[subset-cff] Write out charstrings zerocopy to serializer

src/hb-serialize.hh

@@ -223,6 +223,7 @@ struct hb_serialize_context_t
     this->errors = HB_SERIALIZE_ERROR_NONE;
     this->head = this->start;
     this->tail = this->end;
+    this->zerocopy = nullptr;
     this->debug_depth = 0;
 
     fini ();
@@ -343,8 +344,11 @@ struct hb_serialize_context_t
     current = current->next;
     obj->tail = head;
     obj->next = nullptr;
+    assert (obj->head <= obj->tail);
     unsigned len = obj->tail - obj->head;
-    head = obj->head; /* Rewind head. */
+    head = zerocopy ? zerocopy : obj->head; /* Rewind head. */
+    if (zerocopy)
+      assert (!share);
 
     if (!len)
     {
@@ -368,6 +372,12 @@ struct hb_serialize_context_t
     }
 
     tail -= len;
+    if (zerocopy)
+    {
+      zerocopy = nullptr;
+      assert (tail == obj->head);
+    }
+    else
       memmove (tail, obj->head, len);
 
     obj->head = tail;
@@ -580,8 +590,11 @@ struct hb_serialize_context_t
       return false;
     }
 
+    assert (!this->zerocopy);
+    this->zerocopy = this->head;
+
     assert (this->current->head == this->head);
-    this->current->head = this->head = this->tail - size;
+    this->current->head = this->current->tail = this->head = this->tail - size;
     return true;
   }
 
@@ -721,7 +734,7 @@ struct hb_serialize_context_t
   }
 
   public:
-  char *start, *head, *tail, *end;
+  char *start, *head, *tail, *end, *zerocopy;
   unsigned int debug_depth;
   hb_serialize_error_t errors;
 

src/hb-subset-cff1.cc

@@ -742,9 +742,15 @@ static bool _serialize_cff1 (hb_serialize_context_t *c,
 
   /* CharStrings */
   {
+    c->push<CFF1CharStrings> ();
+
+    unsigned total_size = CFF1CharStrings::total_size (plan.subset_charstrings);
+    if (unlikely (!c->start_zerocopy (total_size)))
+       return false;
+
     CFF1CharStrings  *cs = c->start_embed<CFF1CharStrings> ();
     if (unlikely (!cs)) return false;
-    c->push ();
+
     if (likely (cs->serialize (c, plan.subset_charstrings)))
       plan.info.char_strings_link = c->pop_pack (false);
     else

src/hb-subset-cff2.cc

@@ -361,9 +361,15 @@ static bool _serialize_cff2 (hb_serialize_context_t *c,
 
   /* CharStrings */
   {
+    c->push ();
+
+    unsigned total_size = CFF2CharStrings::total_size (plan.subset_charstrings);
+    if (unlikely (!c->start_zerocopy (total_size)))
+       return false;
+
     CFF2CharStrings  *cs = c->start_embed<CFF2CharStrings> ();
     if (unlikely (!cs)) return false;
-    c->push ();
+
     if (likely (cs->serialize (c, plan.subset_charstrings)))
       plan.info.char_strings_link = c->pop_pack (false);
     else