GzipReader: internal_encoding option overwrites external_encoding

(Ruby version 3.3.1)

Hi!

When I specify the internal_encoding option to GzipReader.open or GzipReader.new, this seems to override whatever was set by the external_encoding option.

Example:

When not specifying internal_encoding, we get the expected external_encoding:

Zlib::GzipReader.open('myfile.gz', external_encoding: 'iso-8859-1') do |gz|
  puts gz.external_encoding
end

output:

ISO-8859-1

But with internal_encoding specified, external_encoding is changed:

Zlib::GzipReader.open('myfile.gz', external_encoding: 'iso-8859-1', internal_encoding: 'utf-8') do |gz|
  puts gz.external_encoding
end

output:

UTF-8

This seems broken, right? I'm not getting the problem with File.open.

Additionally, I noticed GzipReader does not respond to internal_encoding:

Zlib::GzipReader.open('myfile.gz') do |gz|
  puts gz.internal_encoding
end

This throws NoMethodError with message:

undefined method `internal_encoding' for #<Zlib::GzipReader:0x00007f4c616a4e10> (NoMethodError)
Did you mean?  external_encoding

Regards,
Olof

Start by reproducing the two GzipReader examples from the issue on Ruby 3.3.1 and compare their encoding behavior with File.open. Trace the GzipReader API and encoding handling in the zlib repository; done means external_encoding remains ISO-8859-1 when internal_encoding is also supplied and the expected internal_encoding behavior is established.