Avoid IO operations that are affected by the presence or absence of an encoding converter

I think that mixing IO#ungetc with certain operations is not a recommended practice, as it may cause issues depending on the platform or input.
Perhaps it would be better to exclude this from testing as well.
I would like to hear your thoughts.

Background

When I'm working on a patched version of CRuby that uses an encoding converter when opening files with the “r” option, I found the following error occurred on CI.
My working branch is at https://github.com/YO4/ruby/tree/newline_fastpath.
I'm aiming to merge this patch, but I'm not sure if I should just skip the spec.

 1)
  IO#getbyte reads after ungetc without character conversion ERROR
  IOError: byte oriented read for character buffered IO
  D:/a/ruby/ruby/src/spec/ruby/core/io/getbyte_spec.rb:33:in 'IO#getbyte'
  D:/a/ruby/ruby/src/spec/ruby/core/io/getbyte_spec.rb:33:in 'block (2 levels) in <top (required)>'
  D:/a/ruby/ruby/src/spec/ruby/core/io/getbyte_spec.rb:5:in '<top (required)>'
  
  2)
  IO#read with internal encoding not specified reads after ungetc ERROR
  IOError: byte oriented read for character buffered IO
  D:/a/ruby/ruby/src/spec/ruby/core/io/read_spec.rb:694:in 'IO#read'
  D:/a/ruby/ruby/src/spec/ruby/core/io/read_spec.rb:694:in 'block (4 levels) in <top (required)>'
  D:/a/ruby/ruby/src/spec/ruby/core/io/read_spec.rb:655:in '<top (required)>'
  
  3)
  IO#readbyte reads after ungetc without character conversion ERROR
  IOError: byte oriented read for character buffered IO
  D:/a/ruby/ruby/src/spec/ruby/core/io/readbyte_spec.rb:29:in 'IO#readbyte'
  D:/a/ruby/ruby/src/spec/ruby/core/io/readbyte_spec.rb:29:in 'block (2 levels) in <top (required)>'
  D:/a/ruby/ruby/src/spec/ruby/core/io/readbyte_spec.rb:3:in '<top (required)>'
  
  4)
  IO#ungetc adjusts the stream position FAILED
  Expected 1 == 0
  to be truthy but was false
  D:/a/ruby/ruby/src/spec/ruby/core/io/ungetc_spec.rb:91:in 'block (2 levels) in <top (required)>'
  D:/a/ruby/ruby/src/spec/ruby/core/io/ungetc_spec.rb:5:in '<top (required)>'

Also, JRuby already uses an encoding converter for crlf conversion in open(name, “r”).

C:\jruby-10.1.1.0\bin>jruby D:/git/ruby/spec/mspec/bin/mspec-run d:/git/ruby/spec/ruby/core/io/getbyte_spec.rb
WARNING: A restricted method in java.lang.System has been called
WARNING: java.lang.System::load has been called by com.kenai.jffi.internal.StubLoader in an unnamed module
WARNING: Use --enable-native-access=ALL-UNNAMED to avoid a warning for callers in this module
WARNING: Restricted methods will be blocked in a future release unless native access is enabled

jruby 10.1.1.0 (4.0.0) 2026-07-22 aaa95d57e1 OpenJDK 64-Bit Server VM 25.0.1+8-LTS on 25.0.1+8-LTS +indy +jit [x86_64-mswin32]

1)
IO#getbyte reads after ungetc without character conversion ERROR
IOError: byte oriented read for character buffered IO
org/jruby/RubyIO.java:3175:in 'getbyte'
D:/git/ruby/spec/ruby/core/io/getbyte_spec.rb:33:in 'block in <main>'
org/jruby/RubyBasicObject.java:2777:in 'instance_exec'
org/jruby/RubyArrayNative.java:4226:in 'all?'
org/jruby/RubyArrayNative.java:1729:in 'each'
D:/git/ruby/spec/ruby/core/io/getbyte_spec.rb:5:in '<main>'
org/jruby/RubyKernel.java:1413:in 'load'
org/jruby/RubyBasicObject.java:2777:in 'instance_exec'
org/jruby/RubyArrayNative.java:1729:in 'each'
[/ | ==================100%================== | 00:00:00]      0F      1E

Finished in 0.044000 seconds

1 file, 6 examples, 6 expectations, 0 failures, 1 error, 0 tagged

JRuby's CI appears to be green at first glance, but I am unfamiliar with that situation.

Start with the failing examples in spec/ruby/core/io/getbyte_spec.rb, read_spec.rb, readbyte_spec.rb, and ungetc_spec.rb, then run them under CRuby and JRuby. Compare the failures involving IO#ungetc, byte-oriented reads, and encoding conversion to determine which expectations are portable. Done means the affected specs have a justified, consistent outcome across the reported configurations.