Merge branch 'forget'

Conflicts:
	debian/changelog

26 files changed, 685 insertions, 214 deletions

diff --git a/Annex/Branch.hs b/Annex/Branch.hs
index bc3736a9a..9ee281de9 100644
--- a/Annex/Branch.hs
+++ b/Annex/Branch.hs
@@ -1,6 +1,6 @@
 {- management of the git-annex branch
  -
- - Copyright 2011-2012 Joey Hess <joey@kitenet.net>
+ - Copyright 2011-2013 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -22,9 +22,12 @@ module Annex.Branch (
 	commit,
 	files,
 	withIndex,
+	performTransitions,
 ) where
 
 import qualified Data.ByteString.Lazy.Char8 as L
+import qualified Data.Set as S
+import qualified Data.Map as M
 
 import Common.Annex
 import Annex.BranchState
@@ -32,6 +35,7 @@ import Annex.Journal
 import qualified Git
 import qualified Git.Command
 import qualified Git.Ref
+import qualified Git.Sha
 import qualified Git.Branch
 import qualified Git.UnionMerge
 import qualified Git.UpdateIndex
@@ -42,6 +46,12 @@ import Annex.CatFile
 import Annex.Perms
 import qualified Annex
 import Utility.Env
+import Logs
+import Logs.Transitions
+import Logs.Trust.Pure
+import Annex.ReplaceFile
+import qualified Annex.Queue
+import Annex.Branch.Transitions
 
 {- Name of the branch that is used to store git-annex's information. -}
 name :: Git.Ref
@@ -110,6 +120,9 @@ forceUpdate = updateTo =<< siblingBranches
  - later get staged, and might overwrite changes made during the merge.
  - This is only done if some of the Refs do need to be merged.
  -
+ - Also handles performing any Transitions that have not yet been
+ - performed, in either the local branch, or the Refs.
+ -
  - Returns True if any refs were merged in, False otherwise.
  -}
 updateTo :: [(Git.Ref, Git.Branch)] -> Annex Bool
@@ -117,7 +130,8 @@ updateTo pairs = do
 	-- ensure branch exists, and get its current ref
 	branchref <- getBranch
 	dirty <- journalDirty
-	(refs, branches) <- unzip <$> filterM isnewer pairs
+	ignoredrefs <- getIgnoredRefs
+	(refs, branches) <- unzip <$> filterM (isnewer ignoredrefs) pairs
 	if null refs
 		{- Even when no refs need to be merged, the index
 		 - may still be updated if the branch has gotten ahead 
@@ -132,7 +146,9 @@ updateTo pairs = do
 		else lockJournal $ go branchref dirty refs branches
 	return $ not $ null refs
   where
-	isnewer (r, _) = inRepo $ Git.Branch.changed fullname r
+	isnewer ignoredrefs (r, _)
+		| S.member r ignoredrefs = return False
+		| otherwise = inRepo $ Git.Branch.changed fullname r
 	go branchref dirty refs branches = withIndex $ do
 		cleanjournal <- if dirty then stageJournal else return noop
 		let merge_desc = if null branches
@@ -140,23 +156,30 @@ updateTo pairs = do
 			else "merging " ++
 				unwords (map Git.Ref.describe branches) ++ 
 				" into " ++ show name
+		localtransitions <- parseTransitionsStrictly "local"
+			<$> getStale transitionsLog
 		unless (null branches) $ do
 			showSideAction merge_desc
 			mergeIndex refs
-		ff <- if dirty
-			then return False
-			else inRepo $ Git.Branch.fastForward fullname refs
-		if ff
-			then updateIndex branchref
-			else commitBranch branchref merge_desc
-				(nub $ fullname:refs)
+		let commitrefs = nub $ fullname:refs
+		transitioned <- handleTransitions localtransitions commitrefs
+		case transitioned of
+			Nothing -> do
+				ff <- if dirty
+					then return False
+					else inRepo $ Git.Branch.fastForward fullname refs
+				if ff
+					then updateIndex branchref
+					else commitBranch branchref merge_desc commitrefs
+			Just (branchref', commitrefs') ->
+				commitBranch branchref' merge_desc commitrefs'
 		liftIO cleanjournal
 
 {- Gets the content of a file, which may be in the journal, or in the index
  - (and committed to the branch).
  - 
  - Updates the branch if necessary, to ensure the most up-to-date available
- - content is available.
+ - content is returned.
  -
  - Returns an empty string if the file doesn't exist yet. -}
 get :: FilePath -> Annex String
@@ -175,7 +198,10 @@ get' :: FilePath -> Annex String
 get' file = go =<< getJournalFile file
   where
 	go (Just journalcontent) = return journalcontent
-	go Nothing = withIndex $ L.unpack <$> catFile fullname file
+	go Nothing = getRaw file
+	
+getRaw :: FilePath -> Annex String
+getRaw file = withIndex $ L.unpack <$> catFile fullname file
 
 {- Applies a function to modifiy the content of a file.
  -
@@ -253,13 +279,17 @@ commitBranch' branchref message parents = do
 files :: Annex [FilePath]
 files = do
 	update
-	withIndex $ do
-		bfiles <- inRepo $ Git.Command.pipeNullSplitZombie
-			[ Params "ls-tree --name-only -r -z"
-			, Param $ show fullname
-			]
-		jfiles <- getJournalledFiles
-		return $ jfiles ++ bfiles
+	(++)
+		<$> branchFiles
+		<*> getJournalledFiles
+
+{- Files in the branch, not including any from journalled changes,
+ - and without updating the branch. -}
+branchFiles :: Annex [FilePath]
+branchFiles = withIndex $ inRepo $ Git.Command.pipeNullSplitZombie
+	[ Params "ls-tree --name-only -r -z"
+	, Param $ show fullname
+	]
 
