add lsof build deps

Check for it in configure; and add a --force option for people without it
who want to live dangerously.

Assistant.hs

@@ -49,8 +49,10 @@ import Assistant.DaemonStatus
 import Assistant.Watcher
 import Assistant.Committer
 import Assistant.SanityChecker
+import qualified Annex
 import qualified Utility.Daemon
 import Utility.LogFile
+import qualified Build.SysConfig as SysConfig
 
 import Control.Concurrent
 
@@ -64,7 +66,12 @@ startDaemon foreground
 		pidfile <- fromRepo gitAnnexPidFile
 		go $ Utility.Daemon.daemonize logfd (Just pidfile) False
 	where
-		go a = withThreadState $ \st -> do
+		go a
+			| SysConfig.lsof = start a
+			| otherwise =
+				ifM (Annex.getState Annex.force)
+					(start a, needlsof)
+		start a = withThreadState $ \st -> do
 			dstatus <- startDaemonStatus
 			liftIO $ a $ do
 				changechan <- newChangeChan
@@ -78,5 +85,17 @@ startDaemon foreground
 				_ <- forkIO $ sanityCheckerThread st dstatus changechan
 				watchThread st dstatus changechan
 
+		-- this message is optimised away when lsof is available
+		needlsof = error $ unlines
+			[ "The lsof command is needed for watch mode to be safe."
+			, "But this build of git-annex was made without lsof available. Giving up..."
+			, ""
+			, "You can use --force if lsof is available now. Please make very sure it is."
+			, "If run with --force and without lsof available, files can be added to the"
+			, "annex while a process still has them opened for writing. This can"
+			, "corrupt data in the annex, and make fsck complain."
+			, "Use the --force with caution, Luke!"
+			]
+
 stopDaemon :: Annex ()
 stopDaemon = liftIO . Utility.Daemon.stopDaemon =<< fromRepo gitAnnexPidFile