Docker: Avoid committing container every time it's started up.

This was using a lot of disk space. Instead, start the container, and then
use the running container to check if docker is running it with the right
params. In the unlikely case that the params have changed, we still need to
commit the container and restart it. The common case of eg a reboot no
longer commits though.

debian/changelog

@@ -9,6 +9,7 @@ propellor (0.9.0) unstable; urgency=medium
     immediate upgrades to the next stable release.
   * debCdn switched from cdn.debian.net to http.debian.net, which seems to be
     better managed now.
+  * Docker: Avoid committing container every time it's started up.
 
  -- Joey Hess <joeyh@debian.org>  Fri, 10 Oct 2014 11:37:45 -0400
 

src/Propellor/Property/Docker.hs

@@ -314,27 +314,33 @@ runningContainer :: ContainerId -> Image -> [RunParam] -> Property
 runningContainer cid@(ContainerId hn cn) image runps = containerDesc cid $ property "running" $ do
 	l <- liftIO $ listContainers RunningContainers
 	if cid `elem` l
-		then do
-			-- Check if the ident has changed; if so the
-			-- parameters of the container differ and it must
-			-- be restarted.
-			runningident <- liftIO $ getrunningident
-			if runningident == Just ident
-				then noChange
-				else do
-					liftIO $ print ("runningident", runningident, "vs", ident)
-					void $ liftIO $ stopContainer cid
-					restartcontainer
+		then checkident
 		else ifM (liftIO $ elem cid <$> listContainers AllContainers)
-			( restartcontainer
+			( do
+				-- The container exists, but is not
+				-- running. Its parameters may have
+				-- changed, but we cannot tell without
+				-- starting it up first.
+				void $ liftIO $ startContainer cid
+				checkident
 			, go image
 			)
   where
 	ident = ContainerIdent image hn cn runps
 
+	-- Check if the ident has changed; if so the
+	-- parameters of the container differ and it must
+	-- be restarted.
+	checkident = do
+		runningident <- liftIO $ getrunningident
+		if runningident == Just ident
+			then noChange
+			else do
+				void $ liftIO $ stopContainer cid
+				restartcontainer
+
 	restartcontainer = do
 		oldimage <- liftIO $ fromMaybe image <$> commitContainer cid
-		liftIO $ print ("restarting", oldimage)
 		void $ liftIO $ removeContainer cid
 		go oldimage
 
@@ -435,6 +441,9 @@ provisionContainer cid = containerDesc cid $ property "provisioned" $ liftIO $ d
 stopContainer :: ContainerId -> IO Bool
 stopContainer cid = boolSystem dockercmd [Param "stop", Param $ fromContainerId cid ]
 
+startContainer :: ContainerId -> IO Bool
+startContainer cid = boolSystem dockercmd [Param "start", Param $ fromContainerId cid ]
+
 stoppedContainer :: ContainerId -> Property
 stoppedContainer cid = containerDesc cid $ property desc $ 
 	ifM (liftIO $ elem cid <$> listContainers RunningContainers)