followup 0bdb38ba6be208064a514c12a9b80328645689f8

(forgot to amend...)

NEWS.md

@@ -83,6 +83,9 @@ Note: We're only listing outstanding class updates.
     * Ractor.[] and Ractor.[]= are added to access the ractor local storage
       of the current Ractor. [[Feature #20715]]
 
+    * `Ractor.store_if_absent(key){ init }` is added to initialize ractor local
+      variables in thread-safty. [[Feature #20875]]
+
 * Range
 
     * Range#size now raises TypeError if the range is not iterable. [[Misc #18984]]
@@ -401,6 +404,7 @@ details of the default gems or bundled gems.
 [Bug #20795]:     https://bugs.ruby-lang.org/issues/20795
 [Feature #20811]: https://bugs.ruby-lang.org/issues/20811
 [Feature #20860]: https://bugs.ruby-lang.org/issues/20860
+[Feature #20875]: https://bugs.ruby-lang.org/issues/20875
 [Feature #20876]: https://bugs.ruby-lang.org/issues/20876
 [Feature #20884]: https://bugs.ruby-lang.org/issues/20884
 [Feature #20902]: https://bugs.ruby-lang.org/issues/20902

ractor.c

@@ -3677,6 +3677,8 @@ ractor_local_storage_mark(rb_ractor_t *r)
     if (r->idkey_local_storage) {
         rb_id_table_foreach_values(r->idkey_local_storage, idkey_local_storage_mark_i, NULL);
     }
+
+    rb_gc_mark(r->local_storage_store_lock);
 }
 
 static int

ractor.rb

@@ -856,6 +856,20 @@ class Ractor
     Primitive.ractor_local_value_set(sym, val)
   end
 
+  # call-seq:
+  #   Ractor.store_if_absent(key){ init_block }
+  #
+  # If the correponding value is not set, yield a value with
+  # init_block and store the value in thread-safe manner.
+  # This method returns corresponding stored value.
+  #
+  #   (1..10).map{
+  #     Thread.new(it){|i|
+  #       Ractor.store_if_absent(:s){ f(); i }
+  #       #=> return stored value of key :s
+  #     }
+  #   }.map(&:value).uniq.size #=> 1 and f() is called only once
+  #
   def self.store_if_absent(sym)
     Primitive.ractor_local_value_store_if_absent(sym)
   end