Introduce explicit rails server handler option

I mistype `rails server production` instead of `rails server -e
production` expecting to lunch a server in the production environment
all the time. However, the signature of `rails server --help` is:

```
Usage:
  rails server [puma, thin etc] [options]
```

This means that the `production` argument is being interpreted as a Rack
server handler like Puma, Thin or Unicorn.

Should we argue for the `rails server production`? I'm not sure of the
reasons, but the `rails console production` behavior was deprecated in:
https://github.com/rails/rails/pull/29358, so parity with the existing
`rails console production` usage may not hold anymore.

In any case, this PR introduces an explicit option for the Rack servers
configuration. The option is called `--using` (or `-u` for short) to
avoid the `rails server --server` tantrum.

The new interface of `rails server` is:

```
Usage:
  rails server [using] [options]

Options:
  -p, [--port=port]                        # Runs Rails on the specified port - defaults to 3000.
  -b, [--binding=IP]                       # Binds Rails to the specified IP - defaults to 'localhost' in development and '0.0.0.0' in other environments'.
  -c, [--config=file]                      # Uses a custom rackup configuration.
                                           # Default: config.ru
  -d, [--daemon], [--no-daemon]            # Runs server as a Daemon.
  -e, [--environment=name]                 # Specifies the environment to run this server under (development/test/production).
  -u, [--using=name]                       # Specifies the Rack server used to run the application (thin/puma/webrick).
  -P, [--pid=PID]                          # Specifies the PID file.
                                           # Default: tmp/pids/server.pid
  -C, [--dev-caching], [--no-dev-caching]  # Specifies whether to perform caching in development.
      [--early-hints], [--no-early-hints]  # Enables HTTP/2 early hints.
```

As a bonus, if you mistype the server to use, you'll get an
auto-correction message:

```
$ rails s tin
Could not find handler "tin". Maybe you meant "thin" or "cgi"?
Run `rails server --help` for more options.
```

3 files changed, 119 insertions, 20 deletions

diff --git a/railties/CHANGELOG.md b/railties/CHANGELOG.md
index a38888afe5..335eb6c4e3 100644
--- a/railties/CHANGELOG.md
+++ b/railties/CHANGELOG.md
@@ -1,5 +1,23 @@
 ## Rails 6.0.0.alpha (Unreleased) ##
 
+*   Deprecate passing Rack server name as a regular argument to `rails server`.
+
+    Previously:
+
+        $ bin/rails server thin
+
+    There wasn't an explicit option for the Rack server to use, now we have the
+    `--using` option with the `-u` short switch.
+
+    Now:
+
+        $ bin/rails server -u thin
+
+    This change also improves the error message if a missing or mistyped rack
+    server is given.
+
+    *Genadi Samokovarov*
+
 *   Add "rails routes --expanded" option to output routes in expanded mode like
     "psql --expanded". Result looks like:
 
diff --git a/railties/lib/rails/commands/server/server_command.rb b/railties/lib/rails/commands/server/server_command.rb
index e546fe3e4b..44b26d3846 100644
--- a/railties/lib/rails/commands/server/server_command.rb
+++ b/railties/lib/rails/commands/server/server_command.rb
@@ -44,7 +44,6 @@ module Rails
     end
 
     def start
-      print_boot_information
       trap(:INT) { exit }
       create_tmp_directories
       setup_dev_caching
@@ -52,9 +51,14 @@ module Rails
 
       super
     ensure
-      # The '-h' option calls exit before @options is set.
-      # If we call 'options' with it unset, we get double help banners.
-      puts "Exiting" unless @options && options[:daemonize]
+      yield
+    end
+
+    def serveable? # :nodoc:
+      server
+      true
+    rescue LoadError, NameError
+      false
     end
 
     def middleware
@@ -65,6 +69,10 @@ module Rails
       super.merge(@default_options)
     end
 
+    def served_url
+      "#{options[:SSLEnable] ? 'https' : 'http'}://#{host}:#{port}" unless use_puma?
+    end
+
     private
       def setup_dev_caching
         if options[:environment] == "development"
@@ -72,13 +80,6 @@ module Rails
         end
       end
 
-      def print_boot_information
-        url = "on #{options[:SSLEnable] ? 'https' : 'http'}://#{options[:Host]}:#{options[:Port]}" unless use_puma?
-        puts "=> Booting #{ActiveSupport::Inflector.demodulize(server)}"
-        puts "=> Rails #{Rails.version} application starting in #{Rails.env} #{url}"
-        puts "=> Run `rails server -h` for more startup options"
-      end
-
       def create_tmp_directories
         %w(cache pids sockets).each do |dir_to_make|
           FileUtils.mkdir_p(File.join(Rails.root, "tmp", dir_to_make))
@@ -108,9 +109,15 @@ module Rails
 
   module Command
     class ServerCommand < Base # :nodoc:
+      # Hard-coding a bunch of handlers here as we don't have a public way of
+      # querying them from the Rack::Handler registry.
+      RACK_SERVERS = %w(cgi fastcgi webrick lsws scgi thin puma unicorn)
+
       DEFAULT_PORT = 3000
       DEFAULT_PID_PATH = "tmp/pids/server.pid".freeze
 
