[ruby-core:116240] [Ruby master Bug#20189] `rb_str_resize` does not clear coderange when expanding

Issue #20189 has been updated by Eregon (Benoit Daloze).


byroot (Jean Boussier) wrote in #note-5:
> I must admit I'm not very familiar with wide char encodings, but this surprises me a bit. Ruby strings should always have their terminator, so I don't see how expanding a string would change their interpretation.

It's because in UTF-16 if the number of bytes is not a multiple of 2 then it's CR_BROKEN. Same for UTF-32 if not a multiple of 4.
And since `rb_str_resize()` changes the String#bytesize then that condition can change:
```ruby
irb(main):002:0> "a".force_encoding(Encoding::UTF_16LE).valid_encoding?
=> false
irb(main):003:0> "a\x00".force_encoding(Encoding::UTF_16LE).valid_encoding?
=> true
```

----------------------------------------
Bug #20189: `rb_str_resize` does not clear coderange when expanding
https://bugs.ruby-lang.org/issues/20189#change-106261

* Author: tompng (tomoya ishida)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.4.0dev (2024-01-09T07:07:19Z master db476cc71c) [x86_64-linux]
* Backport: 3.0: DONTNEED, 3.1: DONTNEED, 3.2: REQUIRED, 3.3: REQUIRED
----------------------------------------
Expanding string in some encoding (utf16 utf32) can change coderange to either valid or broken, but rb_str_resize does not clear coderange.


This will cause a bug in c-extension libraries that use rb_str_resize.
~~~ruby
# Example for stringio
s = StringIO.new("\0".encode('UTF-16LE'))

s.truncate(1); s.truncate(2); s.string.valid_encoding?
#=> true
s.truncate(1); s.string.valid_encoding?; s.truncate(2); s.string.valid_encoding?
#=> false (expect to be true)
~~~




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