Ractor.make_shareable(obj)

Introduce new method Ractor.make_shareable(obj) which tries to make
obj shareable object. Protocol is here.

(1) If obj is shareable, it is shareable.
(2) If obj is not a shareable object and if obj can be shareable
    object if it is frozen, then freeze obj. If obj has reachable
    objects (rs), do rs.each{|o| Ractor.make_shareable(o)}
    recursively (recursion is not Ruby-level, but C-level).
(3) Otherwise, raise Ractor::Error. Now T_DATA is not a shareable
    object even if the object is frozen.

If the method finished without error, given obj is marked as
a sharable object.

To allow makng a shareable frozen T_DATA object, then set
`RUBY_TYPED_FROZEN_SHAREABLE` as type->flags. On default,
this flag is not set. It means user defined T_DATA objects are
not allowed to become shareable objects when it is frozen.

You can make any object  shareable by setting FL_SHAREABLE flag,
so if you know that the T_DATA object is shareable (== thread-safe),
set this flag, at creation time for example. `Ractor` object is one
example, which is not a frozen, but a shareable object.

1 files changed, 2 insertions, 0 deletions

diff --git a/ractor_pub.h b/ractor_pub.h
index 5347481429..f2869276f6 100644
--- a/ractor_pub.h
+++ b/ractor_pub.h
@@ -36,6 +36,8 @@ rb_ractor_shareable_p(VALUE obj)
     }
 }
 
+VALUE rb_ractor_make_shareable(VALUE obj);
+
 RUBY_SYMBOL_EXPORT_BEGIN
 
 VALUE rb_ractor_stdin(void);