 {- Populates the branch's index file with the current branch contents.
  - 
@@ -361,3 +391,116 @@ stageJournal = withIndex $ do
 		sha <- hashFile h path
 		streamer $ Git.UpdateIndex.updateIndexLine
 			sha FileBlob (asTopFilePath $ fileJournal file)
+
+{- This is run after the refs have been merged into the index,
+ - but before the result is committed to the branch.
+ - Which is why it's passed the contents of the local branches's
+ - transition log before that merge took place.
+ -
+ - When the refs contain transitions that have not yet been done locally,
+ - the transitions are performed on the index, and a new branch
+ - is created from the result, and returned.
+ -
+ - When there are transitions recorded locally that have not been done
+ - to the remote refs, the transitions are performed in the index,
+ - and the existing branch is returned. In this case, the untransitioned
+ - remote refs cannot be merged into the branch (since transitions
+ - throw away history), so none of them are included in the returned
+ - list of refs, and they are added to the list of refs to ignore,
+ - to avoid re-merging content from them again.
+ -}
+handleTransitions :: Transitions -> [Git.Ref] -> Annex (Maybe (Git.Branch, [Git.Ref]))
+handleTransitions localts refs = do
+	m <- M.fromList <$> mapM getreftransition refs
+	let remotets = M.elems m
+	if all (localts ==) remotets
+		then return Nothing
+		else do
+			let allts = combineTransitions (localts:remotets)
+			let (transitionedrefs, untransitionedrefs) =
+				partition (\r -> M.lookup r m == Just allts) refs
+			transitionedbranch <- performTransitions allts (localts /= allts)
+			ignoreRefs untransitionedrefs
+			return $ Just (transitionedbranch, transitionedrefs)
+  where
+  	getreftransition ref = do
+		ts <- parseTransitionsStrictly "remote" . L.unpack
+			<$> catFile ref transitionsLog
+		return (ref, ts)
+
+ignoreRefs :: [Git.Ref] -> Annex ()
+ignoreRefs rs = do
+	old <- getIgnoredRefs
+	let s = S.unions [old, S.fromList rs]
+	f <- fromRepo gitAnnexIgnoredRefs
+	replaceFile f $ \tmp -> liftIO $ writeFile tmp $
+		unlines $ map show $ S.elems s
+
+getIgnoredRefs :: Annex (S.Set Git.Ref)
+getIgnoredRefs = S.fromList . mapMaybe Git.Sha.extractSha . lines <$> content
+  where
+  	content = do
+		f <- fromRepo gitAnnexIgnoredRefs
+		liftIO $ catchDefaultIO "" $ readFile f
+
+{- Performs the specified transitions on the contents of the index file,
+ - commits it to the branch, or creates a new branch, and returns 
+ - the branch's ref. -}
+performTransitions :: Transitions -> Bool -> Annex Git.Ref
+performTransitions ts neednewbranch = do
+	-- For simplicity & speed, we're going to use the Annex.Queue to
+	-- update the git-annex branch, while it usually holds changes
+	-- for the head branch. Flush any such changes.
+	Annex.Queue.flush
+	withIndex $ do
+		run $ mapMaybe getTransitionCalculator $ transitionList ts
+		Annex.Queue.flush
+		if neednewbranch
+			then do
+				committedref <- inRepo $ Git.Branch.commit message fullname []
+				setIndexSha committedref
+				return committedref
+			else do
+				ref <- getBranch
+				commitBranch ref message [fullname]
+				getBranch
+  where
+  	message
+		| neednewbranch = "new branch for transition " ++ tdesc
+		| otherwise = "continuing transition " ++ tdesc
+	tdesc = show $ map describeTransition $ transitionList ts
+
+	{- The changes to make to the branch are calculated and applied to
+	 - the branch directly, rather than going through the journal,
+	 - which would be innefficient. (And the journal is not designed
+	 - to hold changes to every file in the branch at once.)
+	 -
+	 - When a file in the branch is changed by transition code,
+	 - that value is remembered and fed into the code for subsequent
+	 - transitions.
+	 -}
+	run [] = noop
+	run changers = do
+		trustmap <- calcTrustMap <$> getRaw trustLog
+		fs <- branchFiles
+		hasher <- inRepo hashObjectStart
+		forM_ fs $ \f -> do
+			content <- getRaw f
+			apply changers hasher f content trustmap
+		liftIO $ hashObjectStop hasher
+	apply [] _ _ _ _ = return ()
+	apply (changer:rest) hasher file content trustmap =
+		case changer file content trustmap of
+			RemoveFile -> do
+				Annex.Queue.addUpdateIndex
+					=<< inRepo (Git.UpdateIndex.unstageFile file)
+				-- File is deleted; can't run any other
+				-- transitions on it.
+				return ()
+			ChangeFile content' -> do
+				sha <- inRepo $ hashObject BlobObject content'
+				Annex.Queue.addUpdateIndex $ Git.UpdateIndex.pureStreamer $
+					Git.UpdateIndex.updateIndexLine sha FileBlob (asTopFilePath file)
+				apply rest hasher file content' trustmap
+			PreserveFile ->
+				apply rest hasher file content trustmap
diff --git a/Annex/Branch/Transitions.hs b/Annex/Branch/Transitions.hs
new file mode 100644
index 000000000..90002de62
--- /dev/null
+++ b/Annex/Branch/Transitions.hs
@@ -0,0 +1,53 @@
+{- git-annex branch transitions
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Annex.Branch.Transitions (
+	FileTransition(..),
+	getTransitionCalculator
+) where
+
+import Logs
+import Logs.Transitions
+import Logs.UUIDBased as UUIDBased
+import Logs.Presence.Pure as Presence
+import Types.TrustLevel
+import Types.UUID
+
+import qualified Data.Map as M
+
+data FileTransition
+	= ChangeFile String
+	| RemoveFile
+	| PreserveFile
+
+type TransitionCalculator = FilePath -> String -> TrustMap -> FileTransition
+
+getTransitionCalculator :: Transition -> Maybe TransitionCalculator
+getTransitionCalculator ForgetGitHistory = Nothing
+getTransitionCalculator ForgetDeadRemotes = Just dropDead
+
+dropDead :: FilePath -> String -> TrustMap -> FileTransition
+dropDead f content trustmap = case getLogVariety f of
+	Just UUIDBasedLog -> ChangeFile $
+		UUIDBased.showLog id $ dropDeadFromUUIDBasedLog trustmap $ UUIDBased.parseLog Just content
+	Just (PresenceLog _) ->
+		let newlog = Presence.compactLog $ dropDeadFromPresenceLog trustmap $ Presence.parseLog content
+		in if null newlog
+			then RemoveFile
+			else ChangeFile $ Presence.showLog newlog
+	Nothing -> PreserveFile
+
+dropDeadFromUUIDBasedLog :: TrustMap -> UUIDBased.Log String -> UUIDBased.Log String
+dropDeadFromUUIDBasedLog trustmap = M.filterWithKey $ notDead trustmap . const
+
+{- Presence logs can contain UUIDs or other values. Any line that matches
+ - a dead uuid is dropped; any other values are passed through. -}
+dropDeadFromPresenceLog :: TrustMap -> [Presence.LogLine] -> [Presence.LogLine]
+dropDeadFromPresenceLog trustmap = filter $ notDead trustmap (toUUID . Presence.info)
+
+notDead :: TrustMap -> (v -> UUID) -> v -> Bool
+notDead trustmap a v = M.findWithDefault SemiTrusted (a v) trustmap /= DeadTrusted
diff --git a/Annex/TaggedPush.hs b/Annex/TaggedPush.hs
index 44a1a0eb0..039dc0e17 100644
--- a/Annex/TaggedPush.hs
+++ b/Annex/TaggedPush.hs
@@ -13,6 +13,7 @@ import qualified Annex.Branch
 import qualified Git
 import qualified Git.Ref
 import qualified Git.Command
+import qualified Git.Branch
 import Utility.Base64
 
