Fix hang on shutdown in test_refork [changelog skip] (#2442)

* Prevent server from starting if worker receives TERM signal
If a worker receives a `TERM` signal before the server has started,
prevent it from starting at all by clearing `restart_server`.

* Send TERM to fork_worker child processes in Cluster#wait_workers
When `fork_worker` is used, the call to `Process.wait` in `wait_workers`
returns `ECHILD` for forked workers.
Add a call to `w.term` in the rescue clause to match the behavior when
`fork_worker` is not used, ensuring workers shut down properly.

* Add test_command_ignored_before_run
Unit test clarifying behavior of commands sent before Server#run is called.

lib/puma/cluster.rb

@@ -479,7 +479,9 @@ module Puma
         rescue Errno::ECHILD
           begin
             Process.kill(0, w.pid)
-            false # child still alive, but has another parent
+            # child still alive but has another parent (e.g., using fork_worker)
+            w.term if w.term?
+            false
           rescue Errno::ESRCH, Errno::EPERM
             true # child is already terminated
           end

lib/puma/cluster/worker.rb

@@ -91,6 +91,7 @@ module Puma
 
         Signal.trap "SIGTERM" do
           @worker_write << "e#{Process.pid}\n" rescue nil
+          restart_server.clear
           server.stop
           restart_server << false
         end

test/test_puma_server.rb

@@ -1150,4 +1150,15 @@ EOF
       assert thread.join(1)
     end
   end
+
+  def test_command_ignored_before_run
+    @server.stop # ignored
+    @server.run
+    @server.halt
+    done = Queue.new
+    @server.events.register(:state) do |state|
+      done << @server.instance_variable_get(:@status) if state == :done
+    end
+    assert_equal :halt, done.pop
+  end
 end