add remote state logs

This allows a remote to store a piece of arbitrary state associated with a
key. This is needed to support Tahoe, where the file-cap is calculated from
the data stored in it, and used to retrieve a key later. Glacier also would
be much improved by using this.

GETSTATE and SETSTATE are added to the external special remote protocol.

Note that the state is left as-is even when a key is removed from a remote.
It's up to the remote to decide when it wants to clear the state.

The remote state log, $KEY.log.rmt, is a UUID-based log. However,
rather than using the old UUID-based log format, I created a new variant
of that format. The new varient is more space efficient (since it lacks the
"timestamp=" hack, and easier to parse (and the parser doesn't mess with
whitespace in the value), and avoids compatability cruft in the old one.

This seemed worth cleaning up for these new files, since there could be a
lot of them, while before UUID-based logs were only used for a few log
files at the top of the git-annex branch. The transition code has also
been updated to handle these new UUID-based logs.

This commit was sponsored by Daniel Hofer.

Logs.hs

@@ -10,19 +10,21 @@ module Logs where
 import Common.Annex
 import Types.Key
 
-data LogVariety = UUIDBasedLog | PresenceLog Key
+{- There are several varieties of log file formats. -}
+data LogVariety = UUIDBasedLog | NewUUIDBasedLog | PresenceLog Key
 	deriving (Show)
 
 {- Converts a path from the git-annex branch into one of the varieties
  - of logs used by git-annex, if it's a known path. -}
 getLogVariety :: FilePath -> Maybe LogVariety
 getLogVariety f
-	| f `elem` uuidBasedLogs = Just UUIDBasedLog
+	| f `elem` topLevelUUIDBasedLogs = Just UUIDBasedLog
+	| isRemoteStateLog f = Just NewUUIDBasedLog
 	| otherwise = PresenceLog <$> firstJust (presenceLogs f)
 
-{- All the uuid-based logs stored in the git-annex branch. -}
-uuidBasedLogs :: [FilePath]
-uuidBasedLogs =
+{- All the uuid-based logs stored in the top of the git-annex branch. -}
+topLevelUUIDBasedLogs :: [FilePath]
+topLevelUUIDBasedLogs =
 	[ uuidLog
 	, remoteLog
 	, trustLog
@@ -99,16 +101,29 @@ urlLogFileKey path
 isUrlLog :: FilePath -> Bool
 isUrlLog file = urlLogExt `isSuffixOf` file
 
+{- The filename of the remote state log for a given key. -}
+remoteStateLogFile :: Key -> FilePath
+remoteStateLogFile key = hashDirLower key </> keyFile key ++ remoteStateLogExt
+
+remoteStateLogExt :: String
+remoteStateLogExt = ".log.rmt"
+
+isRemoteStateLog :: FilePath -> Bool
+isRemoteStateLog path = remoteStateLogExt `isSuffixOf` path
+
 prop_logs_sane :: Key -> Bool
 prop_logs_sane dummykey = all id
 	[ isNothing (getLogVariety "unknown")
 	, expect isUUIDBasedLog (getLogVariety uuidLog)
 	, expect isPresenceLog (getLogVariety $ locationLogFile dummykey)
 	, expect isPresenceLog (getLogVariety $ urlLogFile dummykey)
+	, expect isNewUUIDBasedLog (getLogVariety $ remoteStateLogFile dummykey)
 	]
   where
   	expect = maybe False
   	isUUIDBasedLog UUIDBasedLog = True
 	isUUIDBasedLog _ = False
+  	isNewUUIDBasedLog NewUUIDBasedLog = True
+	isNewUUIDBasedLog _ = False
 	isPresenceLog (PresenceLog k) = k == dummykey
 	isPresenceLog _ = False