 {- Converts a git branch into a branch that is tagged with a UUID, typically
@@ -50,7 +51,10 @@ taggedPush :: UUID -> Maybe String -> Git.Ref -> Remote -> Git.Repo -> IO Bool
 taggedPush u info branch remote = Git.Command.runBool
         [ Param "push"
         , Param $ Remote.name remote
-        , Param $ refspec Annex.Branch.name
+	{- Using forcePush here is safe because we "own" the tagged branch
+	 - we're pushing; it has no other writers. Ensures it is pushed
+	 - even if it has been rewritten by a transition. -}
+        , Param $ Git.Branch.forcePush $ refspec Annex.Branch.name
         , Param $ refspec branch
         ]
   where
diff --git a/Command/Forget.hs b/Command/Forget.hs
new file mode 100644
index 000000000..d216ae3ca
--- /dev/null
+++ b/Command/Forget.hs
@@ -0,0 +1,52 @@
+{- git-annex command
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Command.Forget where
+
+import Common.Annex
+import Command
+import qualified Annex.Branch as Branch
+import Logs.Transitions
+import qualified Annex
+import qualified Option
+
+import Data.Time.Clock.POSIX
+
+def :: [Command]
+def = [withOptions forgetOptions $ command "forget" paramNothing seek
+		SectionMaintenance "prune git-annex branch history"]
+
+forgetOptions :: [Option]
+forgetOptions = [dropDeadOption]
+
+dropDeadOption :: Option
+dropDeadOption = Option.flag [] "drop-dead" "drop references to dead repositories"
+
+seek :: [CommandSeek]
+seek = [withFlag dropDeadOption $ \dropdead ->
+	withNothing $ start dropdead]
+
+start :: Bool -> CommandStart
+start dropdead = do
+	showStart "forget" "git-annex"
+	now <- liftIO getPOSIXTime
+	let basets = addTransition now ForgetGitHistory noTransitions
+	let ts = if dropdead
+		then addTransition now ForgetDeadRemotes basets
+		else basets
+	next $ perform ts =<< Annex.getState Annex.force
+
+perform :: Transitions -> Bool -> CommandPerform
+perform ts True = do
+	recordTransitions Branch.change ts
+	-- get branch committed before contining with the transition
+	Branch.update
+	void $ Branch.performTransitions ts True
+	next $ return True
+perform _ False = do
+	showLongNote "To forget git-annex branch history, you must specify --force. This deletes metadata!"
+	stop
diff --git a/Command/Log.hs b/Command/Log.hs
index 2d4819f7f..f3a5becb8 100644
--- a/Command/Log.hs
+++ b/Command/Log.hs
@@ -17,7 +17,7 @@ import Data.Char
 
