Added spawnStatusBarAndRemember and cleanupStatusBars

In order to effectively restart status bars without relying
on pipes, spawnStatusBarAndRemember saves the PID of status
bars to kill them with cleanupStatusBars.

Removed checking for docks

Cleaned imports

Use signalProcesGroup instead of inverting the PID

Updated the documentation

Updated the documentation

XMonad/Hooks/DynamicLog.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE FlexibleContexts, PatternGuards #-}
+{-# LANGUAGE FlexibleContexts, PatternGuards, TypeApplications #-}
 
 -----------------------------------------------------------------------------
 -- |
@@ -37,6 +37,10 @@ module XMonad.Hooks.DynamicLog (
     xmonadPropLog,
     xmonadPropLog',
 
+    -- * Specialized spawning and cleaning
+    spawnStatusBarAndRemember,
+    cleanupStatusBars,
+
     -- * Build your own formatter
     dynamicLogWithPP,
     dynamicLogString,
@@ -64,12 +68,15 @@ module XMonad.Hooks.DynamicLog (
 
 import Codec.Binary.UTF8.String (encodeString)
 import Control.Applicative (liftA2)
-import Control.Monad (msum)
+import Control.Exception (try, SomeException)
+import Control.Monad (msum, void)
 import Data.Char (isSpace, ord)
 import Data.List (intersperse, isPrefixOf, sortBy, stripPrefix)
 import Data.Map (Map)
 import Data.Maybe (catMaybes, fromMaybe, isJust, mapMaybe)
 import Data.Ord (comparing)
+import System.Posix.Types (ProcessID)
+import System.Posix.Signals (sigTERM, signalProcessGroup)
 
 import qualified Data.Map        as M
 import qualified XMonad.StackSet as S
@@ -80,8 +87,8 @@ import XMonad
 
 import XMonad.Util.NamedWindows
 import XMonad.Util.Run
-import XMonad.Util.SpawnOnce (spawnOnce)
 import XMonad.Util.WorkspaceCompare
+import qualified XMonad.Util.ExtensibleState as XS
 
 import XMonad.Layout.LayoutModifier
 import XMonad.Hooks.UrgencyHook
@@ -200,7 +207,11 @@ import XMonad.Hooks.ManageDocks
 --
 -- The above has the problem that xmobar will not get restarted whenever you
 -- restart xmonad ('XMonad.Util.SpawnOnce.spawnOnce' will simply prevent your
--- chosen status bar from spawning again).
+-- chosen status bar from spawning again). Using 'statusBarProp', however, takes
+-- care of the necessary plumbing /and/ keeps track of the started status bars, so
+-- they can be correctly restarted with xmonad. This is achieved using
+-- 'spawnStatusBarAndRemember' to start them and 'cleanupStatusBars' to kill
+-- previously started ones.
 --
 -- Even if you don't use a statusbar, you can still use 'dynamicLogString' to
 -- show on-screen notifications in response to some events. For example, to show
@@ -349,7 +360,52 @@ statusBarPropTo :: LayoutClass l Window
 statusBarPropTo prop cmd pp =
     makeStatusBar
         (xmonadPropLog' prop =<< dynamicLogString pp)
-        (spawnOnce cmd)
+        (spawnStatusBarAndRemember cmd)
+
+-- | Spawns a status bar and saves its PID. This is useful when the status bars
+-- should be restarted with xmonad. Use this in combination with 'cleanupStatusBars'.
+--
+-- Note: in some systems, multiple processes might start, even though one command is
+-- provided. This means the first PID, of the group leader, is saved.
+--
+spawnStatusBarAndRemember :: String -- ^ The command used to spawn the status bar
+                          -> X()
+spawnStatusBarAndRemember cmd = do
+  newPid <- spawnPID cmd
+  XS.modify (StatusBarPIDs . (newPid :) . getPIDs)
+
+-- This newtype wrapper, together with the ExtensionClass instance makes use of
+-- the extensible state to save the PIDs bewteen xmonad restarts.
+newtype StatusBarPIDs = StatusBarPIDs { getPIDs :: [ProcessID] }
+  deriving (Show, Read)
+
+instance ExtensionClass StatusBarPIDs where
+  initialValue = StatusBarPIDs []
+  extensionType = PersistentExtension
+
+
+-- | Kills the status bars started with 'spawnStatusBarAndRemember', and resets
+-- the state. This could go for example at the beginning of the startupHook.
+--
+-- Concretely, this function sends a 'sigTERM' to the saved PIDs using
+-- 'signalProcessGroup' to effectively terminate all processes, regardless
+-- of how many were started by using  'spawnStatusBarAndRemember'.
+--
+-- There is one caveat to keep in mind: to keep the implementation simple;
+-- no checks are executed before terminating the processes. This means: if the
+-- started process dies for some reason, and enough time passes for the PIDs
+-- to wrap around, this function might terminate another process that happens
+-- to have the same PID. However, this isn't a typical usage scenario.
+--
+cleanupStatusBars :: X ()
+cleanupStatusBars =
+    getPIDs <$> XS.get
+    >>= (io . mapM_ killPid)
+    >> XS.put (StatusBarPIDs [])
+   where
+    killPid :: ProcessID -> IO ()
+    killPid pidToKill = void $ try @SomeException (signalProcessGroup sigTERM pidToKill)
+
 
 -- | Like 'statusBarProp', but uses pipes instead of property-based logging.
 -- Only use this function if your status bar does not support reading from a