The rack example does not work because of Rack::Lint::LintError

[mopp]

Overview

The rack example does not work due to it causes Rack::Lint::LintError: uppercase character in header name: Content-Type.
The reason is the breaking change for HTTP/2 at Rack 3.0.0.

Response header keys can no longer include uppercase characters.

References

• [3.0.0.beta1] - 2022-08-08
• Make response headers a hash and enforce lower-case keys. rack/rack#1592

Environment

Here is my environment.

> ruby --version
ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]

Gemfile.lock

PATH
  remote: ../..
  specs:
    prometheus-client (4.0.0)

GEM
  remote: https://rubygems.org/
  specs:
    kgio (2.11.4)
    rack (3.0.1)
    raindrops (0.20.0)
    unicorn (6.1.0)
      kgio (~> 2.6)
      raindrops (~> 0.7)

PLATFORMS
  x86_64-linux

DEPENDENCIES
  prometheus-client!
  rack
  unicorn

BUNDLED WITH
   2.3.26

The error details

After curl http://localhost:5000

❯ bundle exec unicorn -c ./unicorn.conf 
I, [2022-11-19T23:20:37.816093 #91195]  INFO -- : Refreshing Gem list
/home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rack-3.0.1/lib/rack/chunked.rb:6: warning: Rack::Chunked is deprecated and will be remove
d in Rack 3.1
I, [2022-11-19T23:20:37.834067 #91195]  INFO -- : listening on addr=0.0.0.0:5000 fd=7
I, [2022-11-19T23:20:37.834157 #91195]  INFO -- : worker=0 spawning...
I, [2022-11-19T23:20:37.834850 #91195]  INFO -- : master process ready
I, [2022-11-19T23:20:37.835144 #91210]  INFO -- : worker=0 spawned pid=91210
I, [2022-11-19T23:20:37.835328 #91210]  INFO -- : worker=0 ready
Rack::Lint::LintError: uppercase character in header name: Content-Type
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rack-3.0.1/lib/rack/lint.rb:653:in `block in check_headers'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rack-3.0.1/lib/rack/lint.rb:637:in `each'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rack-3.0.1/lib/rack/lint.rb:637:in `check_headers'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rack-3.0.1/lib/rack/lint.rb:73:in `response'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rack-3.0.1/lib/rack/lint.rb:35:in `call'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rack-3.0.1/lib/rack/show_exceptions.rb:27:in `call'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rack-3.0.1/lib/rack/common_logger.rb:43:in `call'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rack-3.0.1/lib/rack/chunked.rb:102:in `call'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/rack-3.0.1/lib/rack/content_length.rb:20:in `call'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/unicorn-6.1.0/lib/unicorn/http_server.rb:634:in `process_client'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/unicorn-6.1.0/lib/unicorn/http_server.rb:739:in `worker_loop'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/unicorn-6.1.0/lib/unicorn/http_server.rb:547:in `spawn_missing_workers'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/unicorn-6.1.0/lib/unicorn/http_server.rb:143:in `start'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/unicorn-6.1.0/bin/unicorn:128:in `<top (required)>'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/bin/unicorn:25:in `load'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/bin/unicorn:25:in `<top (required)>'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/cli/exec.rb:58:in `load'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/cli/exec.rb:58:in `kernel_load'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/cli/exec.rb:23:in `run'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/cli.rb:486:in `exec'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/cli.rb:31:in `dispatch'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/cli.rb:25:in `start'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.26/exe/bundle:48:in `block in <top (required)>'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/site_ruby/3.1.0/bundler/friendly_errors.rb:120:in `with_friendly_errors'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/lib/ruby/gems/3.1.0/gems/bundler-2.3.26/exe/bundle:36:in `<top (required)>'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/bin/bundle:25:in `load'
        /home/mopp/.anyenv/envs/rbenv/versions/3.1.2/bin/bundle:25:in `<main>'
127.0.0.1 - - [19/Nov/2022:23:20:40 +0900] "GET / HTTP/1.1" 500 82929 0.0159

[Sinjo]

Thanks for the report @mopp! I've opened a PR to fix it over at #268