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From: "herwin (Herwin W) via ruby-core"
Date: 2025-07-18T17:44:26+00:00
Subject: [ruby-core:122817] [Ruby Bug#21517] Behaviour of StringIO#readpartial with multibyte differs from IO#readpartial
Issue #21517 has been reported by herwin (Herwin W).
----------------------------------------
Bug #21517: Behaviour of StringIO#readpartial with multibyte differs from IO#readpartial
https://bugs.ruby-lang.org/issues/21517
* Author: herwin (Herwin W)
* Status: Open
* ruby -v: ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [x86_64-linux] (+ others)
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
The specs for `IO#readpartial` have the following test (https://github.com/ruby/spec/blob/ed254bae321221f5cd24280beb37e5bddaff6bb6/core/io/readpartial_spec.rb#L41-L47):
```ruby
before :each do
@rd, @wr = IO.pipe
@rd.binmode
@wr.binmode
end
it "reads after ungetc with multibyte characters in the buffer" do
@wr.write("�����/���x = ga��t��")
c = @rd.getc
@rd.ungetc(c)
@rd.readpartial(3).should == "\xE2\x88\x82"
@rd.readpartial(3).should == "\xCF\x86/"
end
```
This behaviour is different in `StringIO#readpartial`. If I add this as a spec:
```ruby
it "reads after ungetc with multibyte characters in the buffer" do
@string = StringIO.new(+"�����/���x = ga��t��")
c = @string.getc
@string.ungetc(c)
@string.readpartial(3).should == "\xE2\x88\x82"
@string.readpartial(3).should == "\xCF\x86/"
end
```
it fails with every supported Ruby version, and even with 3.5-dev of today, with the message `Expected "\xE2\x88\x82" == "���"`
The `getc` + `ungetc` can be removed, but this does not change the behaviour.
--
https://bugs.ruby-lang.org/
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