finish fixing windows timezone madness

Rather than calculating the TSDelta once, and caching it, this now
reads the inode sential file's InodeCache file once, and then each time a
new InodeCache is generated, looks at the sentinal file to get the current
delta.

This way, if the time zone changes while git-annex is running, it will
adapt.

This adds some inneffiency, but only on Windows, and only 1 stat per new
file added. The worst innefficiency is that `git annex status` and
`git annex sync` will now (on Windows) stat the inode sentinal file once per
file in the repo.

It would be more efficient to use getCurrentTimeZone, rather than needing
to stat the sentinal file. This should be easy to do, once the time
package gets my bugfix patch.

This commit was sponsored by Jürgen Lüters.

1 files changed, 31 insertions, 22 deletions

diff --git a/Utility/InodeCache.hs b/Utility/InodeCache.hs
index b0718e0be..b3534487d 100644
--- a/Utility/InodeCache.hs
+++ b/Utility/InodeCache.hs
@@ -107,15 +107,17 @@ readInodeCache s = case words s of
 
 genInodeCache :: FilePath -> TSDelta -> IO (Maybe InodeCache)
 genInodeCache f delta = catchDefaultIO Nothing $
-	toInodeCache delta <$> getFileStatus f
-
-toInodeCache :: TSDelta -> FileStatus -> Maybe InodeCache
-toInodeCache (TSDelta delta) s
-	| isRegularFile s = Just $ InodeCache $ InodeCachePrim
-		(fileID s)
-		(fileSize s)
-		(modificationTime s + delta)
-	| otherwise = Nothing
+	toInodeCache delta =<< getFileStatus f
+
+toInodeCache :: TSDelta -> FileStatus -> IO (Maybe InodeCache)
+toInodeCache (TSDelta getdelta) s
+	| isRegularFile s = do
+		delta <- getdelta
+		return $ Just $ InodeCache $ InodeCachePrim
+			(fileID s)
+			(fileSize s)
+			(modificationTime s + delta)
+	| otherwise = pure Nothing
 
 {- Some filesystem get new random inodes each time they are mounted.
  - To detect this and other problems, a sentinal file can be created.
@@ -130,12 +132,13 @@ data SentinalFile = SentinalFile
 {- On Windows, the mtime of a file appears to change when the time zone is
  - changed. To deal with this, a TSDelta can be used; the delta is added to
  - the mtime when generating an InodeCache. The current delta can be found
- - by looking at the SentinalFile. -}
-newtype TSDelta = TSDelta EpochTime
-	deriving (Show)
+ - by looking at the SentinalFile. Effectively, this makes all InodeCaches
+ - use the same time zone that was in use when the sential file was
+ - originally written. -}
+newtype TSDelta = TSDelta (IO EpochTime)
 
 noTSDelta :: TSDelta
-noTSDelta = TSDelta 0
+noTSDelta = TSDelta (pure 0)
 
 writeSentinalFile :: SentinalFile -> IO ()
 writeSentinalFile s = do
@@ -147,16 +150,15 @@ data SentinalStatus = SentinalStatus
 	{ sentinalInodesChanged :: Bool
 	, sentinalTSDelta :: TSDelta
 	} 
-	deriving (Show)
 
 {- Checks if the InodeCache of the sentinal file is the same
  - as it was when it was originally created.
  -
- - On Windows, there's no change even when there is a nonzero
- - TSDelta between the original and current InodeCaches.
+ - On Windows, time stamp differences are ignored, since they change
+ - with the timezone.
  -
- - If the sential does not exist, returns a dummy value indicating
- - that it's apparently changed.
+ - When the sential file does not exist, InodeCaches canot reliably be
+ - compared, so the assumption is that there is has been a change.
  -}
 checkSentinalFile :: SentinalFile -> IO SentinalStatus
 checkSentinalFile s = do
@@ -172,14 +174,21 @@ checkSentinalFile s = do
 	loadoldcache = catchDefaultIO Nothing $
 		readInodeCache <$> readFile (sentinalCacheFile s)
 	gennewcache = genInodeCache (sentinalFile s) noTSDelta
-	calc (InodeCache (InodeCachePrim inode1 size1 mtime1)) (InodeCache (InodeCachePrim inode2 size2 mtime2)) =
+	calc (InodeCache (InodeCachePrim oldinode oldsize oldmtime)) (InodeCache (InodeCachePrim newinode newsize newmtime)) =
 		SentinalStatus (not unchanged) tsdelta
 	  where
 #ifdef mingw32_HOST_OS
-	  	unchanged = inode1 == inode2 && size1 == size2
-		tsdelta = TSDelta (mtime1 - mtime2)
+	  	unchanged = oldinode == newinode && oldsize == newsize
+		tsdelta = TSDelta $ do
+			-- Run when generating an InodeCache, 
+			-- to get the current delta.
+			mnew <- gennewcache
+			return $ case mnew of
+				Just (InodeCache (InodeCachePrim _ _ currmtime)) ->
+					oldmtime - currmtime
+				Nothing -> 0
 #else
-		unchanged = inode1 == inode2 && size1 == size2 && mtime1 == mtime2
+		unchanged = oldinode == newinode && oldsize == newsize && oldmtime == newmtime
 		tsdelta = noTSDelta
 #endif
 	dummy = SentinalStatus True noTSDelta