Bug #21562: IO#each_codepoint raises using with IO#ungetc

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 3.4.4 (2025-05-14 revision a38531fd3f) +PRISM [x64-mingw-ucrt]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[YO4 (Yoshinao Muramatsu)]

Using IO#each_codepoint together with IO#ungetc causes byte oriented read for character buffered IO (IOError) when encoding conversion is active.

C:\>ruby -e "open('NUL', 'rt') { _1.ungetc('aa'); _1.each_codepoint { |c| p c }}"
97
-e:1:in 'IO#each_codepoint': byte oriented read for character buffered IO (IOError)
        from -e:1:in 'block in <main>'
        from -e:1:in 'Kernel#open'
        from -e:1:in '<main>'

note: 'rt' enables newline conversion for input, and it uses the encoding conversion mechanism.

When encoding conversion is not used, similar operations complete successfully.

C:\>ruby -e "open('NUL', 'rb') { _1.ungetc('aa'); _1.each_codepoint { |c| p c }}"
97
97

Since both IO#each_codepoint and IO#ungetc handle characters, I believe IO#each_codepoint should work on both cases.

[YO4 (Yoshinao Muramatsu)]

PR here
https://github.com/ruby/ruby/pull/14455