implement daemon status serialization to a file

Also afterLastDaemonRun, with 10 minute slop to handle majority of clock
skew issues.

Assistant/DaemonStatus.hs

@@ -6,16 +6,21 @@
 module Assistant.DaemonStatus where
 
 import Common.Annex
+import Utility.TempFile
 
 import Control.Concurrent
 import System.Posix.Types
+import Data.Time.Clock.POSIX
+import Data.Time
+import System.Locale
 
 data DaemonStatus = DaemonStatus
 	-- False when the daemon is performing its startup scan
 	{ scanComplete :: Bool
 	-- Time when a previous process of the daemon was running ok
-	, lastRunning :: Maybe EpochTime
+	, lastRunning :: Maybe POSIXTime
 	}
+	deriving (Show)
 
 type DaemonStatusHandle = MVar DaemonStatus
 
@@ -32,4 +37,46 @@ getDaemonStatus :: DaemonStatusHandle -> Annex DaemonStatus
 getDaemonStatus = liftIO . readMVar
 
 modifyDaemonStatus :: DaemonStatusHandle -> (DaemonStatus -> DaemonStatus) -> Annex ()
-modifyDaemonStatus status a = liftIO $ modifyMVar_ status (return . a)
+modifyDaemonStatus handle a = liftIO $ modifyMVar_ handle (return . a)
+
+{- Don't just dump out the structure, because it will change over time,
+ - and parts of it are not relevant. -}
+writeDaemonStatusFile :: FilePath -> DaemonStatus -> IO ()
+writeDaemonStatusFile file status = 
+	viaTmp writeFile file =<< serialized <$> getPOSIXTime
+	where
+		serialized now = unlines
+			[ "lastRunning:" ++ show now
+			, "scanComplete:" ++ show (scanComplete status)
+			]
+
+readDaemonStatusFile :: FilePath -> IO DaemonStatus
+readDaemonStatusFile file = parse <$> readFile file
+	where
+		parse = foldr parseline newDaemonStatus . lines
+		parseline line status
+			| key == "lastRunning" = parseval readtime $ \v ->
+				status { lastRunning = Just v }
+			| key == "scanComplete" = parseval readish $ \v ->
+				status { scanComplete = v }
+			| otherwise = status -- unparsable line
+			where
+				(key, value) = separate (== ':') line
+				parseval parser a = maybe status a (parser value)
+				readtime s = do
+					d <- parseTime defaultTimeLocale "%s%Qs" s
+					Just $ utcTimeToPOSIXSeconds d
+
+{- Checks if a time stamp was made after the daemon was lastRunning.
+ -
+ - Some slop is built in; this really checks if the time stamp was made
+ - at least ten minutes after the daemon was lastRunning. This is to
+ - ensure the daemon shut down cleanly, and deal with minor clock skew.
+ -
+ - If the daemon has never ran before, this always returns False.
+ -}
+afterLastDaemonRun :: EpochTime -> DaemonStatus -> Bool
+afterLastDaemonRun timestamp status = maybe True (< t) (lastRunning status)
+	where
+		t = realToFrac (timestamp + slop) :: POSIXTime
+		slop = 10 * 60

Assistant/Watcher.hs

@@ -154,10 +154,8 @@ onAddSymlink file filestatus dstatus = go =<< Backend.lookupFile file
 			| scanComplete daemonstatus = addlink link
 			| otherwise = case filestatus of
 				Just s
-					| safe (statusChangeTime s) -> noChange
+					| not (afterLastDaemonRun (statusChangeTime s) daemonstatus) -> noChange
 				_ -> addlink link
-			where
-				safe t = maybe True (> t) (lastRunning daemonstatus)
 
 		{- For speed, tries to reuse the existing blob for
 		 - the symlink target. -}

Locations.hs

@@ -24,6 +24,7 @@ module Locations (
 	gitAnnexIndexLock,
 	gitAnnexIndexDirty,
 	gitAnnexPidFile,
+	gitAnnexDaemonStatusFile,
 	gitAnnexLogFile,
 	gitAnnexSshDir,
 	gitAnnexRemotesDir,
@@ -151,6 +152,10 @@ gitAnnexIndexDirty r = gitAnnexDir r </> "index.dirty"
 gitAnnexPidFile :: Git.Repo -> FilePath
 gitAnnexPidFile r = gitAnnexDir r </> "daemon.pid"
 
+{- Status file for daemon mode. -}
+gitAnnexDaemonStatusFile :: Git.Repo -> FilePath
+gitAnnexDaemonStatusFile r = gitAnnexDir r </> "daemon.status"
+
 {- Log file for daemon mode. -}
 gitAnnexLogFile :: Git.Repo -> FilePath
 gitAnnexLogFile r = gitAnnexDir r </> "daemon.log"