Throw RuntimeError if getting/setting ractor local storage for non-main ractor

[Bug #19367]

bootstraptest/test_ractor.rb

@@ -1564,6 +1564,24 @@ assert_equal '1', %q{
   }.take
 }
 
+# Ractor-local storage
+assert_equal '2', %q{
+  Ractor.new {
+    fails = 0
+    begin
+      Ractor.main[:key] # cannot get ractor local storage from non-main ractor
+    rescue => e
+      fails += 1 if e.message =~ /Cannot get ractor local/
+    end
+    begin
+      Ractor.main[:key] = 'val'
+    rescue => e
+      fails += 1 if e.message =~ /Cannot set ractor local/
+    end
+    fails
+  }.take
+}
+
 ###
 ### Synchronization tests
 ###

ractor.rb

@@ -835,15 +835,21 @@ class Ractor
     end
   end
 
-  # get a value from ractor-local storage of current Ractor
+  # get a value from ractor-local storage for current Ractor
   # Obsolete and use Ractor.[] instead.
   def [](sym)
+    if (self != Ractor.current)
+      raise RuntimeError, "Cannot get ractor local storage for non-current ractor"
+    end
     Primitive.ractor_local_value(sym)
   end
 
-  # set a value in ractor-local storage of current Ractor
+  # set a value in ractor-local storage for current Ractor
   # Obsolete and use Ractor.[]= instead.
   def []=(sym, val)
+    if (self != Ractor.current)
+      raise RuntimeError, "Cannot set ractor local storage for non-current ractor"
+    end
     Primitive.ractor_local_value_set(sym, val)
   end