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+[[!tag type/widget]]
+
+This plugin provides the msc [[ikiwiki/directive]].
+This directive allows embedding [mscgen](http://www.mcternan.me.uk/mscgen/)
+message sequence chart graphs in a page.
+
+Here's an mscgen source example.
+
+    \[[!msc src="""
+      arcgradient = 8;
+
+      a [label="Client"],b [label="Server"];
+
+      a=>b [label="data1"];
+      a-xb [label="data2"];
+      a=>b [label="data3"];
+      a<=b [label="ack1, nack2"];
+      a=>b [label="data2", arcskip="1"];
+      |||;
+      a<=b [label="ack3"];
+      |||;
+    """]]
+
+And here's the resulting graph.
+
+[[!msc src=<<"""
+  arcgradient = 8;
+
+  a [label="Client"],b [label="Server"];
+
+  a=>b [label="data1"];
+  a-xb [label="data2"];
+  a=>b [label="data3"];
+  a<=b [label="ack1, nack2"];
+  a=>b [label="data2", arcskip="1"];
+  |||;
+  a<=b [label="ack3"];
+  |||;
+"""]]
+
+Security implications: to be determined.
+
+This plugin uses the [[!cpan Digest::SHA]] perl module.
+
+This plugin borrows heavily from the [[graphviz|plugins/graphviz]] plugin written by [[JoshTriplett]].
+
+I couldn't upload an attachment, so here's the plugin source.
+
+[[!format perl """
+#!/usr/bin/perl
+# mscgen plugin for ikiwiki: render mscgen source as an image.
+# Terry Golubiewski
+# Derived from graphviz plugin by Josh Triplett.
+package IkiWiki::Plugin::mscgen;
+
+use warnings;
+use strict;
+use IkiWiki 3.00;
+use IPC::Open2;
+
+sub import {
+	hook(type => "getsetup", id => "mscgen", call => \&getsetup);
+	hook(type => "preprocess", id => "msc", call => \&graph);
+}
+
+sub getsetup () {
+	return
+		plugin => {
+			safe => 1,
+			rebuild => undef,
+			section => "widget",
+		},
+}
+
+sub render_graph (\%) {
+	my %params = %{(shift)};
+
+	my $src = "msc {\n";
+	$src .= $params{src};
+	$src .= "\n}\n";
+
+	# Use the sha1 of the mscgen code as part of its filename.
+	eval q{use Digest::SHA};
+	error($@) if $@;
+	my $dest=$params{page}."/msc-".
+		IkiWiki::possibly_foolish_untaint(Digest::SHA::sha1_hex($src)).
+		".png";
+	will_render($params{page}, $dest);
+
+	if (! -e "$config{destdir}/$dest") {
+		my $pid;
+		my $sigpipe=0;
+		$SIG{PIPE}=sub { $sigpipe=1 };
+		$pid=open2(*IN, *OUT, 'mscgen', '-Tpng', '-i-', '-o-');
+
+		# open2 doesn't respect "use open ':utf8'"
+		binmode (OUT, ':utf8');
+
+		print OUT $src;
+		close OUT;
+
+		my $png;
+		{
+			local $/ = undef;
+			$png = <IN>;
+		}
+		close IN;
+
+		waitpid $pid, 0;
+		$SIG{PIPE}="DEFAULT";
+		error gettext("failed to run mscgen") if $sigpipe;
+
+		if (! $params{preview}) {
+			writefile($dest, $config{destdir}, $png, 1);
+		}
+		else {
+			# in preview mode, embed the image in a data uri
+			# to avoid temp file clutter
+			eval q{use MIME::Base64};
+			error($@) if $@;
+			return "<img src=\"data:image/png;base64,".
+				encode_base64($png)."\" />";
+		}
+	}
+
+	return "<img src=\"".urlto($dest, $params{destpage})."\" />\n";
+}
+
+sub graph (@) {
+	my %params=@_;
+	$params{src} = "" unless defined $params{src};
+	return render_graph(%params);
+}
+
+1
+"""]]