[subset] fix subsetting of faces created via hb_face_create_for_tables.

Fixes #3609.

src/hb-subset.cc

@@ -78,6 +78,85 @@ using OT::Layout::GSUB::GSUB;
  * retain glyph ids option and configure the subset to pass through the layout tables untouched.
  */
 
+/*
+ * The list of tables that the subsetter can perform subsetting for.
+ */
+static hb_tag_t handled_tables[] {
+  HB_OT_TAG_glyf,
+  HB_OT_TAG_hdmx,
+  HB_OT_TAG_name,
+  HB_OT_TAG_head,
+  HB_OT_TAG_hhea,
+  HB_OT_TAG_hmtx,
+  HB_OT_TAG_vhea,
+  HB_OT_TAG_vmtx,
+  HB_OT_TAG_maxp,
+  HB_OT_TAG_sbix,
+  HB_OT_TAG_loca,
+  HB_OT_TAG_cmap,
+  HB_OT_TAG_OS2,
+  HB_OT_TAG_post,
+  HB_OT_TAG_COLR,
+  HB_OT_TAG_CPAL,
+  HB_OT_TAG_CBLC,
+  HB_OT_TAG_CBDT,
+  HB_OT_TAG_MATH,
+  HB_OT_TAG_cff1,
+  HB_OT_TAG_cff2,
+  HB_OT_TAG_VORG,
+  HB_OT_TAG_GDEF,
+  HB_OT_TAG_GSUB,
+  HB_OT_TAG_GPOS,
+  HB_OT_TAG_gvar,
+  HB_OT_TAG_HVAR,
+  HB_OT_TAG_VVAR
+};
+
+static bool _table_is_empty (const hb_face_t *face, hb_tag_t tag)
+{
+  hb_blob_t* blob = hb_face_reference_table (face, tag);
+  bool result = (blob == hb_blob_get_empty ());
+  hb_blob_destroy (blob);
+  return result;
+}
+
+static unsigned int
+_get_table_tags (const hb_face_t *face,
+                 unsigned int  start_offset,
+                 unsigned int *table_count, /* IN/OUT */
+                 hb_tag_t     *table_tags /* OUT */)
+{
+  unsigned num_tables = hb_face_get_table_tags (face, 0, nullptr, nullptr);
+  if (num_tables)
+    return hb_face_get_table_tags (face, start_offset, table_count, table_tags);
+
+  // If face has 0 tables associated with it, assume that it has built from
+  // hb_face_create_tables and thus is unable to list its tables. Fallback to
+  // checking each table type we can handle for existence instead.
+  auto it = hb_array_t<hb_tag_t> (handled_tables);
+
+  while (bool (it) && start_offset > 0)
+  {
+    if (!_table_is_empty (face, *it))
+      start_offset--;
+
+    it++;
+  }
+
+  unsigned num_written = 0;
+  while (bool (it) && num_written < *table_count)
+  {
+    if (!_table_is_empty (face, *it))
+      table_tags[num_written++] = *it;
+
+    it++;
+  }
+
+  *table_count = num_written;
+  return 0;
+}
+
+
 static unsigned
 _plan_estimate_subset_table_size (hb_subset_plan_t *plan,
 				  unsigned table_len,
@@ -226,7 +305,7 @@ _is_table_present (hb_face_t *source, hb_tag_t tag)
 {
   hb_tag_t table_tags[32];
   unsigned offset = 0, num_tables = ARRAY_LENGTH (table_tags);
-  while ((hb_face_get_table_tags (source, offset, &num_tables, table_tags), num_tables))
+  while ((_get_table_tags (source, offset, &num_tables, table_tags), num_tables))
   {
     for (unsigned i = 0; i < num_tables; ++i)
       if (table_tags[i] == tag)
@@ -388,7 +467,7 @@ hb_subset_plan_execute_or_fail (hb_subset_plan_t *plan)
   unsigned offset = 0, num_tables = ARRAY_LENGTH (table_tags);
   hb_vector_t<char> buf;
   buf.alloc (4096 - 16);
-  while ((hb_face_get_table_tags (plan->source, offset, &num_tables, table_tags), num_tables))
+  while ((_get_table_tags (plan->source, offset, &num_tables, table_tags), num_tables))
   {
     for (unsigned i = 0; i < num_tables; ++i)
     {

test/api/test-subset.c

@@ -193,6 +193,41 @@ test_subset_plan (void)
   hb_face_destroy (face_ac);
 }
 
+static hb_blob_t*
+_ref_table (hb_face_t *face, hb_tag_t tag, void *user_data)
+{
+  return hb_face_reference_table ((hb_face_t*) user_data, tag);
+}
+
+static void
+test_subset_create_for_tables_face (void)
+{
+  hb_face_t *face_abc = hb_test_open_font_file ("fonts/Roboto-Regular.abc.ttf");
+  hb_face_t *face_ac = hb_test_open_font_file ("fonts/Roboto-Regular.ac.ttf");
+  hb_face_t *face_create_for_tables = hb_face_create_for_tables (
+      _ref_table,
+      face_abc,
+      NULL);
+
+  hb_set_t *codepoints = hb_set_create();
+  hb_set_add (codepoints, 97);
+  hb_set_add (codepoints, 99);
+
+  hb_subset_input_t* input = hb_subset_test_create_input (codepoints);
+  hb_set_destroy (codepoints);
+
+  hb_face_t* face_abc_subset = hb_subset_or_fail (face_create_for_tables, input);
+
+  hb_subset_test_check (face_ac, face_abc_subset, HB_TAG ('l','o','c', 'a'));
+  hb_subset_test_check (face_ac, face_abc_subset, HB_TAG ('g','l','y','f'));
+
+  hb_subset_input_destroy (input);
+  hb_face_destroy (face_abc_subset);
+  hb_face_destroy (face_create_for_tables);
+  hb_face_destroy (face_abc);
+  hb_face_destroy (face_ac);
+}
+
 int
 main (int argc, char **argv)
 {
@@ -204,6 +239,7 @@ main (int argc, char **argv)
   hb_test_add (test_subset_set_flags);
   hb_test_add (test_subset_sets);
   hb_test_add (test_subset_plan);
+  hb_test_add (test_subset_create_for_tables_face);
 
   return hb_test_run();
 }