From 0c0d0752f1fefd9b085f191b62946a811763ef1b Mon Sep 17 00:00:00 2001
From: Koichi Sasada <ko1@atdot.net>
Date: Thu, 22 Oct 2020 00:36:53 +0900
Subject: [PATCH] allow to access ivars of frozen shareable objects

Accessing a shareable object is prohibitted because it can cause
race condition, but if the shareable object is frozen, there is no
problem to access ivars.
---
 bootstraptest/test_ractor.rb | 12 ++++++++++++
 variable.c                   |  3 ++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/bootstraptest/test_ractor.rb b/bootstraptest/test_ractor.rb
index 6290b73c36..62b0b523ae 100644
--- a/bootstraptest/test_ractor.rb
+++ b/bootstraptest/test_ractor.rb
@@ -734,6 +734,18 @@ assert_equal 'can not access instance variables of shareable objects from non-ma
   end
 }
 
+# But a sharable object is frozen, it is allowed to access ivars from non-main Ractor
+assert_equal '11', %q{
+  [Object.new, [], ].map{|obj|
+    obj.instance_variable_set('@a', 1)
+    Ractor.make_shareable obj = obj.freeze
+
+    Ractor.new obj do |obj|
+      obj.instance_variable_get('@a')
+    end.take.to_s
+  }.join
+}
+
 # cvar in sharable-objects are not allowed to access from non-main Ractor
 assert_equal 'can not access class variables from non-main Ractors', %q{
   class C
diff --git a/variable.c b/variable.c
index 795ee3fb0a..8ac40efe80 100644
--- a/variable.c
+++ b/variable.c
@@ -922,7 +922,8 @@ generic_ivtbl(VALUE obj, ID id, bool force_check_ractor)
 {
     ASSERT_vm_locking();
 
-    if ((force_check_ractor || rb_is_instance_id(id)) && // not internal ID
+    if ((force_check_ractor || LIKELY(rb_is_instance_id(id)) /* not internal ID */ )  &&
+        !RB_OBJ_FROZEN_RAW(obj) &&
         UNLIKELY(!rb_ractor_main_p()) &&
         UNLIKELY(rb_ractor_shareable_p(obj))) {
         rb_raise(rb_eRuntimeError, "can not access instance variables of shareable objects from non-main Ractors");