 import Common.Annex
 import Command
-import qualified Logs.Location
+import Logs
 import qualified Logs.Presence
 import Annex.CatFile
 import qualified Annex.Branch
@@ -135,7 +135,7 @@ getLog :: Key -> [CommandParam] -> Annex [String]
 getLog key os = do
 	top <- fromRepo Git.repoPath
 	p <- liftIO $ relPathCwdToFile top
-	let logfile = p </> Logs.Location.logFile key
+	let logfile = p </> locationLogFile key
 	inRepo $ pipeNullSplitZombie $
 		[ Params "log -z --pretty=format:%ct --raw --abbrev=40"
 		, Param "--remove-empty"
diff --git a/Command/Sync.hs b/Command/Sync.hs
index 551c2fa69..567e3146b 100644
--- a/Command/Sync.hs
+++ b/Command/Sync.hs
@@ -167,29 +167,45 @@ pushRemote remote branch = go =<< needpush
 			showOutput
 			inRepo $ pushBranch remote branch
 
-{- If the remote is a bare git repository, it's best to push the branch
- - directly to it. On the other hand, if it's not bare, pushing to the
- - checked out branch will fail, and this is why we use the syncBranch.
+{- Pushes a regular branch like master to a remote. Also pushes the git-annex
+ - branch.
+ -
+ - If the remote is a bare git repository, it's best to push the regular 
+ - branch directly to it, so that cloning/pulling will get it.
+ - On the other hand, if it's not bare, pushing to the checked out branch
+ - will fail, and this is why we push to its syncBranch.
  -
  - Git offers no way to tell if a remote is bare or not, so both methods
  - are tried.
  -
  - The direct push is likely to spew an ugly error message, so stderr is
- - elided. Since progress is output to stderr too, the sync push is done
- - first, and actually sends the data. Then the direct push is tried,
- - with stderr discarded, to update the branch ref on the remote.
+ - elided. Since git progress display goes to stderr too, the sync push
+ - is done first, and actually sends the data. Then the direct push is
+ - tried, with stderr discarded, to update the branch ref on the remote.
+ -
+ - The sync push forces the update of the remote synced/git-annex branch.
+ - This is necessary if a transition has rewritten the git-annex branch.
+ - Normally any changes to the git-annex branch get pulled and merged before
+ - this push, so this forcing is unlikely to overwrite new data pushed
+ - in from another repository that is also syncing.
+ -
+ - But overwriting of data on synced/git-annex can happen, in a race.
+ - The only difference caused by using a forced push in that case is that
+ - the last repository to push wins the race, rather than the first to push.
  -}
 pushBranch :: Remote -> Git.Ref -> Git.Repo -> IO Bool
-pushBranch remote branch g = tryIO directpush `after` syncpush
+pushBranch remote branch g = tryIO (directpush g) `after` syncpush g
   where
-	syncpush = Git.Command.runBool (pushparams (refspec branch)) g
-	directpush = Git.Command.runQuiet (pushparams (show $ Git.Ref.base branch)) g
-	pushparams b =
+	syncpush = Git.Command.runBool $ pushparams
+		[ Git.Branch.forcePush $ refspec Annex.Branch.name
+		, refspec branch
+		]
+	directpush = Git.Command.runQuiet $ pushparams
+		[show $ Git.Ref.base branch]
+	pushparams branches =
 		[ Param "push"
 		, Param $ Remote.name remote
-		, Param $ refspec Annex.Branch.name
-		, Param b
-		]
+		] ++ map Param branches
 	refspec b = concat 
 		[ show $ Git.Ref.base b
 		,  ":"
diff --git a/Git/Branch.hs b/Git/Branch.hs
index d4a684016..fed53d767 100644
--- a/Git/Branch.hs
+++ b/Git/Branch.hs
@@ -101,3 +101,7 @@ commit message branch parentrefs repo = do
 	return sha
   where
 	ps = concatMap (\r -> ["-p", show r]) parentrefs
+
+{- A leading + makes git-push force pushing a branch. -}
+forcePush :: String -> String
+forcePush b = "+" ++ b
diff --git a/GitAnnex.hs b/GitAnnex.hs
index 05565e643..1212edf9f 100644
--- a/GitAnnex.hs
+++ b/GitAnnex.hs
@@ -67,6 +67,7 @@ import qualified Command.Map
 import qualified Command.Direct
 import qualified Command.Indirect
 import qualified Command.Upgrade
+import qualified Command.Forget
 import qualified Command.Version
 import qualified Command.Help
 #ifdef WITH_ASSISTANT
@@ -139,6 +140,7 @@ cmds = concat
 	, Command.Direct.def
 	, Command.Indirect.def
 	, Command.Upgrade.def
+	, Command.Forget.def
 	, Command.Version.def
 	, Command.Help.def
 #ifdef WITH_ASSISTANT
diff --git a/Locations.hs b/Locations.hs
index 1cbbb9886..7762afb64 100644
--- a/Locations.hs
+++ b/Locations.hs
@@ -35,6 +35,7 @@ module Locations (
 	gitAnnexJournalLock,
 	gitAnnexIndex,
 	gitAnnexIndexLock,
+	gitAnnexIgnoredRefs,
 	gitAnnexPidFile,
 	gitAnnexDaemonStatusFile,
 	gitAnnexLogFile,
@@ -225,6 +226,10 @@ gitAnnexIndex r = gitAnnexDir r </> "index"
 gitAnnexIndexLock :: Git.Repo -> FilePath
 gitAnnexIndexLock r = gitAnnexDir r </> "index.lck"
 
