fix for Windows file timestamp timezone madness

On Windows, changing the time zone causes the apparent mtime of files to
change. This confuses git-annex, which natually thinks this means the files
have actually been modified (since THAT'S WHAT A MTIME IS FOR, BILL <sheesh>).

Work around this stupidity, by using the inode sentinal file to detect if
the timezone has changed, and calculate a TSDelta, which will be applied
when generating InodeCaches.

This should add no overhead at all on unix. Indeed, I sped up a few
things slightly in the refactoring.

Seems to basically work! But it has a big known problem:
If the timezone changes while the assistant (or a long-running command)
runs, it won't notice, since it only checks the inode cache once, and
so will use the old delta for all new inode caches it generates for new
files it's added. Which will result in them seeming changed the next time
it runs.

This commit was sponsored by Vincent Demeester.

Annex/Content/Direct.hs

@@ -1,10 +1,12 @@
 {- git-annex file content managing for direct mode
  -
- - Copyright 2012-2013 Joey Hess <joey@kitenet.net>
+ - Copyright 2012-2014 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
+{-# LANGUAGE CPP #-}
+
 module Annex.Content.Direct (
 	associatedFiles,
 	associatedFilesRelative,
@@ -27,6 +29,8 @@ module Annex.Content.Direct (
 	inodesChanged,
 	createInodeSentinalFile,
 	addContentWhenNotPresent,
+	withTSDelta,
+	getTSDelta,
 ) where
 
 import Common.Annex
@@ -136,7 +140,7 @@ recordedInodeCache key = withInodeCacheFile key $ \f ->
  -}
 updateInodeCache :: Key -> FilePath -> Annex ()
 updateInodeCache key file = maybe noop (addInodeCache key)
-	=<< liftIO (genInodeCache file)
+	=<< withTSDelta (liftIO . genInodeCache file)
 
 {- Adds another inode to the cache for a key. -}
 addInodeCache :: Key -> InodeCache -> Annex ()
@@ -164,16 +168,16 @@ withInodeCacheFile key a = a =<< calcRepo (gitAnnexInodeCache key)
 {- Checks if a InodeCache matches the current version of a file. -}
 sameInodeCache :: FilePath -> [InodeCache] -> Annex Bool
 sameInodeCache _ [] = return False
-sameInodeCache file old = go =<< liftIO (genInodeCache file)
+sameInodeCache file old = go =<< withTSDelta (liftIO . genInodeCache file)
   where
 	go Nothing = return False
 	go (Just curr) = elemInodeCaches curr old
 
 {- Checks if a FileStatus matches the recorded InodeCache of a file. -}
 sameFileStatus :: Key -> FileStatus -> Annex Bool
-sameFileStatus key status = do
+sameFileStatus key status = withTSDelta $ \delta -> do
 	old <- recordedInodeCache key
-	let curr = toInodeCache status
+	let curr = toInodeCache delta status
 	case (old, curr) of
 		(_, Just c) -> elemInodeCaches c old
 		([], Nothing) -> return True
@@ -217,40 +221,43 @@ addContentWhenNotPresent key contentfile associatedfile = do
  - inodes have changed.
  -}
 inodesChanged :: Annex Bool
-inodesChanged = maybe calc return =<< Annex.getState Annex.inodeschanged
+inodesChanged = sentinalInodesChanged <$> sentinalStatus
+
+withTSDelta :: (TSDelta -> Annex a) -> Annex a
+withTSDelta a = a =<< getTSDelta
+
+getTSDelta :: Annex TSDelta
+#ifdef mingw32_HOST_OS
+getTSDelta = sentinalTSDelta <$> sentinalStatus
+#else
+getTSDelta = pure noTSDelta -- optimisation
+#endif
+
+sentinalStatus :: Annex SentinalStatus
+sentinalStatus = maybe check return =<< Annex.getState Annex.sentinalstatus
   where
-	calc = do
-		scache <- liftIO . genInodeCache
-			=<< fromRepo gitAnnexInodeSentinal
-		scached <- readInodeSentinalFile
-		let changed = case (scache, scached) of
-			(Just c1, Just c2) -> not $ compareStrong c1 c2
-			_ -> True
-		Annex.changeState $ \s -> s { Annex.inodeschanged = Just changed }
-		return changed
-
-readInodeSentinalFile :: Annex (Maybe InodeCache)
-readInodeSentinalFile = do
-	sentinalcachefile <- fromRepo gitAnnexInodeSentinalCache
-	liftIO $ catchDefaultIO Nothing $
-		readInodeCache <$> readFile sentinalcachefile
-
-writeInodeSentinalFile :: Annex ()
-writeInodeSentinalFile = do
-	sentinalfile <- fromRepo gitAnnexInodeSentinal
-	createAnnexDirectory (parentDir sentinalfile)
-	sentinalcachefile <- fromRepo gitAnnexInodeSentinalCache
-	liftIO $ writeFile sentinalfile ""
-	liftIO $ maybe noop (writeFile sentinalcachefile . showInodeCache)
-		=<< genInodeCache sentinalfile
+	check = do
+		sc <- liftIO . checkSentinalFile =<< annexSentinalFile
+		Annex.changeState $ \s -> s { Annex.sentinalstatus = Just sc }
+		return sc
 
 {- The sentinal file is only created when first initializing a repository.
  - If there are any annexed objects in the repository already, creating
  - the file would invalidate their inode caches. -}
 createInodeSentinalFile :: Annex ()
-createInodeSentinalFile =
-	unlessM (alreadyexists <||> hasobjects)
-		writeInodeSentinalFile
+createInodeSentinalFile = unlessM (alreadyexists <||> hasobjects) $ do
+	s <- annexSentinalFile
+	createAnnexDirectory (parentDir (sentinalFile s))
+	liftIO $ writeSentinalFile s
   where
-	alreadyexists = isJust <$> readInodeSentinalFile
+	alreadyexists = liftIO. sentinalFileExists =<< annexSentinalFile
 	hasobjects = liftIO . doesDirectoryExist =<< fromRepo gitAnnexObjectDir
+
+annexSentinalFile :: Annex SentinalFile
+annexSentinalFile = do
+	sentinalfile <- fromRepo gitAnnexInodeSentinal
+	sentinalcachefile <- fromRepo gitAnnexInodeSentinalCache
+	return $ SentinalFile
+		{ sentinalFile = sentinalfile
+		, sentinalCacheFile = sentinalcachefile
+		}