Fix Namespace.current to show its caller's namespace

Calling rb_current_namespace() in rb_namespace_current() means to show
the definition namespace of Namespace.current itself (it's the root always)
but the users' expectation is to show the namespace of the place where
the Namespace.current is called.

Author: tagomoris

eval_intern.h

@@ -299,6 +299,7 @@ VALUE rb_vm_call_cfunc(VALUE recv, VALUE (*func)(VALUE), VALUE arg, VALUE block_
 VALUE rb_vm_call_cfunc_in_namespace(VALUE recv, VALUE (*func)(VALUE, VALUE), VALUE arg1, VALUE arg2, VALUE filename, const rb_namespace_t *ns);
 void rb_vm_frame_flag_set_ns_require(const rb_execution_context_t *ec);
 const rb_namespace_t *rb_vm_current_namespace(const rb_execution_context_t *ec);
+const rb_namespace_t *rb_vm_caller_namespace(const rb_execution_context_t *ec);
 const rb_namespace_t *rb_vm_loading_namespace(const rb_execution_context_t *ec);
 void rb_vm_set_progname(VALUE filename);
 VALUE rb_vm_cbase(void);

namespace.c

@@ -455,11 +455,12 @@ rb_namespace_s_getenabled(VALUE namespace)
 static VALUE
 rb_namespace_current(VALUE klass)
 {
-    const rb_namespace_t *ns = rb_current_namespace();
+    const rb_namespace_t *ns;
 
     if (!rb_namespace_available())
         return Qnil;
 
+    ns = rb_vm_caller_namespace(GET_EC());
     VM_ASSERT(ns && ns->ns_object);
     return ns->ns_object;
 }

vm.c

@@ -3052,13 +3052,21 @@ current_namespace_on_env(const VALUE *ep)
 const rb_namespace_t *
 rb_vm_current_namespace(const rb_execution_context_t *ec)
 {
-    const rb_control_frame_t *cfp;
+    VM_ASSERT(rb_namespace_available());
+    return current_namespace_on_env(ec->cfp->ep);
+}
 
-    if (!rb_namespace_available() || !ec)
-        return rb_root_namespace();
+const rb_namespace_t *
+rb_vm_caller_namespace(const rb_execution_context_t *ec)
+{
+    const rb_control_frame_t *caller_cfp;
 
-    cfp = ec->cfp;
-    return current_namespace_on_env(cfp->ep);
+    VM_ASSERT(rb_namespace_available());
+
+    // The current control frame is MAGIC_CFUNC to call Namespace.current, but
+    // we want to get the current namespace of its caller.
+    caller_cfp = vm_get_ruby_level_caller_cfp(ec, RUBY_VM_PREVIOUS_CONTROL_FRAME(ec->cfp));
+    return current_namespace_on_env(caller_cfp->ep);
 }
 
 const rb_namespace_t *