fix: StringIO#write transcodes strings with a different encoding

[flavorjones]

This is a potential fix for #2839. I've based this approach on the transcoding logic in IO#write.

I'm not sure I've placed the new test in the best place; or if this fix is the best option. This is my first PR to TR! Feedback welcome. ❤️

[eregon]

Thank you for the PR!
I'll address my own comments for convenience.

[flavorjones]

Remaining test failures don't seem related to this PR.

[eregon]

I found another edge case, revealed by this failing CRuby test:

  1) Error:
TestGemCommandsSetupCommand#test_show_release_notes:
Encoding::UndefinedConversionError: "\xCF\x80" to US-ASCII in conversion from UTF-8 to US-ASCII
    <internal:core> core/string.rb:373:in `encode!'
    <internal:core> core/string.rb:417:in `encode'
    <internal:core> core/truffle/io_operations.rb:78:in `write_transcoding'
    /b/b/e/graal/sdk/mxbuild/linux-amd64/GRAALVM_5080722BFA_JAVA17/graalvm-5080722bfa-java17-23.0.0-dev/languages/ruby/lib/truffle/stringio.rb:284:in `write'
    <internal:core> core/truffle/io_operations.rb:67:in `block in puts'
    <internal:core> core/truffle/io_operations.rb:34:in `each'
    <internal:core> core/truffle/io_operations.rb:34:in `puts'
    /b/b/e/graal/sdk/mxbuild/linux-amd64/GRAALVM_5080722BFA_JAVA17/graalvm-5080722bfa-java17-23.0.0-dev/languages/ruby/lib/truffle/stringio.rb:96:in `puts'
    /b/b/e/graal/sdk/mxbuild/linux-amd64/GRAALVM_5080722BFA_JAVA17/graalvm-5080722bfa-java17-23.0.0-dev/languages/ruby/lib/mri/rubygems/user_interaction.rb:331:in `say'
    /b/b/e/graal/sdk/mxbuild/linux-amd64/GRAALVM_5080722BFA_JAVA17/graalvm-5080722bfa-java17-23.0.0-dev/languages/ruby/lib/mri/rubygems/user_interaction.rb:152:in `say'
    /b/b/e/graal/sdk/mxbuild/linux-amd64/GRAALVM_5080722BFA_JAVA17/graalvm-5080722bfa-java17-23.0.0-dev/languages/ruby/lib/mri/rubygems/commands/setup_command.rb:572:in `show_release_notes'
    /b/b/e/main/test/mri/tests/rubygems/test_gem_commands_setup_command.rb:394:in `block in test_show_release_notes'
    /b/b/e/graal/sdk/mxbuild/linux-amd64/GRAALVM_5080722BFA_JAVA17/graalvm-5080722bfa-java17-23.0.0-dev/languages/ruby/lib/mri/rubygems/user_interaction.rb:46:in `use_ui'
    /b/b/e/graal/sdk/mxbuild/linux-amd64/GRAALVM_5080722BFA_JAVA17/graalvm-5080722bfa-java17-23.0.0-dev/languages/ruby/lib/mri/rubygems/user_interaction.rb:69:in `use_ui'
    /b/b/e/main/test/mri/tests/rubygems/test_gem_commands_setup_command.rb:393:in `test_show_release_notes'

It tries to write a UTF-8 String with non-ASCII characters into a StringIO which was set_encoding(Encoding::US_ASCII).

I simplified it to:

$ cruby -rstringio -e 'io=StringIO.new.set_encoding(Encoding::US_ASCII); p io.string.encoding; io.write "* π is tasty"; p io.string.encoding; p io.string'
#<Encoding:US-ASCII>
#<Encoding:UTF-8>
"* π is tasty"

So CRuby seems to magically change the StringIO encoding for this case. Waow.

And I wondered if the BINARY case is handled similarly but no, that doesn't change the StringIO encoding:

$ cruby -rstringio -e 'io=StringIO.new.set_encoding(Encoding::US_ASCII); p io.string.encoding; io.write "* π is tasty".b; p io.string.encoding; p io.string'
#<Encoding:US-ASCII>
#<Encoding:US-ASCII>
"* \xCF\x80 is tasty"

So many hacks and inconsistencies (and maybe some bugs) for StringIO encoding :(

[eregon]

The encoding logic for StringIO#write upstream seems to be https://github.com/ruby/stringio/blob/009896b97378311ecaa88c8fa9c32acd7656648a/ext/stringio/stringio.c#L1498-L1506. So special case both BINARY and US-ASCII for the String encoding, in case conversation fail, move on without converting. Yeah, that's a messy hack, but we can copy it.

[flavorjones]

Thank you for the extra work needed to complete this work, @eregon.