From 6547724630c99c93d4634e79a2087a4639772515 Mon Sep 17 00:00:00 2001
From: Jonathan Kew <jfkthame@gmail.com>
Date: Tue, 2 Jan 2024 12:09:48 +0000
Subject: [PATCH] [colr] Make rejecting paint-graph cycles in COLRv1 a
 compile-time option.

---
 meson.build       |  4 ++++
 meson_options.txt |  1 +
 src/colr.cc       | 13 +++++++++++++
 3 files changed, 18 insertions(+)

diff --git a/meson.build b/meson.build
index 56ce7ac6..a3b1e03c 100644
--- a/meson.build
+++ b/meson.build
@@ -24,6 +24,10 @@ if get_option('synthesize-gvar')
   conf.set('OTS_SYNTHESIZE_MISSING_GVAR', 1)
 endif
 
+if get_option('colr-cycle-check')
+  conf.set('OTS_COLR_CYCLE_CHECK', 1)
+endif
+
 freetype = dependency('freetype2', required: false)
 if freetype.found()
   conf.set('HAVE_FREETYPE', 1)
diff --git a/meson_options.txt b/meson_options.txt
index 36968bee..01b4c923 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -1,3 +1,4 @@
+option('colr-cycle-check', type : 'boolean', value : true, description : 'Reject fonts with cycles in COLRv1 paint graph')
 option('graphite', type : 'boolean', value : true, description : 'Sanitize Graphite tables')
 option('synthesize-gvar', type : 'boolean', value : true, description : 'Synthesize an empty gvar if fvar is present')
 option('fuzzer_ldflags', type: 'string', description : 'Extra LDFLAGS used during linking of fuzzing binaries')
diff --git a/src/colr.cc b/src/colr.cc
index 8931c77c..a09d3ab3 100644
--- a/src/colr.cc
+++ b/src/colr.cc
@@ -193,7 +193,15 @@ bool ParsePaintColrLayers(const ots::Font* font,
                           colrState& state)
 {
   if (setContains(state.visited, data)) {
+#ifdef OTS_COLR_CYCLE_CHECK
+    // A cycle would imply an infinite loop during painting, unless the renderer
+    // detects and breaks it. To be safe, reject the table.
     return OTS_FAILURE_MSG("Cycle detected in PaintColrLayers");
+#else
+    // Just issue a warning and return (as we've already checked this subgraph).
+    OTS_WARNING("Cycle detected in COLRv1 glyph paint graph (PaintColrLayers)\n");
+    return true;
+#endif
   }
   state.visited.insert(data);
 
@@ -393,7 +401,12 @@ bool ParsePaintColrGlyph(const ots::Font* font,
                          colrState& state)
 {
   if (setContains(state.visited, data)) {
+#ifdef OTS_COLR_CYCLE_CHECK
     return OTS_FAILURE_MSG("Cycle detected in PaintColrGlyph");
+#else
+    OTS_WARNING("Cycle detected in COLRv1 glyph paint graph (PaintColrGlyph)\n");
+    return true;
+#endif
   }
   state.visited.insert(data);