Stop using /dev/null for silent ui for WASI platform

[kateinoigakukun]

What was the end-user or developer problem that led to this PR?

WASI doesn't guarantee that /dev/null is present.
So before this patch, we needed to mount host's /dev directory to WASI guest process to avoid ENOTCAPABLE error while require "bundler/setup"

Here is how to reproduce the problem.

# Download prebuilt ruby
curl -LO https://github.com/ruby/ruby.wasm/releases/download/2022-05-11-a/ruby-head-wasm32-unknown-wasi-full.tar.gz
tar xfz ruby-head-wasm32-unknown-wasi-full.tar.gz

# Install the same version of native ruby to avoid bundler version mismatch in "BUNDLED WITH" of Gemfile.lock
rbenv install 3.2.0-dev
rbenv local 3.2.0-dev

# Setup Gemfile
bundle init
bundle add syntax_tree

# Install gems in vendor/bundle to mount it on WASI
bundle config set --local path 'vendor/bundle'
bundle install

cat <<EOS > my_app.rb
require "bundler/setup"
require "syntax_tree"

pp SyntaxTree.parse("1 + 1")
EOS

# Options:
# --mapdir /usr::./head-wasm32-unknown-wasi-full/usr    Map ruby directory
# --mapdir /root::./                                    Map user script directory
# --env BUNDLE_GEMFILE=/root/Gemfile                    Tell bundler where the Gemfile located because WASI has no PWD concept
wasmtime run \
  --mapdir /usr::./head-wasm32-unknown-wasi-full/usr \
  --mapdir /root::./ \
  --env BUNDLE_GEMFILE=/root/Gemfile \
  head-wasm32-unknown-wasi-full/usr/local/bin/ruby -- /root/my_app.rb
/usr/local/lib/ruby/3.2.0+1/rubygems/user_interaction.rb:621:in `initialize': Capabilities insufficient @ rb_sysopen - /dev/null (Errno::ENOTCAPABLE)
        from /usr/local/lib/ruby/3.2.0+1/rubygems/user_interaction.rb:621:in `open'
        from /usr/local/lib/ruby/3.2.0+1/rubygems/user_interaction.rb:621:in `initialize'
        from /usr/local/lib/ruby/3.2.0+1/bundler/ui/rg_proxy.rb:11:in `initialize'
        from /usr/local/lib/ruby/3.2.0+1/bundler.rb:91:in `new'
        from /usr/local/lib/ruby/3.2.0+1/bundler.rb:91:in `ui='
        from /usr/local/lib/ruby/3.2.0+1/bundler.rb:87:in `ui'
        from /usr/local/lib/ruby/3.2.0+1/bundler/setup.rb:10:in `<top (required)>'
        from <internal:/usr/local/lib/ruby/3.2.0+1/rubygems/core_ext/kernel_require.rb>:85:in `require'
        from <internal:/usr/local/lib/ruby/3.2.0+1/rubygems/core_ext/kernel_require.rb>:85:in `require'
        from /root/my_app.rb:2:in `<main>'

If you add --mapdir /dev::/dev option to wasmtime command as a workaround, it runs expectedly.

What is your fix for the problem, implemented in this PR?

This patch removes the use of /dev/null in Gem::SilentUI to avoid the workaround.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

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[deivid-rodriguez]

This sounds good to me. I'd like to add a test to simulate what's happening in WASI and cover this change, but I understand it could be tricky. Only nit is, could we move the initialize method to still be the first method in the class, like it's done in other classes in this file?

[kateinoigakukun]

OK, I've added a test by faking File.open and adjusted coding-style

[deivid-rodriguez]

Thank you!

I rebased and squashed both commits (to not break future bisections), and will be merging as soon as CI passes.