From: Joey Hess Date: Fri, 2 Oct 2009 19:56:44 +0000 (-0400) Subject: split out dependency type issue into its own todo X-Git-Url: https://sipb.mit.edu/gitweb.cgi/ikiwiki.git/commitdiff_plain/8bb94bb197714fcac1ac48f9b330bef4d17dd800?ds=sidebyside split out dependency type issue into its own todo --- diff --git a/doc/todo/dependency_types.mdwn b/doc/todo/dependency_types.mdwn new file mode 100644 index 000000000..db7d06914 --- /dev/null +++ b/doc/todo/dependency_types.mdwn @@ -0,0 +1,83 @@ +Moved this relevant discussion to here from +[[tracking_bugs_with_dependencies]]: --[[Joey]] + +>> it seems that there are two types of dependency, and ikiwiki +>> currently only handles one of them. The first type is "Rebuild this +>> page when any of these other pages changes" - ikiwiki handles this. +>> The second type is "rebuild this page when set of pages referred to by +>> this pagespec changes" - ikiwiki doesn't seem to handle this. I +>> suspect that named pagespecs would make that second type of dependency +>> more important. I'll try to come up with a good example. -- [[Will]] + +>>> Hrm, I was going to build an example of this with backlinks, but it +>>> looks like that is handled as a special case at the moment (line 458 of +>>> render.pm). I'll see if I can breapk +>>> things another way. Fixing this properly would allow removal of that special case. -- [[Will]] + +>>>> I can't quite understand the distinction you're trying to draw +>>>> between the two types of dependencies. Backlinks are a very special +>>>> case though and I'll be suprised if they fit well into pagespecs. +>>>> --[[Joey]] + +>>>>> The issue is that the existential pagespec matching allows you to build things that have similar +>>>>> problems to backlinks. +>>>>> e.g. the following inline: + + \[[!inline pages="define(~done, link(done)) and link(~done)" archive=yes]] + +>>>>> includes any page that links to a page that links to done. Now imagine I add a new link to 'done' on +>>>>> some random page somewhere - a page which some other page links to which didn't previously get included - the set of pages accepted by the pagespec, and hence the set of +>>>>> pages inlined, will change. But, there is no dependency anywhere on the page that I altered, so +>>>>> ikiwiki will not rebuild the page with the inline in it. What is happening is that the page that I altered affects +>>>>> the set of pages matched by the pagespec without itself being matched by the pagespec, and hence included in the dependency list. + +>>>>> To make this work well, I think you need to recognise two types of dependencies for each page (and no +>>>>> special cases for particular types of links, eg backlinks). The first type of dependency says, "The content of +>>>>> this page depends upon the content of these other pages". The `add_depends()` in the shortcuts +>>>>> plugin is of this form: any time the shortcuts page is edited, any page with a shortcut on it +>>>>> is rebuilt. The inline plugin also needs to add dependencies of this form to detect when the inlined +>>>>> content changes. By contrast, the map plugin does not need a dependency of this form, because it +>>>>> doesn't actually care about the content of any pages, just which pages it needs to include (which we'll handle next). + +>>>>> The second type of dependency says, "The content of this page depends upon the exact set of pages matched +>>>>> by this pagespec". The first type of dependency was about the content of some pages, the second type is about +>>>>> which pages get matched by a pagespec. This is the type of dependency tracking that the map plugin needs. +>>>>> If the set of pages matched by map pagespec changes, then the page with the map on it needs to be rebuilt to show a different list of pages. +>>>>> Inline needs this type of dependency as well as the previous type - This type handles a change in which pages +>>>>> are inlined, the previous type handles a change in the content of any of those pages. Shortcut does not need this type of +>>>>> dependency. Most of the places that use `add_depends()` seem to need this type of dependency rather than the first type. + +>>>>>> Note that inline and map currently achieve the second type of dependency by +>>>>>> explicitly calling `add_depends` for each page the displayed. +>>>>>> If any of those pages are removed, the regular pagespec would not +>>>>>> match them -- since they're gone. However, the explicit dependency +>>>>>> on them does cause them to match. It's an ugly corner I'd like to +>>>>>> get rid of. --[[Joey]] + +>>>>> Implementation Details: The first type of dependency can be handled very similarly to the current +>>>>> dependency system. You just need to keep a list of pages that the content depends upon. You could +>>>>> keep that list as a pagespec, but if you do this you might want to check that the pagespec doesn't change, +>>>>> possibly by adding a dependency of the second type along with the dependency of the first type. + +>>>>>> An example of the current system not tracking enough data is +>>>>>> described in [[bugs/transitive_dependencies]]. +>>>>>> --[[Joey]] + +>>>>> The second type of dependency is a little more tricky. For each page, we'd need a list of pagespecs that +>>>>> the page depended on, and for each pagespec you'd want to store the list of pages that currently match it. +>>>>> On refresh, you'd need to check each pagespec to see if the set of pages that match it has changed, and if +>>>>> that set has changed, then rebuild the dependent page(s). Oh, and for this second type of dependency, I +>>>>> don't think you can merge pagespecs. If I wanted to know if either "\*" or "link(done)" changes, then just checking +>>>>> to see if the set of pages matched by "\* or link(done)" changes doesn't work. + +>>>>> The current system works because even though you usually want dependencies of the second type, the set of pages +>>>>> referred to by a pagespec can only change if one of those pages itself changes. i.e. A dependency check of the +>>>>> first type will catch a dependency change of the second type with current pagespecs. +>>>>> This doesn't work with backlinks, and it doesn't work with existential matching. Backlinks are currently special-cased. I don't know +>>>>> how to special-case existential matching - I suspect you're better off just getting the dependency tracking right. + +>>>>> I also tried to come up with other possible solutions: e.g. can we find the dependencies for a pagespec? That +>>>>> would be the set of pages where a change on one of those pages could lead to a change in the set of pages matched by the pagespec. +>>>>> For old-style pagespecs without backlinks, the dependency set for a pagespec is the same as the set of pages the pagespec matches. +>>>>> Unfortunately, with existential matching, the set of pages that each +>>>>> pagespec depends upon can quickly become "*", which is not very useful. -- [[Will]]