Add enhanced default bin/dev

Fixes rails#52459

The current default `bin/dev` execs the Rails server directly and
doesn't support Procfiles. Therefore, gems like `jsbundling-rails`,
`tailwindcss-rails`, and soon `solid_queue` need to overwrite the
`bin/dev` script with their own version.

With this new version, gems can skip overwriting the `bin/dev` script
and add to the Procfile instead.

It spawns Foreman in the background to allow breakpoints in the Rails
process without Foreman eating the inputs or interleaving its output.

railties/lib/rails/commands/devserver/devserver_command.rb

@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+module Rails
+  module Command
+    class DevserverCommand < Base # :nodoc:
+      DEFAULT_PORT = 3000
+      PROCFILE = "Procfile.dev"
+
+      class_option :verbose, aliases: "-v", type: :boolean, default: false,
+        desc: "Show all process outputs."
+      option :port, aliases: "-p", type: :numeric,
+        desc: "Run Rails on the specified port - defaults to 3000.", banner: :port
+
+      desc "devserver", "Start everything"
+      def perform
+        exec_rails  unless procfile_exists?
+
+        Signal.trap("INT") { }
+
+        install_foreman
+
+        begin
+          start_foreman
+          start_rails
+        ensure
+          stop
+        end
+      end
+
+      private
+        attr_reader :foreman_pgid, :pid
+
+        def start_foreman
+          @foreman_pgid =
+            Process.spawn(
+              env,
+              *%W(foreman start -f Procfile.dev -m all=1,web=0 --env /dev/null),
+              in: "/dev/null",
+              out: (verbose ? STDOUT : "/dev/null"),
+              err: (verbose ? STDOUT : "/dev/null"),
+              pgroup: true,
+            ).then(&Process.method(:getpgid))
+        end
+
+        def install_foreman
+          if system(env, *%w(gem list --no-installed --exact --silent foreman))
+            puts "Installing foreman..."
+            system(env, *%w(gem install foreman))
+          end
+        end
+
+        def start_rails
+          @pid = Process.spawn(*%W(bundle exec rails server --port=#{port}))
+          Process.wait(pid)
+          @pid = false
+        end
+
+        def exec_rails
+          Process.exec(env, *%W(bundle exec rails server --port=#{port}))
+        end
+
+        def stop
+          Process.kill("INT", -foreman_pgid) if foreman_pgid
+          Process.wait(-foreman_pgid) if foreman_pgid
+
+          begin
+            Process.kill("KILL", pid) if pid
+          rescue Errno::ESRCH
+          end
+        end
+
+        def procfile_exists?
+          File.exist?(Rails::Command.application_root.join(PROCFILE))
+        end
+
+        def env
+          {
+            "BUNDLER_SETUP" => nil,
+            "RUBYOPT" => nil,
+          }
+        end
+
+        def verbose
+          options[:verbose]
+        end
+
+        def port
+          options[:port] || ENV.fetch("PORT", DEFAULT_PORT).to_i
+        end
+    end
+  end
+end

railties/lib/rails/generators/rails/app/templates/bin/dev.tt

@@ -1 +1,36 @@
-exec "./bin/rails", "server", *ARGV
+# !/usr/bin/env ruby
+
+ENV["PORT"] ||= "3000"
+VERBOSE = ARGV.include?("--verbose") || ARGV.include?("-v") || ENV["VERBOSE"]
+
+PROCFILE = 'Procfile.dev'
+Process.exec(*%w[bundle exec rails server], *ARGV) unless File.exist?(PROCFILE)
+
+Signal.trap("INT") { }
+
+if system(*%w[gem list --no-installed --exact --silent foreman])
+  puts "Installing foreman..."
+  system(*%w[gem install foreman])
+end
+
+begin
+  foreman_pid = Process.spawn(
+    *%w[foreman start -f Procfile.dev -m all=1,web=0 --env /dev/null],
+    in: "/dev/null",
+    out: (VERBOSE ? STDOUT : "/dev/null"),
+    err: (VERBOSE ? STDOUT : "/dev/null"),
+    pgroup: true,
+  )
+  foreman_pgid = Process.getpgid(foreman_pid)
+  pid = Process.spawn(*%w[bundle exec rails server])
+
+  Process.wait(pid)
+  pid = nil
+ensure
+  Process.kill("INT", -foreman_pgid)
+  Process.wait(-foreman_pgid)
+  begin
+    Process.kill("KILL", pid) if pid
+  rescue Errno::ESRCH
+  end
+end