Improve systemd notification support

This takes the work by @acmh and improves on it. This is done by
squashing all commits and rebasing it. Then the following changes were
made:

* Dropped SD_NOTIFY env var. There is aleady the NOTIFY_SOCKET env var
  presented by systemd and is redundant.
* Move code is pushed in Puma::Systemd
* on_reload now emits RELOADING=1 notification to systemd
* Drop lower bound check on usec. Systemd can only be configured in
  seconds and it's hard to misconfigure. The actual code should be safe.

docs/systemd.md

@@ -25,20 +25,17 @@ After=network.target
 
 [Service]
 # Puma supports systemd's `Type=notify` and watchdog service
-# monitoring, since you have installed [sd_notify](https://github.com/agis/ruby-sdnotify) gem. 
+# monitoring, since you have installed [sd_notify](https://github.com/agis/ruby-sdnotify) gem.
 # If you are using an earlier version of Puma, change this to `Type=simple`
-# and remove the `WatchdogSec` and `Environment="SD_NOTIFY=true` line.
+# and remove the `WatchdogSec` line.
 Type=notify
 
-# Preferably configure a non-privileged user
-# User=
-
-# Allows Systemd integration
-Environment="SD_NOTIFY=true"
-
 # If your Puma process locks up, systemd's watchdog will restart it within seconds.
 WatchdogSec=10
 
+# Preferably configure a non-privileged user
+# User=
+
 # The path to the your application code root directory.
 # Also replace the "<YOUR_APP_PATH>" place holders below with this path.
 # Example /home/username/myapp

lib/puma/events.rb

@@ -133,18 +133,26 @@ def debug_error(error, req=nil, text="")
       @error_logger.debug(error: error, req: req, text: text)
     end
 
-    def on_stopped(&block)
-      register(:on_stopped, &block)
-    end
-
     def on_booted(&block)
       register(:on_booted, &block)
     end
 
+    def on_restart(&block)
+      register(:on_restart, &block)
+    end
+
+    def on_stopped(&block)
+      register(:on_stopped, &block)
+    end
+
     def fire_on_booted!
       fire(:on_booted)
     end
 
+    def fire_on_restart!
+      fire(:on_restart)
+    end
+
     def fire_on_stopped!
       fire(:on_stopped)
     end

lib/puma/launcher.rb

@@ -244,6 +244,7 @@ def reload_worker_directory
     end
 
     def restart!
+      @events.fire_on_restart!
       @config.run_hooks :on_restart, self, @events
 
       if Puma.jruby?
@@ -330,22 +331,20 @@ def prune_bundler
     #
 
     def integrate_with_systemd
-      return unless ENV["SD_NOTIFY"]
       return unless ENV["NOTIFY_SOCKET"]
 
-      require 'sd_notify'
-      return log "Systemd integration failed. It looks like you're trying to use systemd notify but don't have sd_notify gem installed" unless defined?(SdNotify)
-
-      require 'puma/systemd'
+      begin
+        require 'puma/systemd'
+      rescue LoadError
+        log "Systemd integration failed. It looks like you're trying to use systemd notify but don't have sd_notify gem installed"
+        return
+      end
 
       log "* Enabling systemd notification integration"
 
-      @events.on_booted { SdNotify.ready }
-      @events.on_stopped { SdNotify.stopping }
-
       systemd = Systemd.new(@events)
-
-      systemd.start_watchdog if SdNotify.watchdog?
+      systemd.hook_events
+      systemd.start_watchdog
     end
 
     def spec_for_gem(gem_name)

lib/puma/systemd.rb

@@ -1,21 +1,23 @@
 # frozen_string_literal: true
 
-require 'puma/events'
+require 'sd_notify'
 
 module Puma
   class Systemd
     def initialize(events)
       @events = events
     end
 
+    def hook_events
+      @events.on_booted { SdNotify.ready }
+      @events.on_stopped { SdNotify.stopping }
+      @events.on_restart { SdNotify.reloading }
+    end
+
     def start_watchdog
-      usec = Integer(ENV["WATCHDOG_USEC"])
-      return warn "systemd Watchdog too fast: #{usec}" if usec < 1_000_000
+      ping_f = watchdog_sleep_time
+      return unless ping_f
 
-      sec_f = usec / 1_000_000.0
-      # "It is recommended that a daemon sends a keep-alive notification message
-      # to the service manager every half of the time returned here."
-      ping_f = sec_f / 2
       log "Pinging systemd watchdog every #{ping_f.round(1)} sec"
       Thread.new do
         loop do
@@ -25,6 +27,19 @@ def start_watchdog
       end
     end
 
+    private
+
+    def watchdog_sleep_time
+      return unless SdNotify.watchdog?
+
+      usec = Integer(ENV["WATCHDOG_USEC"])
+
+      sec_f = usec / 1_000_000.0
+      # "It is recommended that a daemon sends a keep-alive notification message
+      # to the service manager every half of the time returned here."
+      sec_f / 2
+    end
+
     def log(str)
       @events.log str
     end

test/test_integration_systemd.rb

@@ -8,15 +8,15 @@ def setup
     skip "Skipped because Systemd support is linux-only" if windows? || osx?
     skip UNIX_SKT_MSG unless UNIX_SKT_EXIST
 
+    super
+
     ::Dir::Tmpname.create("puma_socket") do |sockaddr|
       @sockaddr = sockaddr
       @socket = Socket.new(:UNIX, :DGRAM, 0)
       socket_ai = Addrinfo.unix(sockaddr)
       @socket.bind(socket_ai)
       ENV["NOTIFY_SOCKET"] = sockaddr
     end
-
-    ENV["SD_NOTIFY"] = "1"
   end
 
   def teardown
@@ -25,13 +25,12 @@ def teardown
     File.unlink(@sockaddr) if @sockaddr
     @socket = nil
     @sockaddr = nil
-    ENV["SD_NOTIFY"] = nil
     ENV["NOTIFY_SOCKET"] = nil
     ENV["WATCHDOG_USEC"] = nil
   end
 
   def socket_message
-    @socket.recvfrom(10)[0]
+    @socket.recvfrom(15)[0]
   end
 
   def test_systemd_integration
@@ -42,8 +41,13 @@ def test_systemd_integration
     cli_server "test/rackup/hello.ru"
     assert_equal(socket_message, "READY=1")
 
+    connection = connect
+    restart_server connection
+    assert_equal(socket_message, "RELOADING=1")
+    assert_equal(socket_message, "READY=1")
+
     stop_server
-    assert_match(socket_message, "STOPPING=1")
+    assert_equal(socket_message, "STOPPING=1")
   end
 
   def test_systemd_watchdog