ikiwiki/doc/bugs/transitive_dependencies.mdwn

If a sidebar contains a map, or inline (etc), one would expect a
add/remove of any of the mapped/inlined pages to cause a full wiki
rebuild. But this does not happen.

If page A inlines page B, which inlines page C, a change to C will cause B
to be updated, but A will not "notice" that this means A needs to be
updated.

One way to look at this bug is that it's a bug in where dependencies are
recorded when preprocessing the rendered or sidebar page. The current code
does:

	add_depends($params{page}, $somepage);

Where `$params{page}` is page B. If this is changed to `$params{destpage}`,
then the dependency is added to page A, and updates to C cause it to
change. This does result in the page A's getting lots more dependency info
recorded than before (essentially a copy of all the B's dependency info).

It's also a fragile, since all plugins that handle dependencies have to be
changed, and do this going forward. And it seems non-obvious that this should
be done. Or really, whether to use `page` or `destpage` there. Currently,
making the "wrong" choice and using `destpage` instead of `page` (which nearly
everything uses) will just result in semi-redundant dependency info being
recorded. If we make destpage mandatory to fix this, goofing up will lead to
this bug coming back. Ugh.

----

## rebuild = change approach

[[!template id=gitbranch branch=origin/transitive-dependencies author="[[joey]]"]]

Another approach to fix it is to say that anything that causes a
rebuild of B is treated as a change of B. Then when C is changed, B is
rebuilt due to dependencies, and in turn this means A is rebuilt because B
"changed". 

This is essentially what is done with wikilinks now, and why, if a sidebar
links to page C, add/remove of C causes all pages to be rebuilt, as seen 
here:

	removing old page meep
	building sidebar.mdwn, which links to meep
	building TourBusStop.mdwn, which depends on sidebar
	building contact.mdwn, which depends on sidebar
	...

Downsides here:

* Means a minimum of 2x as much time spent resolving dependencies,
  at least in my simple implementation, which re-runs the dependency
  resolution loop until no new pages are rebuilt.
  (I added an optimisation that gets it down to 1.5X as much work on
  average, still 2x as much worst case. I suppose building a directed
  graph and traversing it would be theoretically more efficient.)
* Causes extra work for some transitive dependencies that we don't
  actually care about. This is amelorated, but not solved by 
  the current work on [[todo/dependency_types]].
  For example, changing index causes
  plugins/brokenlinks to update in the first pass; if there's a second
  pass, plugins/map is no longer updated (contentless dependencies FTW),
  but plugins is, 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, and could be solved by adding more dependency types.)

[[done]] --[[Joey]] 

> Some questions/comments...  I've thought about this a lot for [[todo/tracking_bugs_with_dependencies]].
> 
>  * When you say that anything that causes a rebuild of B is treated as a change of B, are you: i) Treating
> any rebuild as a change, or ii) Treating any rebuild that gives a new result as a change?  Option ii) would
> lead to fewer rebuilds.  Implementation is easy: when you're about to rebuild a page, load the old rendered html in.  Do the rebuild.  Compare
> the new and old html.  If there is a difference, then mark that page as having changed.  If there is no difference
> then you don't need to mark that pages as changed, even though it has been rebuilt.  (This would ignore pages in meta-data that don't
> cause changes in html, but I don't think that is a huge issue.)

>> That is a good idea. I will have to look at it to see if the overhead of
>> reading back in the html of every page before building actually is a
>> win though. So far, I've focused on avoiding unnecessary rebuilds, and
>> there is still some room for more dependency types doing so.
>> (Particularly for metadata dependencies..) --[[Joey]] 

>  * The second comment I have relates to cycles in transitive dependencies.  At the moment I don't think this is
> possible, but with some additions it may well become so.  This could be problematic as it could lead to a)
> updates that never complete, or b) it being theoretically unclear what the final result should be (i.e. you
> can construct logical paradoxes in the system).  I think the point above about marking things as changed only when
> the output actually changes fixes any cases that are well defined.  For logical paradoxes and infinite loops (e.g.
> two pages that include each other), you might want to put a limit on the number of times you'll rebuild a page in any
> given run of ikiwiki.  Say, only allow a page to rebuild twice on any run, regardless of whether a page it depends on changes.
> This is not a perfect solution, but would be a good approximation. -- [[Will]]

>> Ikiwiki only builds any given output file once per run, already. --[[Joey]]