cron locking

Propellor/Property/Cron.hs

@@ -3,14 +3,19 @@ module Propellor.Property.Cron where
 import Propellor
 import qualified Propellor.Property.File as File
 import qualified Propellor.Property.Apt as Apt
+import Utility.SafeCommand
 
 import Data.Char
 
 type CronTimes = String
 
--- | Installs a cron job, run as a specificed user, in a particular
---directory. Note that the Desc must be unique, as it is used for the 
---cron.d/ filename.
+-- | Installs a cron job, run as a specified user, in a particular
+-- directory. Note that the Desc must be unique, as it is used for the 
+-- cron.d/ filename.
+-- 
+-- Only one instance of the cron job is allowed to run at a time, no matter
+-- how long it runs. This is accomplished using flock locking of the cron
+-- job file.
 job :: Desc -> CronTimes -> UserName -> FilePath -> String -> Property
 job desc times user cddir command = cronjobfile `File.hasContent`
 	[ "# Generated by propellor"
@@ -18,11 +23,14 @@ job desc times user cddir command = cronjobfile `File.hasContent`
 	, "SHELL=/bin/sh"
 	, "PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin"
 	, ""
-	, times ++ "\t" ++ user ++ "\t" ++ "cd " ++ cddir ++ " && " ++ command
+	, times ++ "\t" ++ user ++ "\t"
+		++ "flock -n " ++ shellEscape cronjobfile
+		++ " -c sh -c " ++ shellEscape cmdline
 	]
 	`requires` Apt.serviceInstalledRunning "cron"
 	`describe` ("cronned " ++ desc)
   where
+	cmdline = "cd " ++ cddir ++ " && " ++ command
 	cronjobfile = "/etc/cron.d/" ++ map sanitize desc
 	sanitize c
 		| isAlphaNum c = c

debian/changelog

@@ -8,6 +8,8 @@ propellor (0.3.1) UNRELEASED; urgency=medium
   * Properties can now be satisfied differently on different operating
     systems.
   * Standard apt configuration for stable now includes backports.
+  * Cron jobs generated by propellor use flock(1) to avoid multiple
+    instances running at a time.
 
  -- Joey Hess <joeyh@debian.org>  Fri, 11 Apr 2014 15:00:11 -0400