Fix Supervisor monitoring in Puma's plugin

Puma fires up the background hooks before the ```on_booted``` event. Since the
plugin registers the ```monitor_solid_queue``` after Puma is fully booted, it
doesn't get executed.

The fix is to move the ```monitor_solid_queue``` out of the ```on_booted```
and check whether the SolidQueue has started in the ```solid_queue_dead?```.

Author: kolbasov

lib/puma/plugin/solid_queue.rb

@@ -7,15 +7,15 @@ def start(launcher)
     @log_writer = launcher.log_writer
     @puma_pid = $$
 
+    in_background do
+      monitor_solid_queue
+    end
+
     launcher.events.on_booted do
       @solid_queue_pid = fork do
         Thread.new { monitor_puma }
         SolidQueue::Supervisor.start(mode: :all)
       end
-
-      in_background do
-        monitor_solid_queue
-      end
     end
 
     launcher.events.on_stopped { stop_solid_queue }
@@ -51,12 +51,18 @@ def monitor(process_dead, message)
     end
 
     def solid_queue_dead?
-      Process.waitpid(solid_queue_pid, Process::WNOHANG)
+      if solid_queue_started?
+        Process.waitpid(solid_queue_pid, Process::WNOHANG)
+      end
       false
     rescue Errno::ECHILD, Errno::ESRCH
       true
     end
 
+    def solid_queue_started?
+      solid_queue_pid.present?
+    end
+
     def puma_dead?
       Process.ppid != puma_pid
     end

test/integration/puma/plugin_test.rb

@@ -26,7 +26,8 @@ class PumaPluginTest < ActiveSupport::TestCase
   end
 
   teardown do
-    terminate_process(@pid, signal: :INT)
+    terminate_process(@pid, signal: :INT) if process_exists?(@pid)
+
     wait_for_registered_processes 0, timeout: 1.second
 
     JobResult.delete_all
@@ -49,4 +50,13 @@ class PumaPluginTest < ActiveSupport::TestCase
     wait_for_jobs_to_finish_for(2.seconds)
     assert_equal 1, JobResult.where(queue_name: :background, status: "completed", value: :puma_plugin).count
   end
+
+  test "stop puma when solid queue's supervisor dies" do
+    supervisor = find_processes_registered_as("Supervisor").first
+
+    signal_process(supervisor.pid, :KILL)
+    wait_for_process_termination_with_timeout(@pid)
+
+    assert_not process_exists?(@pid)
+  end
 end