fix: cap total paint-graph node visits in COLRv1 painting (#225)

The recursion stack bounds path depth and forbids repeats along one path, but not fan-out; layers_count is a u8, so a small font can force an enormous traversal. Adds a per-call visit budget.

Author: scadastrangelove

src/tables/colr.rs

@@ -886,6 +886,7 @@ impl<'a> Table<'a> {
         let mut recursion_stack = RecursionStack {
             stack: [0; 64],
             len: 0,
+            visits_left: MAX_PAINT_VISITS,
         };
 
         self.paint_impl(
@@ -1004,6 +1005,10 @@ impl<'a> Table<'a> {
             return None;
         }
 
+        // Total-visit budget exceeded (a DAG can revisit the same offset via different sibling
+        // branches without ever cycling on the active path -- see `RecursionStack::visits_left`).
+        recursion_stack.consume_visit().ok()?;
+
         recursion_stack.push(offset).ok()?;
         let result = self.parse_paint_impl(
             offset,
@@ -1836,8 +1841,21 @@ struct RecursionStack {
     // The limit of 64 is chosen arbitrarily and not from the spec. But we have to stop somewhere...
     stack: [usize; 64],
     len: usize,
+    // `stack`/`contains` only detect a cycle on the CURRENT root-to-leaf path (an entry is popped
+    // as soon as its call returns), so a paint graph shaped as a DAG -- the same offset reachable
+    // through multiple sibling branches, none of which individually cycles -- is not caught by it.
+    // A chain of `PaintColrLayers` records where every layer slot at each level points at one
+    // shared next-level record forces `branching^depth` calls while never re-entering the active
+    // path. This budget bounds the total number of `parse_paint` calls for one top-level `paint`
+    // call, independent of the DAG's branching factor.
+    visits_left: u32,
 }
 
+// The limit of 100_000 total paint-graph node visits is chosen arbitrarily, mirroring
+// `glyf::MAX_COMPONENT_VISITS` -- real color-font paint graphs are expected to have at most a few
+// hundred nodes.
+const MAX_PAINT_VISITS: u32 = 100_000;
+
 impl RecursionStack {
     #[inline]
     pub fn is_empty(&self) -> bool {
@@ -1869,6 +1887,17 @@ impl RecursionStack {
         debug_assert!(!self.is_empty());
         self.len -= 1;
     }
+
+    #[inline]
+    pub fn consume_visit(&mut self) -> Result<(), ()> {
+        match self.visits_left.checked_sub(1) {
+            Some(left) => {
+                self.visits_left = left;
+                Ok(())
+            }
+            None => Err(()),
+        }
+    }
 }
 
 #[cfg(feature = "variable-fonts")]