mdwn: Automatically detect which libdiscount flags to use

Unconditionally passing arbitrary numbers as flags turns out to be a
bad idea, because some of the "unused" values have historically had
side-effects internal to libdiscount. Detect whether the known flags
work by rendering short Markdown snippets the first time we htmlize,
checking whether each known flag is both necessary and sufficient.

Signed-off-by: Simon McVittie <smcv@debian.org>

1 files changed, 50 insertions, 45 deletions

diff --git a/IkiWiki/Plugin/mdwn.pm b/IkiWiki/Plugin/mdwn.pm
index 66116ae01..eefa29a97 100644
--- a/IkiWiki/Plugin/mdwn.pm
+++ b/IkiWiki/Plugin/mdwn.pm
@@ -89,6 +89,55 @@ sub htmlize (@) {
 		    (! exists $config{nodiscount} || ! $config{nodiscount})) {
 			eval q{use Text::Markdown::Discount};
 			if (! $@) {
+				my $markdown = \&Text::Markdown::Discount::markdown;
+				my $always_flags = 0;
+
+				# Disable Pandoc-style % Title, % Author, % Date
+				# Use the meta plugin instead
+				$always_flags |= Text::Markdown::Discount::MKD_NOHEADER();
+
+				# Disable Unicodification of quote marks, em dashes...
+				# Use the typography plugin instead
+				$always_flags |= Text::Markdown::Discount::MKD_NOPANTS();
+
+				# Workaround for discount's eliding of <style> blocks.
+				# https://rt.cpan.org/Ticket/Display.html?id=74016
+				if (Text::Markdown::Discount->can('MKD_NOSTYLE')) {
+					$always_flags |= Text::Markdown::Discount::MKD_NOSTYLE();
+				}
+				elsif ($markdown->('<style>x</style>', 0) !~ '<style>' &&
+					$markdown->('<style>x</style>', 0x00400000) =~ m{<style>x</style>}) {
+					$always_flags |= 0x00400000;
+				}
+
+				# Enable fenced code blocks in libmarkdown >= 2.2.0
+				# https://bugs.debian.org/888055
+				if (Text::Markdown::Discount->can('MKD_FENCEDCODE')) {
+					$always_flags |= Text::Markdown::Discount::MKD_FENCEDCODE();
+				}
+				elsif ($markdown->("~~~\nx\n~~~", 0) !~ m{<pre\b} &&
+					$markdown->("~~~\nx\n~~~", 0x02000000) =~ m{<pre\b}) {
+					$always_flags |= 0x02000000;
+				}
+
+				# PHP Markdown Extra-style term\n: definition -> <dl>
+				if (Text::Markdown::Discount->can('MKD_DLEXTRA')) {
+					$always_flags |= Text::Markdown::Discount::MKD_DLEXTRA();
+				}
+				elsif ($markdown->("term\n: def\n", 0) !~ m{<dl>} &&
+					$markdown->("term\n: def\n", 0x01000000) =~ m{<dl>}) {
+					$always_flags |= 0x01000000;
+				}
+
+				# Allow dashes and underscores in tag names
+				if (Text::Markdown::Discount->can('MKD_GITHUBTAGS')) {
+					$always_flags |= Text::Markdown::Discount::MKD_GITHUBTAGS();
+				}
+				elsif ($markdown->('<foo_bar>', 0) !~ m{<foo_bar} &&
+					$markdown->('<foo_bar>', 0x08000000) =~ m{<foo_bar\b}) {
+					$always_flags |= 0x08000000;
+				}
+
 				$markdown_sub=sub {
 					my $t=shift;
 
@@ -96,15 +145,7 @@ sub htmlize (@) {
 					# https://rt.cpan.org/Ticket/Display.html?id=73657
 					return "" if $t=~/^\s*$/;
 
-					my $flags=0;
-
-					# Disable Pandoc-style % Title, % Author, % Date
-					# Use the meta plugin instead
-					$flags |= Text::Markdown::Discount::MKD_NOHEADER();
-
-					# Disable Unicodification of quote marks, em dashes...
-					# Use the typography plugin instead
-					$flags |= Text::Markdown::Discount::MKD_NOPANTS();
+					my $flags=$always_flags;
 
 					if ($config{mdwn_footnotes}) {
 						$flags |= Text::Markdown::Discount::MKD_EXTRA_FOOTNOTE();
@@ -114,42 +155,6 @@ sub htmlize (@) {
 						$flags |= Text::Markdown::Discount::MKD_NOALPHALIST();
 					}
 
-					# Workaround for discount's eliding
-					# of <style> blocks.
-					# https://rt.cpan.org/Ticket/Display.html?id=74016
-					if (Text::Markdown::Discount->can("MKD_NOSTYLE")) {
-						$flags |= Text::Markdown::Discount::MKD_NOSTYLE();
-					}
-					else {
-						# This is correct for the libmarkdown.so.2 ABI
-						$flags |= 0x00400000;
-					}
-
-					# Enable fenced code blocks in libmarkdown >= 2.2.0
-					# https://bugs.debian.org/888055
-					if (Text::Markdown::Discount->can("MKD_FENCEDCODE")) {
-						$flags |= Text::Markdown::Discount::MKD_FENCEDCODE();
-					}
-					else {
-						$flags |= 0x02000000;
-					}
-
-					# PHP Markdown Extra-style term\n: definition -> <dl>
-					if (Text::Markdown::Discount->can("MKD_DLEXTRA")) {
-						$flags |= Text::Markdown::Discount::MKD_DLEXTRA();
-					}
-					else {
-						$flags |= 0x01000000;
-					}
-
-					# Allow dashes and underscores in tag names
-					if (Text::Markdown::Discount->can("MKD_GITHUBTAGS")) {
-						$flags |= Text::Markdown::Discount::MKD_GITHUBTAGS();
-					}
-					else {
-						$flags |= 0x08000000;
-					}
-
 					return Text::Markdown::Discount::markdown($t, $flags);
 				}
 			}