Add a build_affected hook so trail doesn't have to inject

In principle, building any pages affected by links, backlinks etc.
could work the same way.

IkiWiki/Plugin/trail.pm

@@ -17,6 +17,7 @@ sub import {
 	hook(type => "preprocess", id => "trailitems", call => \&preprocess_trailitems, scan => 1);
 	hook(type => "preprocess", id => "traillink", call => \&preprocess_traillink, scan => 1);
 	hook(type => "pagetemplate", id => "trail", call => \&pagetemplate);
+	hook(type => "build_affected", id => "trail", call => \&build_affected);
 }
 
 =head1 Page state
@@ -275,14 +276,9 @@ sub trails_differ {
 
 my $done_prerender = 0;
 
-my %origsubs;
-
 sub prerender {
 	return if $done_prerender;
 
-	$origsubs{render_backlinks} = \&IkiWiki::render_backlinks;
-	inject(name => "IkiWiki::render_backlinks", call => \&render_backlinks);
-
 	%trail_to_members = ();
 	%member_to_trails = ();
 
@@ -368,18 +364,14 @@ sub prerender {
 	$done_prerender = 1;
 }
 
-# This is called at about the right time that we can hijack it to render
-# extra pages.
-sub render_backlinks ($) {
-	my $blc = shift;
+sub build_affected {
+	my %affected;
 
 	foreach my $member (keys %rebuild_trail_members) {
-		next unless exists $pagesources{$member};
-
-		IkiWiki::render($pagesources{$member}, sprintf(gettext("building %s, its previous or next page has changed"), $member));
+		$affected{$member} = sprintf(gettext("building %s, its previous or next page has changed"), $member);
 	}
 
-	$origsubs{render_backlinks}($blc);
+	return %affected;
 }
 
 sub title_of ($) {

IkiWiki/Render.pm

@@ -800,6 +800,14 @@ sub refresh () {
 		derender_internal($file);
 	}
 
+	run_hooks(build_affected => sub {
+		my %affected = shift->();
+		while (my ($page, $message) = each %affected) {
+			next unless exists $pagesources{$page};
+			render($pagesources{$page}, $message);
+		}
+	});
+
 	my ($backlinkchanged, $linkchangers)=calculate_changed_links($changed,
 		$del, $oldlink_targets);
 

doc/plugins/write.mdwn

@@ -356,6 +356,22 @@ when the page is being previewed.)
 The function is passed named parameters: "page" and "content", and 
 should return the formatted content.
 
+### build_affected
+
+	hook(type => "build_affected", id => "foo", call => \&build_affected);
+
+This hook is called after the directly changed pages have been built,
+and can cause extra pages to be built. If links and backlinks were provided
+by a plugin, this would be where that plugin would rebuild pages whose
+backlinks have changed, for instance. The [[trail]] plugin uses this hook
+to rebuild pages whose next or previous page has changed.
+
+The function should currently ignore its parameters. It returns a list with
+an even number of items (a hash in list context), where the first item of
+each pair is a page name to be rebuilt (if it was not already rebuilt), and
+the second is a log message resembling
+`building plugins/write because the phase of the moon has changed`.
+
 ### delete
 
 	hook(type => "delete", id => "foo", call => \&delete);