highlighter: Avoid reusing parse tree when adding an injection range

Author: the-mikedavis

highlighter/src/parse.rs

@@ -95,9 +95,31 @@ impl LayerData {
         parser
             .set_included_ranges(&self.ranges)
             .map_err(|_| Error::InvalidRanges)?;
-        let tree = parser
-            .parse(source, self.parse_tree.as_ref())
-            .ok_or(Error::Timeout)?;
+
+        // HACK:
+        // This is a workaround for a bug within the lexer (in the C library) or maybe within
+        // tree-sitter-markdown which needs more debugging. When adding a new range to a combined
+        // injection and passing the old tree, if the old tree doesn't already cover a wider range
+        // than the newly added range, some assumptions are violated in the lexer and it tries to
+        // access some invalid memory, resulting in a segfault. This workaround avoids that
+        // situation by avoiding passing the old tree when the old tree's range doesn't cover the
+        // total range of `self.ranges`.
+        //
+        // See <https://github.com/helix-editor/helix/pull/12972#issuecomment-2725410409>.
+        let tree = self.parse_tree.as_ref().filter(|tree| {
+            let included_ranges_range = self.ranges.first().map(|r| r.start_byte).unwrap_or(0)
+                ..self.ranges.last().map(|r| r.end_byte).unwrap_or(u32::MAX);
+            // Allow re-parsing the root layer even though the range is larger. The root always
+            // covers `0..u32::MAX`:
+            if included_ranges_range == (0..u32::MAX) {
+                return true;
+            }
+            let tree_range = tree.root_node().byte_range();
+            tree_range.start <= included_ranges_range.start
+                && tree_range.end >= included_ranges_range.end
+        });
+
+        let tree = parser.parse(source, tree).ok_or(Error::Timeout)?;
         self.parse_tree = Some(tree);
         Ok(())
     }