Add watchdog_interval method and tests

This adds a `watchdog_interval` method as well as tests for it and for `watchdog?` (which previously had no tests). I also took the liberty of revising the docs for `watchdog?` to (I hope) be a little clearer.

Fixes agis#6.

lib/sd_notify.rb

@@ -59,33 +59,43 @@ def self.fdstore(unset_env=false)
     notify(FDSTORE, unset_env)
   end
 
-  # If the $WATCHDOG_USEC environment variable is set,
-  # and the $WATCHDOG_PID variable is unset or set to the PID of the current
-  # process
+  # Determine whether the systemd's watchdog is enabled for your program. If
+  # enabled, systemd will restart (or take some configured action) on your
+  # service when it goes N seconds without receiving a `WATCHDOG` notification
+  # from your program.
+  #
+  # See #watchdog for sending notifications and #watchdog_interval for how
+  # frequently to send them.
   #
   # @return [Boolean] true if the service manager expects watchdog keep-alive
   #   notification messages to be sent from this process.
   #
   # @note Unlike sd_watchdog_enabled(3), this method does not mutate the
   #   environment.
   def self.watchdog?
-    wd_usec = ENV["WATCHDOG_USEC"]
-    wd_pid = ENV["WATCHDOG_PID"]
-
-    return false if !wd_usec
-
-    begin
-      wd_usec = Integer(wd_usec)
-    rescue
-      return false
-    end
+    return false if watchdog_interval == 0.0
 
-    return false if wd_usec <= 0
+    wd_pid = ENV["WATCHDOG_PID"]
     return true if !wd_pid || wd_pid == $$.to_s
 
     false
   end
 
+  # Get the expected number of seconds between watchdog notifications. If
+  # systemd's watchdog manager is enabled, it will take action if it does not
+  # receive notifications at least this often from your program.
+  #
+  # @return [Float] the frequency (in seconds) at which the service manager
+  #   expects watchdog keep-alive notification messages from this process.
+  #
+  # @note Unlike sd_watchdog_enabled(3), this returns seconds, not microseconds.
+  def self.watchdog_interval
+    wd_usec = Integer(ENV["WATCHDOG_USEC"])
+    wd_usec.positive? ? wd_usec / 1e6 : 0.0
+  rescue StandardError
+    0.0
+  end
+
   # Notify systemd with the provided state, via the notification socket, if
   # any.
   #

test/sd_notify_test.rb

@@ -30,11 +30,50 @@ def test_sd_notify_ready_unset
     assert_nil(ENV["NOTIFY_SOCKET"])
   end
 
+  def test_sd_notify_watchdog_disabled
+    setup_socket
+
+    assert_equal(false, SdNotify.watchdog?)
+  end
+
+  def test_sd_notify_watchdog_enabled
+    ENV["WATCHDOG_USEC"] = "5_000_000"
+    ENV["WATCHDOG_PID"] = $$.to_s
+    setup_socket
+
+    assert_equal(true, SdNotify.watchdog?)
+  end
+
+  def test_sd_notify_watchdog_enabled_for_a_different_process
+    ENV["WATCHDOG_USEC"] = "5_000_000"
+    ENV["WATCHDOG_PID"] = ($$ + 1).to_s
+    setup_socket
+
+    assert_equal(false, SdNotify.watchdog?)
+  end
+
+  def test_sd_notify_watchdog_interval_disabled
+    setup_socket
+
+    assert_equal(0.0, SdNotify.watchdog_interval)
+  end
+
+  def test_sd_notify_watchdog_interval_enabled
+    ENV["WATCHDOG_USEC"] = "5_000_000"
+    ENV["WATCHDOG_PID"] = $$.to_s
+    setup_socket
+
+    assert_equal(5.0, SdNotify.watchdog_interval)
+  end
+
   def teardown
     @socket.close if @socket
     File.unlink(@sockaddr) if @sockaddr
     @socket = nil
     @sockaddr = nil
+    ENV.delete("NOTIFY_SOCKET")
+    ENV.delete("WATCHDOG_USEC")
+    ENV.delete("WATCHDOG_PID")
   end
 
   private