should not use rb_ary_modify()

ractor_copy() used rb_ary_modify() to make sure this array is not
sharing anything, but it also checks frozen flag. So frozen arrays
raises an error. To solve this issue, this patch introduces new
function rb_ary_cancel_sharing() which makes sure the array does not
share another array and it doesn't check frozen flag.
[Bug #17343]

A test is quoted from https://github.com/ruby/ruby/pull/3817

array.c

@@ -562,9 +562,8 @@ rb_ary_modify_check(VALUE ary)
 }
 
 void
-rb_ary_modify(VALUE ary)
+rb_ary_cancel_sharing(VALUE ary)
 {
-    rb_ary_modify_check(ary);
     if (ARY_SHARED_P(ary)) {
         long shared_len, len = RARRAY_LEN(ary);
         VALUE shared_root = ARY_SHARED_ROOT(ary);
@@ -603,6 +602,13 @@ rb_ary_modify(VALUE ary)
     ary_verify(ary);
 }
 
+void
+rb_ary_modify(VALUE ary)
+{
+    rb_ary_modify_check(ary);
+    rb_ary_cancel_sharing(ary);
+}
+
 static VALUE
 ary_ensure_room_for_push(VALUE ary, long add_len)
 {

bootstraptest/test_ractor.rb

@@ -1022,6 +1022,13 @@ assert_equal 'can not make a Proc shareable because it accesses outer variables
   end
 }
 
+# Ractor deep copies frozen objects
+assert_equal '[true, false]', %q{
+  Ractor.new([[]].freeze) { |ary|
+    [ary.frozen?, ary.first.frozen? ]
+  }.take
+}
+
 ###
 ### Synchronization tests
 ###

internal/array.h

@@ -29,6 +29,8 @@ VALUE rb_ary_tmp_new_fill(long capa);
 VALUE rb_ary_at(VALUE, VALUE);
 size_t rb_ary_memsize(VALUE);
 VALUE rb_to_array_type(VALUE obj);
+void rb_ary_cancel_sharing(VALUE ary);
+
 static inline VALUE rb_ary_entry_internal(VALUE ary, long offset);
 static inline bool ARY_PTR_USING_P(VALUE ary);
 static inline void RARY_TRANSIENT_SET(VALUE ary);

ractor.c

@@ -2314,7 +2314,7 @@ obj_traverse_replace_i(VALUE obj, struct obj_traverse_replace_data *data)
 
       case T_ARRAY:
         {
-            rb_ary_modify(obj);
+            rb_ary_cancel_sharing(obj);
 #if USE_TRANSIENT_HEAP
             if (data->move) rb_ary_transient_heap_evacuate(obj, TRUE);
 #endif
@@ -2537,7 +2537,8 @@ copy_leave(VALUE obj, struct obj_traverse_replace_data *data)
     return traverse_cont;
 }
 
-static VALUE ractor_copy(VALUE obj)
+static VALUE
+ractor_copy(VALUE obj)
 {
     VALUE val = rb_obj_traverse_replace(obj, copy_enter, copy_leave, false);
     if (val != Qundef) {