[ruby-core:119633] [Ruby master Bug#20819] IO#readline does not process newlines correctly for non-ASCII compatible encodings

[ruby-core:119633] [Ruby master Bug#20819] IO#readline does not process newlines correctly for non-ASCII compatible encodings

[javanthropus (Jeremy Bopp) via ruby-core]

Issue #20819 has been reported by javanthropus (Jeremy Bopp).

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Bug #20819: IO#readline does not process newlines correctly for non-ASCII compatible encodings
https://bugs.ruby-lang.org/issues/20819

* Author: javanthropus (Jeremy Bopp)
* Status: Open
* ruby -v: ruby 3.3.4 (2024-07-09 revision be1089c8ec) [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
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When not performing character conversion, IO#readline only processes newline characters as ASCII when reading paragraphs.  However, when character conversion is involved, even when converting between 2 ASCII incompatible encodings, newline handling is correct.

```ruby
require "tempfile"

Tempfile.open(binmode: true) do |f|
  f.set_encoding("utf-16le")
  f.write("\n\n\n\nhello\n\nworld")
  f.rewind

  # No character conversion case.
  # Expecting "hello\n\n".encode(Encoding::UTF_16LE)
  f.readline("")   # => "\0".force_encoding(Encoding::UTF_16LE) + "\n\n\nhello\n\nworld".encode(Encoding::UTF_16LE)

  f.set_encoding("utf-16le:utf-32le")
  f.rewind

  # Character conversion case.
  f.readline("")   # => "hello\n\n".encode(Encoding::UTF_32LE)
end
```

In the failing case, a newline character appears in the first byte of the input due to the UTF-16LE encoding.  This is discarded per the normal behavior of reading paragraphs, but the following null byte is not consumed as required to consume the entire newline character in UTF-16LE encoding.  This leads to a leading and invalid null byte in the output of IO#readline.  Furthermore, the newlines between "hello" and "world" are not seen as a pair of newline characters sufficient to end the first paragraph because they are not ASCII newlines and instead have a null byte between them.



-- 
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