prevent multiple concurrent provisioning inside docker container

Lock a lock file while provisioning inside, otherwise propellor could be
running to init the container when the system has just booted, or the
container was just started from being stopped, and at the same time,
propellor run outside the container chains into it to provision.

Previously, simplesh prevented this in a different way.

1 files changed, 15 insertions, 0 deletions

diff --git a/src/Propellor/Engine.hs b/src/Propellor/Engine.hs
index a3fc0f30..3fa9ffc0 100644
--- a/src/Propellor/Engine.hs
+++ b/src/Propellor/Engine.hs
@@ -8,11 +8,15 @@ import Data.Monoid
 import Control.Applicative
 import System.Console.ANSI
 import "mtl" Control.Monad.Reader
+import Control.Exception (bracket)
+import System.PosixCompat
+import System.Posix.IO
 
 import Propellor.Types
 import Propellor.Message
 import Propellor.Exception
 import Propellor.Info
+import Utility.Exception
 
 runPropellor :: Host -> Propellor a -> IO a
 runPropellor host a = runReaderT (runWithHost a) host
@@ -47,3 +51,14 @@ fromHost l hn getter = case findHost l hn of
 	Nothing -> return Nothing
 	Just h -> liftIO $ Just <$>
 		runReaderT (runWithHost getter) h
+
+onlyProcess :: FilePath -> IO a -> IO a
+onlyProcess lockfile a = bracket lock unlock (const a)
+  where
+	lock = do
+		l <- createFile lockfile stdFileMode
+		setLock l (WriteLock, AbsoluteSeek, 0, 0)
+			`catchIO` const alreadyrunning
+		return l
+	unlock = closeFd
+	alreadyrunning = error "Propellor is already running on this host!"