[GR-18163] Add $SAFE access warning

PullRequest: truffleruby/2195

Author: bjfish

spec/tags/language/safe_tags.txt

@@ -15,5 +15,4 @@ fails:The $SAFE variable can be manually lowered
 fails:The $SAFE variable is not Proc local
 fails:The $SAFE variable is not lambda local
 fails:The $SAFE variable is global like regular global variables
-fails:The $SAFE variable warn when access
 fails:The $SAFE variable warn when set

spec/tags/truffle/safe_tags.txt

@@ -1 +1 @@
-slow:The $SAFE variable does not warn when set to 0 and remembers the value
+slow:The $SAFE variable warns when set to 0 and remembers the value

spec/truffle/safe_spec.rb

@@ -9,8 +9,8 @@
 require_relative '../ruby/spec_helper'
 
 describe "The $SAFE variable" do
-  it "does not warn when set to 0 and remembers the value" do
-    ruby_exe("$SAFE = 0; puts $SAFE; puts Thread.current.safe_level", args: "2>&1").should == "0\n0\n"
+  it "warns when set to 0 and remembers the value" do
+    ruby_exe("$SAFE = 0; puts $SAFE; puts Thread.current.safe_level", args: "2>&1").should include("warning: $SAFE will become a normal global variable in Ruby 3.0\n0\n0\n")
   end
 
   it "raises an error when set to 1" do

src/main/ruby/truffleruby/core/thread.rb

@@ -371,7 +371,10 @@ def marshal_dump
 
 Truffle::KernelOperations.define_hooked_variable(
   :$SAFE,
-  -> { Thread.current.safe_level },
+  -> {
+    warn '$SAFE will become a normal global variable in Ruby 3.0', uplevel: 1
+    Thread.current.safe_level
+  },
   -> level {
     raise SecurityError, 'Setting $SAFE is no longer supported.' unless level == 0
   }