split off todo item for multiple dependency types

doc/bugs/transitive_dependencies.mdwn

@@ -59,6 +59,6 @@ Downsides here:
   pass, plugins/map is then updated, because it depends on plugins/brokenlinks.
   (Of course, this is just a special case of the issue that a real
   modification to plugins/brokenlinks causes an unnecessary update of plugins/map,
-  because we have only one kind of dependency.)
+  because we have [[only_one_kind_of_dependency|todo/dependency_types]].)
 
 --[[Joey]] 

doc/todo/dependency_types.mdwn

@@ -1,5 +1,24 @@
-Moved this relevant discussion to here from
-[[tracking_bugs_with_dependencies]]: --[[Joey]]
+Ikiwiki currently only has one type of dependency between pages
+(plus wikilinks special cased in on the side). This has resulted in various
+problems, and it's seemed for a long time to me that ikiwiki needs to get
+smarter about what types of dependencies are supported.
+
+### unnecessary work
+
+The current single dependency type causes the depending page to be rebuilt
+whenever a matching dependency is added, removed, or *modified*. But a
+great many things don't care about the modification case, and often cause
+unnecessary page rebuilds:
+
+* meta only cares if the pages are added or removed. Content change does
+  not matter (unless show=title is used).
+* brokenlinks, orphans, pagecount, ditto
+* inline in archive mode cares about page title, author changing, but
+  not content. (Ditto for meta with show=title.)
+* Causes extra work when solving the [[bug/transitive_dependencies]]
+  problem.
+
+### two types of dependencies needed for [[tracking_bugs_with_dependencies]]
 
 >>  it seems that there are two types of dependency, and ikiwiki
 >>  currently only handles one of them.  The first type is "Rebuild this
@@ -81,3 +100,26 @@ Moved this relevant discussion to here from
 >>>>> 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]]
+
+### proposal
+
+I propose the following. --[[Joey]] 
+
+* Add a second type of dependency, call it an "contentless dependency".
+* `add_depends` defaults to adding a regular ("full") dependency, as
+  before. (So nothing breaks.)
+* `add_depends($page, $spec, content => 0)` adds an contentless dependency.
+* Contentless dependencies are stored in `%depends_contentless` and
+  `%depends_contentless_simple`, which are stored in the index similarly
+  to the existing hashes.
+* `refresh` only looks at added/removed pages when resolving contentless
+  dependencies.
+
+This seems straightforwardly doable. I'd like [[Will]]'s feedback on it, if
+possible. The type types of dependencies I am proposing are not identical
+to the two types he talks about above, but I hope are close enough that
+they can be used.
+
+This doesn't deal with the stuff that only depend on the metadata of a
+page, as collected in the scan pass, changing. But it does leave a window
+open for adding such a dependency type later.

doc/todo/tracking_bugs_with_dependencies.mdwn

@@ -360,7 +360,10 @@ account all comments above (which doesn't mean it is above reproach :) ).  --[[W
 > --[[Joey]] 
 
 >>  There is one issue that I've been thinking about that I haven't raised anywhere (or checked myself), and that is how this all interacts with page dependencies.
->>  Firstly, I'm not sure anymore that the `pagespec_merge` function will continue to work in all cases.
+>> 
+>>> I've moved the discussion of that to [[dependency_types]]. --[[Joey]] 
+>>
+>>  I'm not sure anymore that the `pagespec_merge` function will continue to work in all cases.
 
 >>> The problem I can see there is that if two pagespecs
 >>> get merged and both use `~foo` but define it differently,
@@ -413,86 +416,7 @@ account all comments above (which doesn't mean it is above reproach :) ).  --[[W
 >>> My [[remove-pagespec-merge|should_optimise_pagespecs]] branch has now
 >>> solved all this by deleting the offending function :-) --[[smcv]]
 
->>  Secondly, 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]]
 
 Patch updated to use closures rather than inline generated code for named pagespecs.  Also includes some new use of ErrorReason where appropriate. -- [[Will]]