+{- List of refs that should not be merged into the git-annex branch. -}
+gitAnnexIgnoredRefs :: Git.Repo -> FilePath
+gitAnnexIgnoredRefs r = gitAnnexDir r </> "ignoredrefs"
+
 {- Pid file for daemon mode. -}
 gitAnnexPidFile :: Git.Repo -> FilePath
 gitAnnexPidFile r = gitAnnexDir r </> "daemon.pid"
diff --git a/Logs.hs b/Logs.hs
new file mode 100644
index 000000000..6339efa13
--- /dev/null
+++ b/Logs.hs
@@ -0,0 +1,110 @@
+{- git-annex log file names
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs where
+
+import Common.Annex
+import Types.Key
+
+data LogVariety = UUIDBasedLog | 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
+	| otherwise = PresenceLog <$> firstJust (presenceLogs f)
+
+{- All the uuid-based logs stored in the git-annex branch. -}
+uuidBasedLogs :: [FilePath]
+uuidBasedLogs =
+	[ uuidLog
+	, remoteLog
+	, trustLog
+	, groupLog 
+	, preferredContentLog
+	]
+
+{- All the ways to get a key from a presence log file -}
+presenceLogs :: FilePath -> [Maybe Key]
+presenceLogs f =
+	[ urlLogFileKey f
+	, locationLogFileKey f
+	]
+
+uuidLog :: FilePath
+uuidLog = "uuid.log"
+
+remoteLog :: FilePath
+remoteLog = "remote.log"
+
+trustLog :: FilePath
+trustLog = "trust.log"
+
+groupLog :: FilePath
+groupLog = "group.log"
+
+preferredContentLog :: FilePath
+preferredContentLog = "preferred-content.log"
+
+{- The pathname of the location log file for a given key. -}
+locationLogFile :: Key -> String
+locationLogFile key = hashDirLower key ++ keyFile key ++ ".log"
+
+{- Converts a pathname into a key if it's a location log. -}
+locationLogFileKey :: FilePath -> Maybe Key
+locationLogFileKey path
+	| ["remote", "web"] `isPrefixOf` splitDirectories dir = Nothing
+        | ext == ".log" = fileKey base
+        | otherwise = Nothing
+  where
+	(dir, file) = splitFileName path
+        (base, ext) = splitAt (length file - 4) file
+
+{- The filename of the url log for a given key. -}
+urlLogFile :: Key -> FilePath
+urlLogFile key = hashDirLower key </> keyFile key ++ urlLogExt
+
+{- Old versions stored the urls elsewhere. -}
+oldurlLogs :: Key -> [FilePath]
+oldurlLogs key =
+	[ "remote/web" </> hashDirLower key </> key2file key ++ ".log"
+	, "remote/web" </> hashDirLower key </> keyFile key ++ ".log"
+	]
+
+urlLogExt :: String
+urlLogExt = ".log.web"
+
+{- Converts a url log file into a key.
+ - (Does not work on oldurlLogs.) -}
+urlLogFileKey :: FilePath -> Maybe Key
+urlLogFileKey path
+	| ext == urlLogExt = fileKey base
+	| otherwise = Nothing
+  where
+  	file = takeFileName path
+	(base, ext) = splitAt (length file - extlen) file
+	extlen = length urlLogExt
+
+{- Does not work on oldurllogs. -}
+isUrlLog :: FilePath -> Bool
+isUrlLog file = urlLogExt `isSuffixOf` file
+
+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)
+	]
+  where
+  	expect = maybe False
+  	isUUIDBasedLog UUIDBasedLog = True
+	isUUIDBasedLog _ = False
+	isPresenceLog (PresenceLog k) = k == dummykey
+	isPresenceLog _ = False
diff --git a/Logs/Group.hs b/Logs/Group.hs
index ee3b75b86..3f88b627d 100644
--- a/Logs/Group.hs
+++ b/Logs/Group.hs
@@ -21,16 +21,13 @@ import qualified Data.Set as S
 import Data.Time.Clock.POSIX
 
 import Common.Annex
+import Logs
 import qualified Annex.Branch
 import qualified Annex
 import Logs.UUIDBased
 import Types.Group
 import Types.StandardGroups
 
-{- Filename of group.log. -}
-groupLog :: FilePath
-groupLog = "group.log"
-
 {- Returns the groups of a given repo UUID. -}
 lookupGroups :: UUID -> Annex (S.Set Group)
 lookupGroups u = (fromMaybe S.empty . M.lookup u) . groupsByUUID <$> groupMap
diff --git a/Logs/Location.hs b/Logs/Location.hs
index 0f57b6663..1289af321 100644
--- a/Logs/Location.hs
+++ b/Logs/Location.hs
@@ -20,12 +20,11 @@ module Logs.Location (
 	loggedLocations,
 	loggedKeys,
 	loggedKeysFor,
-	logFile,
-	logFileKey
 ) where
 
 import Common.Annex
 import qualified Annex.Branch
+import Logs
 import Logs.Presence
 import Annex.UUID
 
