For some time now, in circumstances that I've had enormous troubles
trying to track, I've seen feeds getting removed by ikiwiki when
apparently unrelated pages got changed, with the message:

> removing somepath/somepage/somefeed, no longer built by some/unrelated/page

I've finally been able to find how and why it happens. The situation is
the following:

* page A has an inline directive that (directly) generates a feed F
* page B inlines A, thus (indirectly) generating F again
* page B is rendered after page A

The feed removal happens when changes are made to prevent B from
inlining A; for example, because B is a tag page and A is untagged B, or
because B includes A through a pagespec that no longer matches A. In
this case, this happens:

* page A is built, rendering F
* page B is built, _not_ rendering F, which it used to render
* F is removed because it is not built by B anymore

Note that although this issue is triggered (for me) from the changes I
proposed last year to allow feed generation from nested inlines
coalescing it to be page-based instead of destpage-based
(bb8f76a4a04686def8cc6f21bcca80cb2cc3b2c9 and
72c8f01b36c841b0e83a2ad7ad1365b9116075c5) there is potential for it
popping up in other cases.

Specifically, the logic for the removal of dependent pages currently
relies on the assumption that each output has a single generator. My
changes caused this assumption to be violated, hence the error, but
other cases may pop up for other plugins in the future.

I have a [patch] fixing this issue (for feeds specifically, i.e. only
the problem I am actually having) on top of my `mystuff` branch, but
since that also has heaps of other unrelated stuff, you may want to just
[pick it from my gitweb][gw].

[gw]: (http://git.oblomov.eu/ikiwiki/patch/671cb26cf50643827f258270d9ac8ad0b1388a65)

The patch changes the `will_render()` for feeds to be based on the page
rather than on the destpage, matching the fact that for nested inlines
it's the inner page that is ultimately responsible for generating the
feed.

I've noticed that it requires at least _two_ full rebuilds before the
index is again in a sensible state. (On the first rebuild, all feeds
from nested inlines are actually _removed_.)

While the patch is needed because there are legitimate cases in which
nested feeds are needed (for example, I have an index page that inlines
index pages for subsection of my site, and I want _those_ feed from
being visible), there are other cases when one may want to skip feed
generation from nested inlines.

--[[GiuseppeBilotta]]
