This is a plugin that prompts the user for a title for a post, and then 
redirects the user to post in a hierarchically organized directory
structure. It supports limiting blog posts only to certain users, and 
this applies to creating new as well as editing old ones. It's a little
clumsy so far:

* Although Joey said specifically not to, I export the printheader
  function so that the plugin can create a form.
* The form doesn't invoke the formbuilder hooks; I couldn't figure
  out a way to do that easily from inside the plugin (unless I
  exported run_hooks and that didn't seem right).
* This invokes a new hook, authcgi, although Joey said that
  "the best approach was adding a flag that makes a cgi hook get 
  postponed and called after a session is set up". I don't know
  if this is a good idea; authenticated CGI plugins need sessions,
  and unauthenticated CGI plugins don't, so it seems like a new hook
  is a better idea.
* This creates a lot of broken parents unless you use something 
  like [[missingparents.pm]].

I don't expect that this will be applied; it's more like a first draft.
I would like to hear good ideas on how to solve the first two problems.
Maybe make the hook return a form, which is printed by CGI.pm? --Ethan

<pre>
Index: IkiWiki/CGI.pm
===================================================================
--- IkiWiki/CGI.pm	(revision 3968)
+++ IkiWiki/CGI.pm	(working copy)
@@ -684,6 +684,9 @@
 		}
 	}
 	
+	run_hooks(authcgi => sub { shift->($q, $session); });
+	my $do=$q->param('do'); # in case the hook changed it
+
 	if (defined $session->param("name") &&
 	    userinfo_get($session->param("name"), "banned")) {
 		print $q->header(-status => "403 Forbidden");
Index: IkiWiki/Plugin/blogpost.pm
===================================================================
--- IkiWiki/Plugin/blogpost.pm	(revision 0)
+++ IkiWiki/Plugin/blogpost.pm	(revision 0)
@@ -0,0 +1,97 @@
+#!/usr/bin/perl
+# blogpost plugin: interprets cgi "blogpost" commands as create commands.
+package IkiWiki::Plugin::blogpost;
+
+use warnings;
+use strict;
+use POSIX;
+use IkiWiki 2.00;
+
+sub import { #{{{
+	hook(type => "checkconfig", id => "blogpost", call => \&checkconfig);
+	hook(type => "authcgi", id => "blogpost", call => \&authcgi);
+	hook(type => "canedit", id => "blogpost", call => \&canedit);
+} # }}}
+
+sub checkconfig () { #{{{
+	if (! defined $config{blogformat}){
+		$config{blogformat} = 'posts/%Y/%m/%d/$title';
+	}
+
+	if (! defined $config{blogpagespec}){
+		my $spec = $config{blogformat};
+		$spec =~ s/%./\*/g;
+		$spec =~ s/\$title/*/;
+		$config{blogpagespec} = "$spec and ! $spec/*";
+	}
+	
+	if (! defined $config{blogusers}) {
+		$config{blogusers} = (); # disallow all posting by default
+	}
+} #}}}
+
+sub authcgi ($$) { #{{{
+	my $cgi=shift;
+	my $session=shift;
+
+	return unless (defined $cgi->param('do') && $cgi->param('do') eq "blogpost");
+	my $user=$session->param("name");
+	error ("not allowed to blog, $user") unless
+	    $config{blogusers} eq "*" ||
+		grep {$_ eq $user} $config{blogusers};
+	eval q{use CGI::FormBuilder};
+	error($@) if $@;
+	my @fields=qw(title);
+	my $form = CGI::FormBuilder->new(
+		fields => \@fields,
+		title => "post title",
+		name => "post title",
+		header => 1,
+		charset => "utf-8",
+		method => 'POST',
+		required => 'NONE',
+		javascript => 0,
+		params => $cgi,
+		action => $config{cgiurl},
+		header => 0,
+		template => {type => 'div'},
+		stylesheet => $config{url}."/style.css",
+	);
+	my $buttons=["Blog!"];
+	$form->field(name => "do", type => "hidden", value => "blogpost",
+		force => 1);
+	if (! $form->submitted){
+		printheader($session);
+		print misctemplate($form->title, $form->render(submit => $buttons));
+		exit;
+	}
+	else {
+		my $page = blogpage($form->field("title"));
+		$cgi->param("do", "create");
+		$cgi->param("page", $page);
+	}
+
+} #}}}
+
+sub blogpage ($) { #{{{
+	my $title=shift;
+	my $page=POSIX::strftime $config{blogformat}, localtime;
+	$page =~ s/\$title/$title/;
+	return $page;
+} #}}}
+
+sub canedit ($$$) { #{{{
+	my $page=shift;
+	my $cgi=shift;
+	my $session=shift;
+
+	return undef unless pagespec_match($page, $config{blogpagespec});
+	my $user=$session->param("name");
+	IkiWiki::needsignin($cgi, $session) unless defined $user;
+
+	return "" if ($config{blogusers} eq "*" ||
+		      grep {$_ eq $user} $config{blogusers});
+	return ("not allowed to blog, $user");
+} #}}}
+
+1
Index: IkiWiki.pm
===================================================================
--- IkiWiki.pm	(revision 3968)
+++ IkiWiki.pm	(working copy)
@@ -17,6 +17,7 @@
 our @EXPORT = qw(hook debug error template htmlpage add_depends pagespec_match
                  bestlink htmllink readfile writefile pagetype srcfile pagename
                  displaytime will_render gettext urlto targetpage
+                 misctemplate printheader
                  %config %links %renderedfiles %pagesources %destsources);
 our $VERSION = 2.00; # plugin interface version, next is ikiwiki version
 our $version='unknown'; # VERSION_AUTOREPLACE done by Makefile, DNE
</pre>