@@ -37,19 +36,19 @@ logStatus key status = do
 
 {- Log a change in the presence of a key's value in a repository. -}
 logChange :: Key -> UUID -> LogStatus -> Annex ()
-logChange key (UUID u) s = addLog (logFile key) =<< logNow s u
+logChange key (UUID u) s = addLog (locationLogFile key) =<< logNow s u
 logChange _ NoUUID _ = noop
 
 {- Returns a list of repository UUIDs that, according to the log, have
  - the value of a key.
  -}
 loggedLocations :: Key -> Annex [UUID]
-loggedLocations key = map toUUID <$> (currentLog . logFile) key
+loggedLocations key = map toUUID <$> (currentLog . locationLogFile) key
 
 {- Finds all keys that have location log information.
  - (There may be duplicate keys in the list.) -}
 loggedKeys :: Annex [Key]
-loggedKeys = mapMaybe (logFileKey . takeFileName) <$> Annex.Branch.files
+loggedKeys = mapMaybe locationLogFileKey <$> Annex.Branch.files
 
 {- Finds all keys that have location log information indicating
  - they are present for the specified repository. -}
@@ -62,15 +61,3 @@ loggedKeysFor u = filterM isthere =<< loggedKeys
 		us <- loggedLocations k
 		let !there = u `elem` us
 		return there
-
-{- The filename of the log file for a given key. -}
-logFile :: Key -> String
-logFile key = hashDirLower key ++ keyFile key ++ ".log"
-
-{- Converts a log filename into a key. -}
-logFileKey :: FilePath -> Maybe Key
-logFileKey file
-	| ext == ".log" = fileKey base
-	| otherwise = Nothing
-  where
-	(base, ext) = splitAt (length file - 4) file
diff --git a/Logs/PreferredContent.hs b/Logs/PreferredContent.hs
index 8005fc0d3..947a31875 100644
--- a/Logs/PreferredContent.hs
+++ b/Logs/PreferredContent.hs
@@ -24,6 +24,7 @@ import Data.Time.Clock.POSIX
 import Common.Annex
 import qualified Annex.Branch
 import qualified Annex
+import Logs
 import Logs.UUIDBased
 import Limit
 import qualified Utility.Matcher
@@ -35,10 +36,6 @@ import Logs.Group
 import Logs.Remote
 import Types.StandardGroups
 
-{- Filename of preferred-content.log. -}
-preferredContentLog :: FilePath
-preferredContentLog = "preferred-content.log"
-
 {- Changes the preferred content configuration of a remote. -}
 preferredContentSet :: UUID -> String -> Annex ()
 preferredContentSet uuid@(UUID _) val = do
diff --git a/Logs/Presence.hs b/Logs/Presence.hs
index ec5cec209..516d59618 100644
--- a/Logs/Presence.hs
+++ b/Logs/Presence.hs
@@ -12,36 +12,18 @@
  -}
 
 module Logs.Presence (
-	LogStatus(..),
-	LogLine(LogLine),
+	module X,
 	addLog,
 	readLog,
-	getLog,
-	parseLog,
-	showLog,
 	logNow,
-	compactLog,
-	currentLog,
-	prop_parse_show_log,
+	currentLog
 ) where
 
 import Data.Time.Clock.POSIX
-import Data.Time
-import System.Locale
-import qualified Data.Map as M
 
+import Logs.Presence.Pure as X
 import Common.Annex
 import qualified Annex.Branch
-import Utility.QuickCheck
-
-data LogLine = LogLine {
-	date :: POSIXTime,
-	status :: LogStatus,
-	info :: String
-} deriving (Eq, Show)
-
-data LogStatus = InfoPresent | InfoMissing
-	deriving (Eq, Show, Bounded, Enum)
 
 addLog :: FilePath -> LogLine -> Annex ()
 addLog file line = Annex.Branch.change file $ \s -> 
@@ -52,29 +34,6 @@ addLog file line = Annex.Branch.change file $ \s ->
 readLog :: FilePath -> Annex [LogLine]
 readLog = parseLog <$$> Annex.Branch.get
 
