+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+ pass, plugins/map is no longer updated (contentless dependencies FTW),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]]