環境

macOS Catalina 10.15.5
Rails 6.0.3

Railsチュートリアルとそれに付随するいろいろを書いていきます。
Githubに慣れたいので、チュートリアルは第6版に準拠しています。

1.5.1 Herokuのセットアップとデプロイ

リスト 1.18

Gemfile

source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

gem 'rails',      '6.0.3'
gem 'puma',       '4.3.4'
gem 'sass-rails', '5.1.0'
gem 'webpacker',  '4.0.7'
gem 'turbolinks', '5.2.0'
gem 'jbuilder',   '2.9.1'
gem 'bootsnap',   '1.4.5', require: false

group :development, :test do
  gem 'sqlite3', '1.4.1'
  gem 'byebug',  '11.0.1', platforms: [:mri, :mingw, :x64_mingw]
end

group :development do
  gem 'web-console',           '4.0.1'
  gem 'listen',                '3.1.5'
  gem 'spring',                '2.1.0'
  gem 'spring-watcher-listen', '2.0.1'
end

group :test do
  gem 'capybara',           '3.28.0'
  gem 'selenium-webdriver', '3.142.4'
  gem 'webdrivers',         '4.1.2'
end

group :production do
  gem 'pg', '1.1.4'
end

上記のように記載変更して$ bundle install --without production実行。

しかし以下のエラー発生。

You have requested:
  spring = 2.1.0

The bundle currently has spring locked at 2.1.1.
Try running `bundle update spring`

If you are updating multiple gems in your Gemfile at once,
try passing them all to `bundle update`

エラーに従いbundle update springしたが、
今度はgemlistのpumaに関するエラー。

An error occurred while installing puma (4.3.4), and Bundler cannot continue.
Make sure that `gem install puma -v '4.3.4' --source 'https://rubygems.org/'` succeeds before
bundling.

% gem install puma -v 4.3.4を実行。

Fetching puma-4.3.4.gem
Building native extensions. This could take a while...
ERROR:  Error installing puma:
	ERROR: Failed to build gem native extension.

    current directory: /Users/user/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/puma-4.3.4/ext/puma_http11
/Users/user/.rbenv/versions/2.7.0/bin/ruby -I /Users/user/.rbenv/versions/2.7.0/lib/ruby/2.7.0 -r ./siteconf20200913-23274-ktgz78.rb extconf.rb
checking for BIO_read() in -lcrypto... yes
checking for SSL_CTX_new() in -lssl... yes
checking for openssl/bio.h... yes
checking for DTLS_method() in openssl/ssl.h... yes
checking for TLS_server_method() in openssl/ssl.h... yes
checking for SSL_CTX_set_min_proto_version in openssl/ssl.h... yes
creating Makefile

current directory: /Users/user/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/puma-4.3.4/ext/puma_http11
make "DESTDIR=" clean

current directory: /Users/user/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/puma-4.3.4/ext/puma_http11
make "DESTDIR="
compiling http11_parser.c
ext/puma_http11/http11_parser.c:44:18: warning: unused variable 'puma_parser_en_main' [-Wunused-const-variable]
static const int puma_parser_en_main = 1;
                 ^
1 warning generated.
compiling io_buffer.c
compiling mini_ssl.c
mini_ssl.c:145:7: warning: unused variable 'min' [-Wunused-variable]
  int min, ssl_options;
      ^
mini_ssl.c:299:40: warning: function 'raise_error' could be declared with attribute 'noreturn' [-Wmissing-noreturn]
void raise_error(SSL* ssl, int result) {
                                       ^
2 warnings generated.
compiling puma_http11.c
puma_http11.c:203:22: error: implicitly declaring library function 'isspace' with type 'int (int)' [-Werror,-Wimplicit-function-declaration]
  while (vlen > 0 && isspace(value[vlen - 1])) vlen--;
                     ^
puma_http11.c:203:22: note: include the header <ctype.h> or explicitly provide a declaration for 'isspace'
1 error generated.
make: *** [puma_http11.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/user/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/puma-4.3.4 for inspection.
Results logged to /Users/user/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/extensions/x86_64-darwin-19/2.7.0/puma-4.3.4/gem_make.out

改めて$ bundle installしたが以下ループ。
$ gem listでもpuma　4.3.4になっている。

いろいろ調べた結果、下記記事を参考。
https://qiita.com/aiandrox/items/9389696ebc3cc6d3422e

puma4.3.6で対応したそうなのでGemfile変更。

Gemfile

gem 'puma',       '4.3.6'

$ bundle updateの後、
問題なく$ bundle installが通った。