-{- Parses a log file. Unparseable lines are ignored. -}
-parseLog :: String -> [LogLine]
-parseLog = mapMaybe parseline . lines
-  where
-	parseline l = LogLine
-		<$> (utcTimeToPOSIXSeconds <$> parseTime defaultTimeLocale "%s%Qs" d)
-		<*> parsestatus s
-		<*> pure rest
-	  where
-		(d, pastd) = separate (== ' ') l
-		(s, rest) = separate (== ' ') pastd
-	parsestatus "1" = Just InfoPresent
-	parsestatus "0" = Just InfoMissing
-	parsestatus _ = Nothing
-
-{- Generates a log file. -}
-showLog :: [LogLine] -> String
-showLog = unlines . map genline
-  where
-	genline (LogLine d s i) = unwords [show d, genstatus s, i]
-	genstatus InfoPresent = "1"
-	genstatus InfoMissing = "0"
-
 {- Generates a new LogLine with the current date. -}
 logNow :: LogStatus -> String -> Annex LogLine
 logNow s i = do
@@ -84,39 +43,3 @@ logNow s i = do
 {- Reads a log and returns only the info that is still in effect. -}
 currentLog :: FilePath -> Annex [String]
 currentLog file = map info . filterPresent <$> readLog file
-
-{- Given a log, returns only the info that is are still in effect. -}
-getLog :: String -> [String]
-getLog = map info . filterPresent . parseLog
-
-{- Returns the info from LogLines that are in effect. -}
-filterPresent :: [LogLine] -> [LogLine]
-filterPresent = filter (\l -> InfoPresent == status l) . compactLog
-
-{- Compacts a set of logs, returning a subset that contains the current
- - status. -}
-compactLog :: [LogLine] -> [LogLine]
-compactLog = M.elems . foldr mapLog M.empty
-
-type LogMap = M.Map String LogLine
-
-{- Inserts a log into a map of logs, if the log has better (ie, newer)
- - information than the other logs in the map -}
-mapLog :: LogLine -> LogMap -> LogMap
-mapLog l m
-	| better = M.insert i l m
-	| otherwise = m
-  where
-	better = maybe True newer $ M.lookup i m
-	newer l' = date l' <= date l
-	i = info l
-
-instance Arbitrary LogLine where
-	arbitrary = LogLine
-		<$> arbitrary
-		<*> elements [minBound..maxBound]
-		<*> arbitrary `suchThat` ('\n' `notElem`)
-
-prop_parse_show_log :: [LogLine] -> Bool
-prop_parse_show_log l = parseLog (showLog l) == l
-
diff --git a/Logs/Presence/Pure.hs b/Logs/Presence/Pure.hs
new file mode 100644
index 000000000..ffeb78b26
--- /dev/null
+++ b/Logs/Presence/Pure.hs
@@ -0,0 +1,84 @@
+{- git-annex presence log, pure operations
+ -
+ - Copyright 2010-2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs.Presence.Pure where
+
+import Data.Time.Clock.POSIX
+import Data.Time
+import System.Locale
+import qualified Data.Map as M
+
+import Common.Annex
+import Utility.QuickCheck
+
+data LogLine = LogLine {
+	date :: POSIXTime,
+	status :: LogStatus,
+	info :: String
+} deriving (Eq, Show)
+
+data LogStatus = InfoPresent | InfoMissing
+	deriving (Eq, Show, Bounded, Enum)
+
+{- Parses a log file. Unparseable lines are ignored. -}
+parseLog :: String -> [LogLine]
+parseLog = mapMaybe parseline . lines
+  where
+	parseline l = LogLine
+		<$> (utcTimeToPOSIXSeconds <$> parseTime defaultTimeLocale "%s%Qs" d)
+		<*> parsestatus s
+		<*> pure rest
+	  where
+		(d, pastd) = separate (== ' ') l
+		(s, rest) = separate (== ' ') pastd
+	parsestatus "1" = Just InfoPresent
+	parsestatus "0" = Just InfoMissing
+	parsestatus _ = Nothing
+
+{- Generates a log file. -}
+showLog :: [LogLine] -> String
+showLog = unlines . map genline
+  where
+	genline (LogLine d s i) = unwords [show d, genstatus s, i]
+	genstatus InfoPresent = "1"
+	genstatus InfoMissing = "0"
+
+{- Given a log, returns only the info that is are still in effect. -}
+getLog :: String -> [String]
+getLog = map info . filterPresent . parseLog
+
+{- Returns the info from LogLines that are in effect. -}
+filterPresent :: [LogLine] -> [LogLine]
+filterPresent = filter (\l -> InfoPresent == status l) . compactLog
+
+{- Compacts a set of logs, returning a subset that contains the current
+ - status. -}
+compactLog :: [LogLine] -> [LogLine]
+compactLog = M.elems . foldr mapLog M.empty
+
+type LogMap = M.Map String LogLine
+
+{- Inserts a log into a map of logs, if the log has better (ie, newer)
+ - information than the other logs in the map -}
+mapLog :: LogLine -> LogMap -> LogMap
+mapLog l m
+	| better = M.insert i l m
+	| otherwise = m
+  where
+	better = maybe True newer $ M.lookup i m
+	newer l' = date l' <= date l
+	i = info l
+
+instance Arbitrary LogLine where
+	arbitrary = LogLine
+		<$> arbitrary
+		<*> elements [minBound..maxBound]
+		<*> arbitrary `suchThat` ('\n' `notElem`)
+
+prop_parse_show_log :: [LogLine] -> Bool
+prop_parse_show_log l = parseLog (showLog l) == l
+
diff --git a/Logs/Remote.hs b/Logs/Remote.hs
index 89792b054..48ee9eb60 100644
--- a/Logs/Remote.hs
+++ b/Logs/Remote.hs
@@ -25,12 +25,9 @@ import Data.Char
 import Common.Annex
 import qualified Annex.Branch
 import Types.Remote
+import Logs
 import Logs.UUIDBased
 
