Add ancestor_extension field.

src/node.h

@@ -82,6 +82,14 @@ struct cmark_node {
 
   cmark_syntax_extension *extension;
 
+  /**
+   * Used during cmark_render() to cache the most recent non-NULL
+   * extension, if you go up the parent chain like this:
+   *
+   * node->parent->...parent->extension
+   */
+  cmark_syntax_extension *ancestor_extension;
+
   union {
     int ref_ix;
     int def_count;

src/render.c

@@ -31,13 +31,7 @@ static void S_out(cmark_renderer *renderer, cmark_node *node,
   cmark_chunk remainder = cmark_chunk_literal("");
   int k = renderer->buffer->size - 1;
 
-  cmark_syntax_extension *ext = NULL;
-  cmark_node *n = node;
-  while (n && !ext) {
-    ext = n->extension;
-    if (!ext)
-      n = n->parent;
-  }
+  cmark_syntax_extension *ext = node->ancestor_extension;
   if (ext && !ext->commonmark_escape_func)
     ext = NULL;
 
@@ -182,6 +176,11 @@ char *cmark_render(cmark_mem *mem, cmark_node *root, int options, int width,
 
   while ((ev_type = cmark_iter_next(iter)) != CMARK_EVENT_DONE) {
     cur = cmark_iter_get_node(iter);
+    if (cur->extension) {
+      cur->ancestor_extension = cur->extension;
+    } else if (cur->parent) {
+      cur->ancestor_extension = cur->parent->ancestor_extension;
+    }
     if (!render_node(&renderer, cur, ev_type, options)) {
       // a false value causes us to skip processing
       // the node's contents.  this is used for