Ensure ServerProcess is started only once

lib/tailwindcss/server_process.rb

@@ -6,20 +6,12 @@ def self.start
       new.start
     end
 
-    def start
+    def initialize
       @server = Server.new(self)
-      @pid = fork do
-        monitor_server
-        exit_hook
-        # Using IO.popen(command, 'r+') will avoid watch_command read from $stdin.
-        # If we use system(*command) instead, IRB and Debug can't read from $stdin
-        # correctly bacause some keystrokes will be taken by watch_command.
-        IO.popen(Commands.watch_command, 'r+') do |io|
-          IO.copy_stream(io, $stdout)
-        end
-      end
-      Process.detach pid
+    end
 
+    def start
+      @pid = existing_process || start_process
       server.monitor_process
       server.exit_hook
     end
@@ -41,6 +33,37 @@ def dead?
 
     private
 
+    def existing_process
+      if (pid = Pidfile.pid)
+        begin
+          Process.kill 0, pid
+          pid
+        rescue Errno::ESRCH
+          # Process does not exist
+        rescue Errno::EPERM
+          # Ignore process owned by another user
+        end
+      end
+    end
+
+    def start_process
+      pid = fork do
+        Pidfile.write
+        monitor_server
+        exit_hook
+        # Using IO.popen(command, 'r+') will avoid watch_command read from $stdin.
+        # If we use system(*command) instead, IRB and Debug can't read from $stdin
+        # correctly bacause some keystrokes will be taken by watch_command.
+        IO.popen(Commands.watch_command, 'r+') do |io|
+          IO.copy_stream(io, $stdout)
+        end
+      ensure
+        Pidfile.delete
+      end
+      Process.detach pid
+      pid
+    end
+
     def monitor_server
       Thread.new do
         loop do
@@ -60,6 +83,33 @@ def exit_hook
       end
     end
 
+    module Pidfile
+      def self.path
+        Rails.root.join("tmp", "pids", "tailwindcss.txt")
+      end
+
+      def self.read
+        File.read(path, mode: "rb:UTF-8")
+      rescue Errno::ENOENT
+        # File does not exist
+      end
+
+      def self.write
+        File.write(path, Process.pid, mode: "wb:UTF-8")
+      end
+
+      def self.delete
+        File.exist?(path) && File.delete(path)
+      end
+
+      def self.pid
+        Integer(read)
+      rescue ArgumentError, TypeError
+        # Invalid content
+        delete
+      end
+    end
+
     class Server
       attr_reader :process, :pid