add metadata command to get/set metadata

Adds metadata log, and command.

Note that unsetting field values seems to currently be broken.
And in general this has had all of 2 minutes worth of testing.

This commit was sponsored by Julien Lefrique.

Logs/MetaData.hs

@@ -0,0 +1,135 @@
+{- git-annex general metadata storage log
+ -
+ - A line of the log will look like "timestamp field [+-]value [...]"
+ -
+ - Note that unset values are preserved. Consider this case:
+ -
+ - We have:
+ -
+ - 100 foo +x
+ - 200 foo -x
+ -
+ - An unmerged remote has:
+ -
+ - 150 foo +x
+ - 
+ - After union merge, because the foo -x was preserved, we know that
+ - after the other remote redundantly set foo +x, it was unset,
+ - and so foo currently has no value.
+ -
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+
+module Logs.MetaData (
+	getCurrentMetaData,
+	getMetaData,
+	setMetaData,
+	unsetMetaData,
+	addMetaData,
+	currentMetaData,
+) where
+
+import Common.Annex
+import Types.MetaData
+import qualified Annex.Branch
+import Logs
+import Logs.SingleValue
+
+import qualified Data.Set as S
+import Data.Time.Clock.POSIX
+
+instance SingleValueSerializable MetaData where
+	serialize = Types.MetaData.serialize
+	deserialize = Types.MetaData.deserialize
+
+getMetaData :: Key -> Annex (Log MetaData)
+getMetaData = readLog . metaDataLogFile
+
+{- Go through the log from oldest to newest, and combine it all
+ - into a single MetaData representing the current state. -}
+getCurrentMetaData :: Key -> Annex MetaData
+getCurrentMetaData = currentMetaData . collect <$$> getMetaData
+  where
+	collect = foldl' unionMetaData newMetaData . map value . S.toAscList
+
+setMetaData :: Key -> MetaField -> String -> Annex ()
+setMetaData = setMetaData' True
+
+unsetMetaData :: Key -> MetaField -> String -> Annex ()
+unsetMetaData = setMetaData' False
+
+setMetaData' :: Bool -> Key -> MetaField -> String -> Annex ()
+setMetaData' isset k field s = addMetaData k $
+	updateMetaData field (mkMetaValue (CurrentlySet isset) s) newMetaData
+
+{- Adds in some metadata, which can override existing values, or unset
+ - them, but otherwise leaves any existing metadata as-is. -}
+addMetaData :: Key -> MetaData -> Annex ()
+addMetaData k metadata = do
+        now <- liftIO getPOSIXTime
+	Annex.Branch.change (metaDataLogFile k) $
+		showLog . simplifyLog 
+			. S.insert (LogEntry now metadata) 
+			. parseLog
+
+{- Simplify a log, removing historical values that are no longer
+ - needed. 
+ -
+ - This is not as simple as just making a single log line with the newest
+ - state of all metadata. Consider this case:
+ -
+ - We have:
+ -
+ - 100 foo +x bar +y
+ - 200 foo -x
+ -
+ - An unmerged remote has:
+ -
+ - 150 bar +z baz +w
+ -
+ - If what we have were simplified to "200 foo -x bar +y" then when the line
+ - from the remote became available, it would be older than the simplified
+ - line, and its change to bar would not take effect. That is wrong.
+ -
+ - Instead, simplify it to:                   (this simpliciation is optional)
+ -
+ - 100 bar +y                                 (100 foo +x bar +y)
+ - 200 foo -x
+ -
+ - Now merging with the remote yields:
+ -
+ - 100 bar +y                                 (100 foo +x bar +y)
+ - 150 bar +z baz +w
+ - 200 foo -x
+ -
+ - Simplifying again:
+ -
+ - 150 bar +z baz +w
+ - 200 foo -x
+ -
+ - In practice, there is little benefit to making simplications to lines
+ - that only remove some values, while leaving others on the line.
+ - Since lines are kept in git, that likely increases the size of the
+ - git repo (depending on compression), rather than saving any space.
+ -
+ - So, the only simplication that is actually done is to throw out an
+ - old line when all the values in it have been overridden by lines that
+ - came before
+ -}
+simplifyLog :: Log MetaData -> Log MetaData
+simplifyLog s = case S.toDescList s of
+	(newest:rest) -> S.fromList $ go [newest] (value newest) rest
+	_ -> s
+  where
+	go c _ [] = c
+	go c newer (l:ls)
+		| older `hasUniqueMetaData` newer =
+			go (l:c) (unionMetaData older newer) ls
+		| otherwise = go c newer ls
+	  where
+		older = value l