+      argument :using, optional: true
+
       class_option :port, aliases: "-p", type: :numeric,
         desc: "Runs Rails on the specified port - defaults to 3000.", banner: :port
       class_option :binding, aliases: "-b", type: :string,
@@ -122,6 +129,8 @@ module Rails
         desc: "Runs server as a Daemon."
       class_option :environment, aliases: "-e", type: :string,
         desc: "Specifies the environment to run this server under (development/test/production).", banner: :name
+      class_option :using, aliases: "-u", type: :string,
+        desc: "Specifies the Rack server used to run the application (thin/puma/webrick).", banner: :name
       class_option :pid, aliases: "-P", type: :string, default: DEFAULT_PID_PATH,
         desc: "Specifies the PID file."
       class_option "dev-caching", aliases: "-C", type: :boolean, default: nil,
@@ -132,19 +141,28 @@ module Rails
       def initialize(args = [], local_options = {}, config = {})
         @original_options = local_options
         super
-        @server = self.args.shift
+        @using = deprecated_positional_rack_server(using) || options[:using]
         @log_stdout = options[:daemon].blank? && (options[:environment] || Rails.env) == "development"
       end
 
       def perform
         set_application_directory!
         prepare_restart
+
         Rails::Server.new(server_options).tap do |server|
           # Require application after server sets environment to propagate
           # the --environment option.
           require APP_PATH
           Dir.chdir(Rails.application.root)
-          server.start
+
+          if server.serveable?
+            print_boot_information(server.server, server.served_url)
+            server.start do
+              say "Exiting" unless options[:daemon]
+            end
+          else
+            say rack_server_suggestion(using)
+          end
         end
       end
 
@@ -152,7 +170,7 @@ module Rails
         def server_options
           {
             user_supplied_options: user_supplied_options,
-            server:                @server,
+            server:                using,
             log_stdout:            @log_stdout,
             Port:                  port,
             Host:                  host,
@@ -226,7 +244,7 @@ module Rails
         end
 
         def restart_command
-          "bin/rails server #{@server} #{@original_options.join(" ")} --restart"
+          "bin/rails server #{using} #{@original_options.join(" ")} --restart"
         end
 
         def early_hints
@@ -238,12 +256,50 @@ module Rails
         end
 
         def self.banner(*)
-          "rails server [puma, thin etc] [options]"
+          "rails server [thin/puma/webrick] [options]"
         end
 
         def prepare_restart
           FileUtils.rm_f(options[:pid]) if options[:restart]
         end
+
+        def deprecated_positional_rack_server(value)
+          if value
+            ActiveSupport::Deprecation.warn(<<-MSG.squish)
+              Passing the Rack server name as a regular argument is deprecated
+              and will be removed in the next Rails version. Please, use the -u
+              option instead.
+            MSG
+            value
+          end
+        end
+
+        def rack_server_suggestion(server)
+          if server.in?(RACK_SERVERS)
+            <<~MSG
+              Could not load server "#{server}". Maybe you need to the add it to the Gemfile?
+
+                gem "#{server}"
+
+              Run `rails server --help` for more options.
+            MSG
+          else
+            suggestions = Rails::Command::Spellchecker.suggest(server, from: RACK_SERVERS).map(&:inspect)
+
+            <<~MSG
+              Could not find server "#{server}". Maybe you meant #{suggestions.first} or #{suggestions.second}?
+              Run `rails server --help` for more options.
+            MSG
+          end
+        end
+
+        def print_boot_information(server, url)
+          say <<~MSG
+            => Booting #{ActiveSupport::Inflector.demodulize(server)}
+            => Rails #{Rails.version} application starting in #{Rails.env} #{url}
+            => Run `rails server --help` for more startup options
+          MSG
+        end
     end
   end
 end
diff --git a/railties/test/commands/server_test.rb b/railties/test/commands/server_test.rb
index 33715ea75f..c84be439e4 100644
--- a/railties/test/commands/server_test.rb
+++ b/railties/test/commands/server_test.rb
@@ -1,15 +1,15 @@
 # frozen_string_literal: true
 
-require "abstract_unit"
+require "isolation/abstract_unit"
 require "env_helpers"
 require "rails/command"
 require "rails/commands/server/server_command"
 
-class Rails::ServerTest < ActiveSupport::TestCase
+class Rails::Command::ServerCommandTest < ActiveSupport::TestCase
   include EnvHelpers
 
   def test_environment_with_server_option
-    args = ["thin", "-e", "production"]
+    args = ["-u", "thin", "-e", "production"]
     options = parse_arguments(args)
     assert_equal "production", options[:environment]
     assert_equal "thin", options[:server]
@@ -22,6 +22,24 @@ class Rails::ServerTest < ActiveSupport::TestCase
     assert_nil options[:server]
   end
 
+  def test_explicit_using_option
+    args = ["-u", "thin"]
+    options = parse_arguments(args)
+    assert_equal "thin", options[:server]
+  end
+
+  def test_using_server_mistype
+    assert_match(/Could not find server "tin". Maybe you meant "thin" or "cgi"/, run_command("--using", "tin"))
+  end
+
+  def test_using_positional_argument_deprecation
+    assert_match(/DEPRECATION WARNING/, run_command("tin"))
+  end
+
+  def test_using_known_server_that_isnt_in_the_gemfile
+    assert_match(/Could not load server "unicorn". Maybe you need to the add it to the Gemfile/, run_command("-u", "unicorn"))
+  end
+
   def test_daemon_with_option
     args = ["-d"]
     options = parse_arguments(args)
@@ -35,7 +53,7 @@ class Rails::ServerTest < ActiveSupport::TestCase
   end
 
   def test_server_option_without_environment
-    args = ["thin"]
+    args = ["-u", "thin"]
     with_rack_env nil do
       with_rails_env nil do
         options = parse_arguments(args)
@@ -222,6 +240,13 @@ class Rails::ServerTest < ActiveSupport::TestCase
   end
 
   private
+    def run_command(*args)
+      build_app
+      rails "server", *args
+    ensure
+      teardown_app
+    end
+
     def parse_arguments(args = [])
       Rails::Command::ServerCommand.new([], args).server_options
     end