[subset] During LigatureSubstFormat1 subsetting always place Coverage last.

In Windows 7 on Chrome if the coverage table comes before any of the LigatureSet or Ligature subtables the font won't load. This changes the packing order to always place the Coverage table last. Virtual links are used to ensure the repacker maintains the desired ordering.

Coincidentally fontTools also does the same thing (a3f988fbf6/Lib/fontTools/ttLib/tables/otTables.py (L1137)) to reduce overflows during packing.

src/hb-ot-layout-gsub-table.hh

@@ -861,13 +861,15 @@ struct Ligature
     return_trace (true);
   }
 
-  bool subset (hb_subset_context_t *c) const
+  bool subset (hb_subset_context_t *c, unsigned coverage_idx) const
   {
     TRACE_SUBSET (this);
     const hb_set_t &glyphset = *c->plan->glyphset_gsub ();
     const hb_map_t &glyph_map = *c->plan->glyph_map;
 
     if (!intersects (&glyphset) || !glyphset.has (ligGlyph)) return_trace (false);
+    // Ensure Coverage table is always packed after this.
+    c->serializer->add_virtual_link (coverage_idx);
 
     auto it =
       + hb_iter (component)
@@ -968,16 +970,21 @@ struct LigatureSet
     return_trace (true);
   }
 
-  bool subset (hb_subset_context_t *c) const
+  bool subset (hb_subset_context_t *c, unsigned coverage_idx) const
   {
     TRACE_SUBSET (this);
     auto *out = c->serializer->start_embed (*this);
     if (unlikely (!c->serializer->extend_min (out))) return_trace (false);
 
     + hb_iter (ligature)
-    | hb_filter (subset_offset_array (c, out->ligature, this))
+    | hb_filter (subset_offset_array (c, out->ligature, this, coverage_idx))
     | hb_drain
     ;
+
+    if (bool (out->ligature))
+      // Ensure Coverage table is always packed after this.
+      c->serializer->add_virtual_link (coverage_idx);
+
     return_trace (bool (out->ligature));
   }
 
@@ -1092,15 +1099,38 @@ struct LigatureSubstFormat1
     if (unlikely (!c->serializer->extend_min (out))) return_trace (false);
     out->format = format;
 
-    hb_sorted_vector_t<hb_codepoint_t> new_coverage;
-    + hb_zip (this+coverage, ligatureSet)
+    // Due to a bug in some older versions of windows 7 the Coverage table must be
+    // packed after the LigatureSet and Ligature tables, so serialize Coverage first
+    // which places it last in the packed order.
+    hb_set_t new_coverage;
+    + hb_zip (this+coverage, hb_iter (ligatureSet) | hb_map (hb_add (this)))
     | hb_filter (glyphset, hb_first)
-    | hb_filter (subset_offset_array (c, out->ligatureSet, this), hb_second)
+    | hb_filter ([&] (const LigatureSet& _) {
+      return _.intersects (&glyphset);
+    }, hb_second)
     | hb_map (hb_first)
-    | hb_map (glyph_map)
-    | hb_sink (new_coverage)
+    | hb_sink (new_coverage);
+
+    if (!c->serializer->push<Coverage> ()
+        ->serialize (c->serializer,
+                     + new_coverage.iter () | hb_map_retains_sorting (glyph_map)))
+    {
+      c->serializer->pop_discard ();
+      return_trace (false);
+    }
+
+    unsigned coverage_idx = c->serializer->pop_pack ();
+     c->serializer->add_link (out->coverage, coverage_idx);
+
+    + hb_zip (this+coverage, ligatureSet)
+    | hb_filter (new_coverage, hb_first)
+    | hb_map (hb_second)
+    // to ensure that the repacker always orders the coverage table after the LigatureSet
+    // and LigatureSubtable's they will be linked to the Coverage table via a virtual link
+    // the coverage table object idx is passed down to facilitate this.
+    | hb_apply (subset_offset_array (c, out->ligatureSet, this, coverage_idx))
     ;
-    out->coverage.serialize_serialize (c->serializer, new_coverage.iter ());
+
     return_trace (bool (new_coverage));
   }