[subset] Add hb_subset_or_fail () to public subset api.

src/hb-subset.cc

@@ -304,13 +304,31 @@ _subset_table (hb_subset_plan_t *plan, hb_tag_t tag)
  **/
 hb_face_t *
 hb_subset (hb_face_t *source, const hb_subset_input_t *input)
+{
+  hb_face_t* result = hb_subset_or_fail (source, input);
+  if (unlikely (!result)) return hb_face_get_empty ();
+  return result;
+}
+
+/**
+ * hb_subset_or_fail:
+ * @source: font face data to be subset.
+ * @input: input to use for the subsetting.
+ *
+ * Subsets a font according to provided input. Returns nullptr
+ * if the subset operation fails.
+ *
+ * Since: REPLACE
+ **/
+hb_face_t *
+hb_subset_or_fail (hb_face_t *source, const hb_subset_input_t *input)
 {
   if (unlikely (!input || !source)) return hb_face_get_empty ();
 
   hb_subset_plan_t *plan = hb_subset_plan_create (source, input);
   if (unlikely (plan->in_error ())) {
     hb_subset_plan_destroy (plan);
-    return hb_face_get_empty ();
+    return nullptr;
   }
 
   hb_set_t tags_set;
@@ -331,7 +349,7 @@ hb_subset (hb_face_t *source, const hb_subset_input_t *input)
   }
 end:
 
-  hb_face_t *result = success ? hb_face_reference (plan->dest) : hb_face_get_empty ();
+  hb_face_t *result = success ? hb_face_reference (plan->dest) : nullptr;
 
   hb_subset_plan_destroy (plan);
   return result;

src/hb-subset.h

@@ -118,6 +118,9 @@ hb_subset_input_get_no_prune_unicode_ranges (hb_subset_input_t *subset_input);
 HB_EXTERN hb_face_t *
 hb_subset (hb_face_t *source, const hb_subset_input_t *input);
 
+HB_EXTERN hb_face_t *
+hb_subset_or_fail (hb_face_t *source, const hb_subset_input_t *input);
+
 HB_EXTERN hb_bool_t
 hb_subset_input_set_user_data (hb_subset_input_t  *input,
 			       hb_user_data_key_t *key,

util/hb-subset.cc

@@ -101,16 +101,18 @@ struct subset_consumer_t
     hb_face_t *new_face = nullptr;
     for (unsigned i = 0; i < subset_options.num_iterations; i++) {
       hb_face_destroy (new_face);
-      new_face = hb_subset (face, input);
+      new_face = hb_subset_or_fail (face, input);
     }
-    hb_blob_t *result = hb_face_reference_blob (new_face);
 
-    failed = !hb_blob_get_length (result);
+    failed = !new_face;
     if (!failed)
+    {
+      hb_blob_t *result = hb_face_reference_blob (new_face);
       write_file (options.output_file, result);
+      hb_blob_destroy (result);
+    }
 
     hb_subset_input_destroy (input);
-    hb_blob_destroy (result);
     hb_face_destroy (new_face);
     hb_font_destroy (font);
   }