[ruby-core:122535] [Ruby Bug#20998] rb_str_locktmp() changes flags of frozen strings and string literals

15 Jun 2025

      Issue #20998 has been updated by ioquatix (Samuel Williams).

My usage of `rb_str_locktmp` isn't about mutation, it's about making sure the pointer doesn't change. Could compacting GC cause embedded frozen strings to be moved?

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Bug #20998: rb_str_locktmp() changes flags of frozen strings and string literals
https://bugs.ruby-lang.org/issues/20998#change-113761

* Author: Eregon (Benoit Daloze)
* Status: Open
* Assignee: Eregon (Benoit Daloze)
* ruby -v: ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
```ruby
# frozen_string_literal: true
# BOILERPLATE START

require 'tmpdir'
require 'rbconfig'

def inline_c_extension(c_code)
  Dir.mktmpdir('inline_c_extension') do |dir|
    File.write("#{dir}/cext.c", c_code)
    File.write("#{dir}/extconf.rb", <<~RUBY)
    require 'mkmf'
    create_makefile('cext')
    RUBY

    out = IO.popen([RbConfig.ruby, 'extconf.rb'], chdir: dir, &:read)
    raise "ruby extconf.rb failed: #{$?.inspect}\n#{out}" unless $?.success?

    out = IO.popen(['make'], chdir: dir, &:read)
    raise "make failed: #{$?.inspect}\n#{out}" unless $?.success?

    require "#{dir}/cext.#{RbConfig::CONFIG['DLEXT']}"
  end
end

inline_c_extension <<~C
#include "ruby.h"

static VALUE foo(VALUE self, VALUE str) {
  rb_str_locktmp(str);
  return str;
}

void Init_cext(void) {
  VALUE c = rb_define_class("Foo", rb_cObject);
  rb_define_singleton_method(c, "foo", foo, 1);
}
C

# BOILERPLATE END

a = "str"
Foo.foo(a)

# imagine a million lines of code in between

b = "str"

b << "."
# can't modify string; temporarily locked (RuntimeError)
# What? Who "locked" that immutable frozen string literal?
# It should be: can't modify frozen String: "str" (FrozenError)
```

Same problem with:
```ruby
Foo.foo("abc")

# imagine a million lines of code in between

Foo.foo("abc") # temporal locking already locked string (RuntimeError)
```

Related: https://github.com/oracle/truffleruby/issues/3752

It seems a clear bug to mutate a frozen string (with visible side effects), even more so for shared frozen string literals.

I think rb_str_locktmp() should raise (a FrozenError, as it's effectively attempting to mutate it, same as calling `rb_str_modify()`) if called on a frozen string, because it makes little sense, I think "locking a string" only makes sense for mutable strings.

The alternative would be to noop on frozen strings for rb_str_locktmp() and rb_str_unlocktmp(), I think that's susprising, and potentially hiding more bugs, e.g. if one by mistake tries to mutate the RSTRING_PTR() or so, and also rb_str_locktmp() wouldn't imply the flag is set after it returns.
Any attempt to mutate a frozen string should fail, so might as well fail early.

-- 
https://bugs.ruby-lang.org/