Merge pull request #2198 from ruby/ractore_store_if_absent

Add `Ractor.store_if_absent`

Author: soutaro

core/ractor.rbs

@@ -588,6 +588,23 @@ class Ractor
   #
   def self.shareable?: (untyped obj) -> bool
 
+  # <!--
+  #   rdoc-file=ractor.rb
+  #   - 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: (Symbol) { () -> untyped } -> untyped
+
   # <!--
   #   rdoc-file=ractor.rb
   #   - Ractor.yield(msg, move: false) -> nil