From f0c4473107d0c3589479809d8accd79b9c4dba08 Mon Sep 17 00:00:00 2001
From: Aleks Kissinger <aleks0@gmail.com>
Date: Mon, 12 Oct 2020 21:01:11 +0100
Subject: [PATCH] tell markdown parser to ignore properly-formatted math tags

---
 src/Markdown.js | 51 +++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 41 insertions(+), 10 deletions(-)

diff --git a/src/Markdown.js b/src/Markdown.js
index 329dcdd996..564a2ed0a8 100644
--- a/src/Markdown.js
+++ b/src/Markdown.js
@@ -23,19 +23,47 @@ const ALLOWED_HTML_TAGS = ['sub', 'sup', 'del', 'u'];
 // These types of node are definitely text
 const TEXT_NODES = ['text', 'softbreak', 'linebreak', 'paragraph', 'document'];
 
-function is_math_node(node) {
-    return node != null &&
-        node.literal != null &&
-        node.literal.match(/^<((div|span) data-mx-maths="[^"]*"|\/(div|span))>$/) != null;
+// prevent renderer from interpreting contents of AST node
+function freeze_node(walker, node) {
+    const newNode = new commonmark.Node('custom_inline', node.sourcepos);
+    newNode.onEnter = node.literal;
+    node.insertAfter(newNode);
+    node.unlink();
+    walker.resumeAt(newNode.next, true);
+}
+
+// prevent renderer from interpreting contents of latex math tags
+function freeze_math(parsed) {
+    const walker = parsed.walker();
+    let ev;
+    let inMath = false;
+    while ( (ev = walker.next()) ) {
+        const node = ev.node;
+        if (ev.entering) {
+            if (!inMath) {
+                // entering a math tag
+                if (node.literal != null && node.literal.match('^<(div|span) data-mx-maths="[^"]*">$') != null) {
+                    inMath = true;
+                    freeze_node(walker, node);
+                }
+            } else {
+                // math tags should only contain a single code block, with URL-escaped latex as fallback output
+                if (node.literal != null && node.literal.match('^(<code>|</code>|[^<>]*)$')) {
+                    freeze_node(walker, node);
+                // leave when span or div is closed
+                } else if (node.literal == '</span>' || node.literal == '</div>') {
+                    inMath = false;
+                    freeze_node(walker, node);
+                // this case only happens if we have improperly formatted math tags, so bail
+                } else {
+                    inMath = false;
+                }
+            }
+        }
+    }
 }
 
 function is_allowed_html_tag(node) {
-    if (SettingsStore.getValue("feature_latex_maths") &&
-        (is_math_node(node) ||
-         (node.literal.match(/^<\/?code>$/) && is_math_node(node.parent)))) {
-        return true;
-    }
-
     // Regex won't work for tags with attrs, but we only
     // allow <del> anyway.
     const matches = /^<\/?(.*)>$/.exec(node.literal);
@@ -173,6 +201,9 @@ export default class Markdown {
 */
         };
 
+        // prevent strange behaviour when mixing latex math and markdown
+        freeze_math(this.parsed);
+
         return renderer.render(this.parsed);
     }