> now, but sorta precludes anything more complicated. And anything with
>>>> a nested datatype probably needs a customized UI for users to be able
>>>> to deal with it. I don't think websetupability need be a deal-breaker
>>>> for this patch. I personally like special pages like Kathryn is doing
>>>> more than complex setup files. --[[Joey]]

>>>>> I've ran out of time to keep working on this, so I'm just going to
>>>>> submit it as a 'contrib' plugin and leave things at that for now.
>>>>> — [[Jon]]

---------------------------

Based on the above, I have written an experimental plugin called "subset".
It's in my "ikiplugins" repo on github, in the "experimental" branch.
<https://github.com/rubykat/ikiplugins/blob/experimental/IkiWiki/Plugin/subset.pm>

It takes Joey's suggestion of defining the subsets (aliases) as directives;
I took the example of the [[plugins/shortcut]] plugin and designated a single special page as the one where the directives are defined,
though unlike "shortcut" I haven't hardcoded the name of the page; it defaults to "subsets" but it can be re-defined in the config.

I've also added a feature which one might call subset-caching; I had to override `pagespec_match_list` to do it, however.
An extra parameter added to `pagespec_match_list` called `subset` which

* limits the result to look *only* within the set of pages defined by the subset (uses the "list" option to pagespec_match_list to do this)
* caches the result of the subset search so that the second time subset "foo" is used, it uses the stored result of the first search for "foo".

This speeds things up if one is using a particular subset more than once, which one probably is if one bothered to define the subset in the first place.
The speed increase is most dramatic when the site has a large number of pages and the number of pages in the subset is small.
(this is similar to the "trail" concept I used in my [[plugins/contrib/report]] plugin, but not quite the same)

Note that things like [[plugins/map]] can't make use of "subset" (yet) because they don't pass along all the parameters they're given.
But [[plugins/contrib/report]] actually works without alteration because it does pass along all the parameters.

Unfortunately I haven't figured out how to do the dependencies - I'd really appreciate help on that.

--[[KathrynAndersen]]

> > Cool!  I like the caching idea.  I'm not sure about the name.  I don't like defining
> > stuff in pages, but I appreciate this is a matter of taste, and would be happy with
> > supporting both. — [[Jon]]

>>> I've now gone and completely re-done "subset" so that it is less like an alias, but it a bit clearer and simpler:
>>> instead of having a separate "match_" function for every alias, I simply have one function, "match_subset"
>>> which takes the name of the subset.  Thus a \[[!subset name="foo"...]] would be called `subset(foo)` rather than `foo()`.

>>> There are a few reasons for this:<br/>
>>> (a) it's more secure not to be evaluating code on the fly<br/>
>>> (b) it's simpler<br/>
>>> (c) (and this was my main reason) it makes it possible to do caching without having to have a separate "subset" argument.
>>> I've done a bit of a hack for this: basically, the PageSpec is checked to see if the very start of the PageSpec is `subset(foo) and` or if the whole pagespec is just `subset(foo)` and if either of those is true, then it does the subset caching stuff.
>>> The reason I check for "and" is that if it is "subset(foo) or something" then it would be an error to use the subset cache in that case.
>>> The reason I just check the start of the PageSpec is because I don't want to have to do complex parsing of the PageSpec.

>>> As for defining subsets in the config rather than on pages, I perfectly understand that desire, and I could probably add that in.

>>> As for the name "subset"... well, it's even less like an alias now, and "alias" is already a reserved name.  What other names would you suggest?

>>>--[[KathrynAndersen]]

>>>> Regarding my comments:  I wasn't clear what you are/were intending to
>>>> achieve with your modifications.  I've aimed for a self-contained plugin
>>>> which could be merged with ikiwiki proper.  I think I initially took your
>>>> developments as being an evolution of that with the same goal, which is
>>>> why I commented on the (change of) name.  However, I guess your work is
>>>> more of a fork than a continuation,  in which case you can call it
>>>> whatever you like ☺  I like some of the enhancements you've made, but
>>>> having the aliases/subsets/"things" work in any pagespec (inside map, or
>>>> inline) is a deal-breaker for me. — [[Jon]]

>>>>> I'm a bit confused by your statement "having the aliases/subsets/"things" work in any pagespec (inside map, or inline) is a deal-breaker for me".
>>>>> Do you mean that you want them to work in any pagespec, or that you *don't* want them to work in any pagespec? -- [[KathrynAndersen]]

>>>>>> I mean I would want them to work in any pagespec. — [[Jon]]

----

Hi, it's been 7 years since I last looked at this, and I'm surprised to find
that I'd got it up to a merge-request state; I've dusted it off and done some
clean up and testing, but it's working (albeit not via websetup). I've revamped
the docs and rebased the branch. Can someone please consider merging ([[joey]]
or [[smcv]]?) or otherwise feed back on this? Thanks! — [[Jon]] (2018-09-25)

> To hide it from `websetup`, the `example` needs to be a hash reference
> like `example => { images => "*.png or *.jpg or *.gif" }`, I think?
> (Please try it on a websetup-enabled wiki, possibly by copying
> `t/manual/git_revert` to `t/manual/websetup` and adapting it as required.)
>
> For a less magical variant, you could consider using `alias(images)`
> instead of `images()` for the pagespec syntax that is enabled by the
> example above. I'm not sure which way is better.
>
> If `safe_key` fails, you probably want to log a warning, or even fail
> `checkconfig` with a fatal `error`?
>
> If `checkconfig` detects that the given pagespec function already
> exists, for example `title` after loading the meta plugin, you probably
> want to log a warning or fail? It seems you can detect this with
> `defined ref *$subname{CODE}`.
>
> If you define a loop of mutually recursive aliases (or even an alias
> that refers to itself), I think you'll get infinite recursion.
> You can probably bypass that with a construct like:
>
>     my $entered;
>     *{ $subname } = sub {
>         return IkiWiki::ErrorReason->new("Alias $key is defined recursively") if $entered;
>         $entered = 1;
>         my $result = IkiWiki::pagespec_match($path, $value);
>         $entered = 0;
>         return $result;
>     }
>
> (but don't take my word for it, a regression test would tell you whether
> this works.)
>
> --[[smcv]]

----

Thank you for the review (nearly a year ago, I've just noticed!). I've
added checks for the issues you outline above, and test coverage for all
those issues.
I've also decided to rename the plugin (back) to just "alias":
I mooted that right back when I started this but I was worried about
potential ambiguity. That was ten years ago and I think the concern has
prove unfounded. I've left the config key as `pagespec_aliases` though,
as that's one area I think its clearer.

With regards `aliasname()` versus `alias(aliasname)`:
I've given this some thought. Pros and cons of that approach: it would be
a little uglier; you would not inadvertently clash with a PageSpec defined
elsewhere. However, I wonder if someone might actually *want* to define a
PageSpec this way that was the same as that defined by something else: Perhaps,
you have disabled a plugin that defined a PageSpec name and you want to substitute
what it would have expanded to with something else, for example.

I will (after writing this) rebase my branch. Please take another look!
(just in case the rebase and/or the "-" in the branchnames causes problems
with IkiWiki's auto-mirror-pull, the branch is here:
<https://github.com/jmtd/ikiwiki/tree/pagespec-alias>)

*— [[Jon]], 2020-01-10*