implement export.log and resolve export conflicts

Incremental export updates work now too.

This commit was sponsored by Anthony DeRobertis on Patreon.

Logs/Export.hs

@@ -0,0 +1,67 @@
+{- git-annex export log
+ -
+ - Copyright 2017 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs.Export where
+
+import qualified Data.Map as M
+
+import Annex.Common
+import qualified Annex.Branch
+import qualified Git
+import Logs
+import Logs.UUIDBased
+import Annex.UUID
+
+-- | Get the treeish that was exported to a special remote.
+--
+-- If the list contains multiple items, there was an export conflict,
+-- and different trees were exported to the same special remote.
+getExport :: UUID -> Annex [Git.Ref]
+getExport remoteuuid = nub . mapMaybe get . M.elems . simpleMap 
+	. parseLogNew parseExportLog
+	<$> Annex.Branch.get exportLog
+  where
+	get (ExportLog t u)
+		| u == remoteuuid = Just t
+		| otherwise = Nothing
+
+data ExportChange = ExportChange
+	{ oldTreeish :: [Git.Ref]
+	, newTreeish :: Git.Ref
+	}
+
+-- | Record a change in what's exported to a special remote.
+--
+-- Any entries in the log for the oldTreeish will be updated to the
+-- newTreeish. This way, when multiple repositories are exporting to
+-- the same special remote, there's no conflict as long as they move
+-- forward in lock-step.
+recordExport :: UUID -> ExportChange -> Annex ()
+recordExport remoteuuid ec = do
+	c <- liftIO currentVectorClock
+	u <- getUUID
+	let val = ExportLog (newTreeish ec) remoteuuid
+	Annex.Branch.change exportLog $
+		showLogNew formatExportLog 
+			. changeLog c u val 
+			. M.mapWithKey (updateothers c u)
+			. parseLogNew parseExportLog
+  where
+	updateothers c u theiru le@(LogEntry _ (ExportLog t remoteuuid'))
+		| u == theiru || remoteuuid' /= remoteuuid || t `notElem` oldTreeish ec = le
+		| otherwise = LogEntry c (ExportLog (newTreeish ec) theiru)
+
+data ExportLog = ExportLog Git.Ref UUID
+
+formatExportLog :: ExportLog -> String
+formatExportLog (ExportLog treeish remoteuuid) =
+	Git.fromRef treeish ++ " " ++ fromUUID remoteuuid
+
+parseExportLog :: String -> Maybe ExportLog
+parseExportLog s = case words s of
+	(t:u:[]) -> Just $ ExportLog (Git.Ref t) (toUUID u)
+	_ -> Nothing