Error handling improvement for preprocess hooks. It's now safe to call error() from such hooks; it will cause a nicely formatted error message to be inserted into the page.

IkiWiki.pm

@@ -768,21 +768,30 @@ sub preprocess ($$$;$$) { #{{{
 			}
 			my $ret;
 			if (! $scan) {
-				$ret=$hooks{preprocess}{$command}{call}->(
-					@params,
-					page => $page,
-					destpage => $destpage,
-					preview => $preprocess_preview,
-				);
+				$ret=eval {
+					$hooks{preprocess}{$command}{call}->(
+						@params,
+						page => $page,
+						destpage => $destpage,
+						preview => $preprocess_preview,
+					);
+				};
+				if ($@) {
+					chomp $@;
+				 	$ret="[[!$command <span class=\"error\">".
+						gettext("Error").": $@"."</span>]]";
+				}
 			}
 			else {
 				# use void context during scan pass
-				$hooks{preprocess}{$command}{call}->(
-					@params,
-					page => $page,
-					destpage => $destpage,
-					preview => $preprocess_preview,
-				);
+				eval {
+					$hooks{preprocess}{$command}{call}->(
+						@params,
+						page => $page,
+						destpage => $destpage,
+						preview => $preprocess_preview,
+					);
+				};
 				$ret="";
 			}
 			$preprocessing{$page}--;

debian/changelog

@@ -20,6 +20,9 @@ ikiwiki (2.54) UNRELEASED; urgency=low
   * Move yesno function out of inline and into IkiWiki core, not exported.
   * meta: fix title() PageSpec (smcv)
   * Some footer style changes. (smcv)
+  * Error handling improvement for preprocess hooks. It's now safe to call
+    error() from such hooks; it will cause a nicely formatted error message
+    to be inserted into the page.
 
  -- Josh Triplett <josh@freedesktop.org>  Wed, 09 Jul 2008 21:30:33 -0700
 

doc/plugins/write.mdwn

@@ -412,12 +412,13 @@ Aborts with an error message. If the second parameter is passed, it is a
 function that is called after the error message is printed, to do any final
 cleanup.
 
-Note that while any plugin can use this for a fatal error, plugins should
-try to avoid dying on bad input when building a page, as that will halt
-the entire wiki build and make the wiki unusable. So for example, if a
-[[ikiwiki/PreProcessorDirective]] is passed bad parameters, it's better to
-return an error message, which can appear on the wiki page, rather than
-calling error().
+If called inside a preprocess hook, error() does not abort the entire
+wiki build, but instead replaces the [[ikiwiki/PreProcessorDirective]] with
+a version containing the error message.
+
+In other hooks, error() is a fatal error, so use with care. Try to avoid
+dying on bad input when building a page, as that will halt
+the entire wiki build and make the wiki unusable.
 
 #### `template($;@)`