`bundle install` fails on macOS

Start with README.asc and the Gemfile entry for kindlegen, then reproduce the reported bundle install failure on macOS using the versions in the issue. Trace the kindlegen installation and its native-extension download before deciding what project change is appropriate. Done means the documented installation completes successfully on the affected environment.

When running bundle install by following the instructions in README.asc, I encountered the following error:

% bundle install
Fetching gem metadata from https://rubygems.org/...........
Resolving dependencies...
(omitted)
Installing kindlegen 3.1.1 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/sakuramochi/.gem/ruby/3.4.8/gems/kindlegen-3.1.1/ext
/Users/sakuramochi/.rubies/ruby-3.4.8/bin/ruby -rrubygems /Users/sakuramochi/.gem/ruby/3.4.8/gems/rake-13.3.1/exe/rake RUBYARCHDIR\=/Users/sakuramochi/.gem/ruby/3.4.8/extensions/x86_64-darwin-24/3.4.0-static/kindlegen-3.1.1
RUBYLIBDIR\=/Users/sakuramochi/.gem/ruby/3.4.8/extensions/x86_64-darwin-24/3.4.0-static/kindlegen-3.1.1
rake aborted!
Zlib::GzipFile::Error: not in gzip format (Zlib::GzipFile::Error)
/Users/sakuramochi/.gem/ruby/3.4.8/gems/kindlegen-3.1.1/ext/Rakefile:31:in 'Zlib::GzipReader#initialize'
/Users/sakuramochi/.gem/ruby/3.4.8/gems/kindlegen-3.1.1/ext/Rakefile:31:in 'Zlib::GzipReader.open'
/Users/sakuramochi/.gem/ruby/3.4.8/gems/kindlegen-3.1.1/ext/Rakefile:31:in 'block in Object#create_task_for_unix'
/Users/sakuramochi/.gem/ruby/3.4.8/gems/rake-13.3.1/exe/rake:27:in '<main>'
Tasks: TOP => default => install => kindlegen
(See full trace by running task with --trace)
open(https://web.archive.org/web/20200814013519/https://kindlegen.s3.amazonaws.com/KindleGen_Mac_i386_v2_9.zip)
save to KindleGen_Mac_i386_v2_9.zip

rake failed, exit code 1

Gem files will remain installed in /Users/sakuramochi/.gem/ruby/3.4.8/gems/kindlegen-3.1.1 for inspection.
Results logged to /Users/sakuramochi/.gem/ruby/3.4.8/extensions/x86_64-darwin-24/3.4.0-static/kindlegen-3.1.1/gem_make.out

  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/rubygems/ext/builder.rb:139:in 'Gem::Ext::Builder.run'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/rubygems/ext/rake_builder.rb:33:in 'Gem::Ext::RakeBuilder.build'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/rubygems/ext/builder.rb:218:in 'Gem::Ext::Builder#build_extension'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/rubygems/ext/builder.rb:250:in 'block in Gem::Ext::Builder#build_extensions'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/rubygems/ext/builder.rb:249:in 'Array#each'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/rubygems/ext/builder.rb:249:in 'Gem::Ext::Builder#build_extensions'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/rubygems/installer.rb:809:in 'Gem::Installer#build_extensions'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/rubygems_gem_installer.rb:115:in 'Bundler::RubyGemsGemInstaller#build_extensions'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/rubygems_gem_installer.rb:30:in 'Bundler::RubyGemsGemInstaller#install'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/source/rubygems.rb:217:in 'block in Bundler::Source::Rubygems#install'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/rubygems.rb:1052:in 'Gem.time'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/source/rubygems.rb:216:in 'Bundler::Source::Rubygems#install'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/installer/gem_installer.rb:54:in 'Bundler::GemInstaller#install'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/installer/gem_installer.rb:17:in 'Bundler::GemInstaller#install_from_spec'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/installer/parallel_installer.rb:133:in 'Bundler::ParallelInstaller#do_install'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/installer/parallel_installer.rb:124:in 'block in Bundler::ParallelInstaller#worker_pool'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/worker.rb:62:in 'Bundler::Worker#apply_func'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/worker.rb:57:in 'block in Bundler::Worker#process_queue'
  <internal:kernel>:168:in 'Kernel#loop'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/worker.rb:54:in 'Bundler::Worker#process_queue'
  /Users/sakuramochi/.rubies/ruby-3.4.8/lib/ruby/site_ruby/3.4.0/bundler/worker.rb:90:in 'block (2 levels) in Bundler::Worker#create_threads'

An error occurred while installing kindlegen (3.1.1), and Bundler cannot continue.

In Gemfile:
  kindlegen

Environment:

% uname -a
Darwin sakuramochi.local 24.6.0 Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:34 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T8103 x86_64
% ruby --version
ruby 3.4.8 (2025-12-17 revision 995b59f666) +PRISM [x86_64-darwin24]