Freeze $/ and make it ractor safe

[Feature #21109]

By always freezing when setting the global rb_rs variable, we can ensure
it is not modified and can be accessed from a ractor.

We're also making sure it's an instance of String and does not have any
instance variables.

Of course, if $/ is changed at runtime, it may cause surprising behavior
but doing so is deprecated already anyway.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

internal/string.h

@@ -61,6 +61,7 @@ size_t rb_str_size_as_embedded(VALUE);
 bool rb_str_reembeddable_p(VALUE);
 VALUE rb_str_upto_endless_each(VALUE, int (*each)(VALUE, VALUE), VALUE);
 VALUE rb_str_with_debug_created_info(VALUE, VALUE, int);
+VALUE rb_str_frozen_bare_string(VALUE);
 
 /* error.c */
 void rb_warn_unchilled_literal(VALUE str);

io.c

@@ -8673,6 +8673,25 @@ deprecated_str_setter(VALUE val, ID id, VALUE *var)
     *var = val;
 }
 
+static void
+deprecated_rs_setter(VALUE val, ID id, VALUE *var)
+{
+    if (!NIL_P(val)) {
+        if (!RB_TYPE_P(val, T_STRING)) {
+            rb_raise(rb_eTypeError, "value of %"PRIsVALUE" must be String", rb_id2str(id));
+        }
+        if (rb_str_equal(val, rb_default_rs)) {
+            val = rb_default_rs;
+        }
+        else {
+            val = rb_str_frozen_bare_string(val);
+        }
+        rb_enc_str_coderange(val);
+        rb_warn_deprecated("'%s'", NULL, rb_id2name(id));
+    }
+    *var = val;
+}
+
 /*
  *  call-seq:
  *    print(*objects) -> nil
@@ -15713,8 +15732,10 @@ Init_IO(void)
     rb_vm_register_global_object(rb_default_rs);
     rb_rs = rb_default_rs;
     rb_output_rs = Qnil;
-    rb_define_hooked_variable("$/", &rb_rs, 0, deprecated_str_setter);
-    rb_define_hooked_variable("$-0", &rb_rs, 0, deprecated_str_setter);
+    rb_define_hooked_variable("$/", &rb_rs, 0, deprecated_rs_setter);
+    rb_gvar_ractor_local("$/"); // not local but ractor safe
+    rb_define_hooked_variable("$-0", &rb_rs, 0, deprecated_rs_setter);
+    rb_gvar_ractor_local("$-0"); // not local but ractor safe
     rb_define_hooked_variable("$\\", &rb_output_rs, 0, deprecated_str_setter);
 
     rb_define_virtual_variable("$_", get_LAST_READ_LINE, set_LAST_READ_LINE);

ruby.c

@@ -1325,11 +1325,11 @@ proc_0_option(ruby_cmdline_options_t *opt, const char *s)
     if (v > 0377)
         rb_rs = Qnil;
     else if (v == 0 && numlen >= 2) {
-        rb_rs = rb_str_new2("");
+        rb_rs = rb_fstring_lit("");
     }
     else {
         c = v & 0xff;
-        rb_rs = rb_str_new(&c, 1);
+        rb_rs = rb_str_freeze(rb_str_new(&c, 1));
     }
     return s;
 }

spec/ruby/command_line/dash_0_spec.rb

@@ -0,0 +1,13 @@
+require_relative '../spec_helper'
+
+describe "The -0 command line option" do
+  it "sets $/ and $-0" do
+    ruby_exe("puts $/, $-0", options: "-072").should == ":\n:\n"
+  end
+
+  ruby_version_is "3.5" do
+    it "sets $/ and $-0 as a frozen string" do
+      ruby_exe("puts $/.frozen?, $-0.frozen?", options: "-072").should == "true\ntrue\n"
+    end
+  end
+end

spec/ruby/language/predefined_spec.rb

@@ -559,12 +559,39 @@ describe "Predefined global $/" do
     $VERBOSE = @verbose
   end
 
-  it "can be assigned a String" do
-    str = "abc"
-    $/ = str
-    $/.should equal(str)
+  ruby_version_is ""..."3.5" do
+    it "can be assigned a String" do
+      str = +"abc"
+      $/ = str
+      $/.should equal(str)
+    end
   end
 
+  ruby_version_is "3.5" do
+    it "makes a new frozen String from the assigned String" do
+      string_subclass = Class.new(String)
+      str = string_subclass.new("abc")
+      str.instance_variable_set(:@ivar, 1)
+      $/ = str
+      $/.should.frozen?
+      $/.should be_an_instance_of(String)
+      $/.should_not.instance_variable_defined?(:@ivar)
+      $/.should == str
+    end
+
+    it "makes a new frozen String if it's not frozen" do
+      str = +"abc"
+      $/ = str
+      $/.should.frozen?
+      $/.should == str
+    end
+
+    it "assigns the given String if it's frozen and has no instance variables" do
+      str = "abc".freeze
+      $/ = str
+      $/.should equal(str)
+    end
+  end
   it "can be assigned nil" do
     $/ = nil
     $/.should be_nil
@@ -608,10 +635,38 @@ describe "Predefined global $-0" do
     $VERBOSE = @verbose
   end
 
-  it "can be assigned a String" do
-    str = "abc"
-    $-0 = str
-    $-0.should equal(str)
+  ruby_version_is ""..."3.5" do
+    it "can be assigned a String" do
+      str = +"abc"
+      $-0 = str
+      $-0.should equal(str)
+    end
+  end
+
+  ruby_version_is "3.5" do
+    it "makes a new frozen String from the assigned String" do
+      string_subclass = Class.new(String)
+      str = string_subclass.new("abc")
+      str.instance_variable_set(:@ivar, 1)
+      $-0 = str
+      $-0.should.frozen?
+      $-0.should be_an_instance_of(String)
+      $-0.should_not.instance_variable_defined?(:@ivar)
+      $-0.should == str
+    end
+
+    it "makes a new frozen String if it's not frozen" do
+      str = +"abc"
+      $-0 = str
+      $-0.should.frozen?
+      $-0.should == str
+    end
+
+    it "assigns the given String if it's frozen and has no instance variables" do
+      str = "abc".freeze
+      $-0 = str
+      $-0.should equal(str)
+    end
   end
 
   it "can be assigned nil" do

string.c

@@ -1480,6 +1480,14 @@ rb_str_new_frozen_String(VALUE orig)
     return str_new_frozen(rb_cString, orig);
 }
 
+
+VALUE
+rb_str_frozen_bare_string(VALUE orig)
+{
+    if (RB_LIKELY(BARE_STRING_P(orig) && OBJ_FROZEN_RAW(orig))) return orig;
+    return str_new_frozen(rb_cString, orig);
+}
+
 VALUE
 rb_str_tmp_frozen_acquire(VALUE orig)
 {