From da3976b9e2f9abf0f2d37300b70bc8884e4ab2f8 Mon Sep 17 00:00:00 2001
From: Joey Hess <joey@kitenet.net>
Date: Fri, 4 Apr 2014 16:20:20 -0400
Subject: [PATCH] propellor spin

---
 Propellor/CmdLine.hs | 16 +++++++++++++++-
 debian/changelog     |  1 +
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/Propellor/CmdLine.hs b/Propellor/CmdLine.hs
index d10211f..3974f1d 100644
--- a/Propellor/CmdLine.hs
+++ b/Propellor/CmdLine.hs
@@ -8,6 +8,8 @@ import System.Log.Formatter
 import System.Log.Handler (setFormatter, LogHandler)
 import System.Log.Handler.Simple
 import System.PosixCompat
+import Control.Exception (bracket)
+import System.Posix.IO
 
 import Propellor
 import qualified Propellor.Property.Docker as Docker
@@ -71,7 +73,7 @@ defaultMain getprops = do
 	go True cmdline = updateFirst cmdline $ go False cmdline
 	go False (Spin host) = withprops host $ const $ spin host
 	go False (Run host) = ifM ((==) 0 <$> getRealUserID)
-		( withprops host ensureProperties
+		( onlyProcess $ withprops host ensureProperties
 		, go True (Spin host)
 		)
 	go False (Boot host) = withprops host $ boot
@@ -79,6 +81,18 @@ defaultMain getprops = do
 	withprops host a = maybe (unknownhost host) a $
 		headMaybe $ catMaybes $ map (\get -> get host) getprops
 
+onlyProcess :: IO a -> IO a
+onlyProcess a = bracket lock unlock (const a)
+  where
+	lock = do
+		l <- openFd lockfile ReadWrite Nothing defaultFileFlags
+		setLock l (WriteLock, AbsoluteSeek, 0, 0)
+			`catchIO` (const alreadyrunning)
+		return l
+	unlock = closeFd
+	alreadyrunning = error "Propellor is already running on this host!"
+	lockfile = localdir </> ".lock"
+
 unknownhost :: HostName -> IO a
 unknownhost h = errorMessage $ unlines
 	[ "Unknown host: " ++ h
diff --git a/debian/changelog b/debian/changelog
index 0fdd42b..a02c27d 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -2,6 +2,7 @@ propellor (0.2.3) UNRELEASED; urgency=medium
 
   * docker: Fix laziness bug that caused running containers to be
     unnecessarily stopped and committed.
+  * Add locking so only one propellor can run at a time on a host.
 
  -- Joey Hess <joeyh@debian.org>  Fri, 04 Apr 2014 15:58:03 -0400