Do not remove pre-existing socket after USR2+TERM (#2816) (#2817)

The issue was that inherited socket was added to the @unix_paths list even if it was pre-existing.
Test added.

Closed #2816.

lib/puma/binder.rb

@@ -189,7 +189,7 @@ module Puma
           end
 
           if fd = @inherited_fds.delete(str)
-            @unix_paths << path unless abstract
+            @unix_paths << path unless abstract || File.exist?(path)
             io = inherit_unix_listener path, fd
             log_writer.log "* Inherited #{str}"
           elsif sock = @activated_sockets.delete([ :unix, path ]) ||

test/test_integration_cluster.rb

@@ -43,6 +43,26 @@ class TestIntegrationCluster < TestIntegration
     end
   end
 
+  def test_pre_existing_unix_stop_after_restart
+    skip_unless :unix
+
+    File.open(@bind_path, mode: 'wb') { |f| f.puts 'pre existing' }
+
+    cli_server "-w #{workers} -q test/rackup/sleep_step.ru", unix: :unix
+    connection = connect(nil, unix: true)
+    restart_server connection
+
+    connect(nil, unix: true)
+    stop_server
+
+    assert File.exist?(@bind_path)
+
+  ensure
+    if UNIX_SKT_EXIST
+      File.unlink @bind_path if File.exist? @bind_path
+    end
+  end
+
   def test_siginfo_thread_print
     skip_unless_signal_exist? :INFO