-{- Filename of remote.log. -}
-remoteLog :: FilePath
-remoteLog = "remote.log"
-
 {- Adds or updates a remote's config in the log. -}
 configSet :: UUID -> RemoteConfig -> Annex ()
 configSet u c = do
diff --git a/Logs/Transitions.hs b/Logs/Transitions.hs
new file mode 100644
index 000000000..783ce5090
--- /dev/null
+++ b/Logs/Transitions.hs
@@ -0,0 +1,87 @@
+{- git-annex transitions log
+ -
+ - This is used to record transitions that have been performed on the
+ - git-annex branch, and when the transition was first started.
+ -
+ - We can quickly detect when the local branch has already had an transition
+ - done that is listed in the remote branch by checking that the local
+ - branch contains the same transition, with the same or newer start time.
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs.Transitions where
+
+import Data.Time.Clock.POSIX
+import Data.Time
+import System.Locale
+import qualified Data.Set as S
+
+import Common.Annex
+
+transitionsLog :: FilePath
+transitionsLog = "transitions.log"
+
+data Transition
+	= ForgetGitHistory
+	| ForgetDeadRemotes
+	deriving (Show, Ord, Eq, Read)
+
+data TransitionLine = TransitionLine
+	{ transitionStarted :: POSIXTime
+	, transition :: Transition
+	} deriving (Show, Ord, Eq)
+
+type Transitions = S.Set TransitionLine
+
+describeTransition :: Transition -> String
+describeTransition ForgetGitHistory = "forget git history"
+describeTransition ForgetDeadRemotes = "forget dead remotes"
+
+noTransitions :: Transitions
+noTransitions = S.empty
+
+addTransition :: POSIXTime -> Transition -> Transitions -> Transitions
+addTransition ts t = S.insert $ TransitionLine ts t
+
+showTransitions :: Transitions -> String
+showTransitions = unlines . map showTransitionLine . S.elems
+
+{- If the log contains new transitions we don't support, returns Nothing. -}
+parseTransitions :: String -> Maybe Transitions
+parseTransitions = check . map parseTransitionLine . lines
+  where
+  	check l
+		| all isJust l = Just $ S.fromList $ catMaybes l
+		| otherwise = Nothing
+
+parseTransitionsStrictly :: String -> String -> Transitions
+parseTransitionsStrictly source = fromMaybe badsource . parseTransitions
+  where
+	badsource = error $ "unknown transitions listed in " ++ source ++ "; upgrade git-annex!"
+
+showTransitionLine :: TransitionLine -> String
+showTransitionLine (TransitionLine ts t) = unwords [show t, show ts]
+
+parseTransitionLine :: String -> Maybe TransitionLine
+parseTransitionLine s = TransitionLine <$> pdate ds <*> readish ts
+  where
+  	ws = words s
+  	ts = Prelude.head ws
+	ds = unwords $ Prelude.tail ws
+	pdate = parseTime defaultTimeLocale "%s%Qs" >=*> utcTimeToPOSIXSeconds
+
+combineTransitions :: [Transitions] -> Transitions
+combineTransitions = S.unions
+
+transitionList :: Transitions -> [Transition]
+transitionList = map transition . S.elems
+
+{- Typically ran with Annex.Branch.change, but we can't import Annex.Branch
+ - here since it depends on this module. -}
+recordTransitions :: (FilePath -> (String -> String) -> Annex ()) -> Transitions -> Annex ()
+recordTransitions changer t = do
+	changer transitionsLog $
+		showTransitions . S.union t . parseTransitionsStrictly "local"
diff --git a/Logs/Trust.hs b/Logs/Trust.hs
index eb6e42ad7..c6f0ad3ab 100644
--- a/Logs/Trust.hs
+++ b/Logs/Trust.hs
@@ -6,6 +6,7 @@
  -}
 
 module Logs.Trust (
+	module X,
 	trustLog,
 	TrustLevel(..),
 	trustGet,
@@ -16,8 +17,6 @@ module Logs.Trust (
 	lookupTrust,
 	trustMapLoad,
 	trustMapRaw,
-	
-	prop_parse_show_TrustLog,
 ) where
 
 import qualified Data.Map as M
@@ -27,13 +26,11 @@ import Common.Annex
 import Types.TrustLevel
 import qualified Annex.Branch
 import qualified Annex
+import Logs
 import Logs.UUIDBased
 import Remote.List
 import qualified Types.Remote
-
-{- Filename of trust.log. -}
-trustLog :: FilePath
-trustLog = "trust.log"
+import Logs.Trust.Pure as X
 
 {- Returns a list of UUIDs that the trustLog indicates have the
  - specified trust level.
@@ -97,26 +94,4 @@ trustMapLoad = do
 {- Does not include forcetrust or git config values, just those from the
  - log file. -}
 trustMapRaw :: Annex TrustMap
-trustMapRaw = simpleMap . parseLog (Just . parseTrustLog)
-	<$> Annex.Branch.get trustLog
-
-{- The trust.log used to only list trusted repos, without a field for the
- - trust status, which is why this defaults to Trusted. -}
-parseTrustLog :: String -> TrustLevel
-parseTrustLog s = maybe Trusted parse $ headMaybe $ words s
-  where
-	parse "1" = Trusted
-	parse "0" = UnTrusted
-	parse "X" = DeadTrusted
-	parse _ = SemiTrusted
-
-showTrustLog :: TrustLevel -> String
-showTrustLog Trusted = "1"
-showTrustLog UnTrusted = "0"
-showTrustLog DeadTrusted = "X"
-showTrustLog SemiTrusted = "?"
-
-prop_parse_show_TrustLog :: Bool
-prop_parse_show_TrustLog = all check [minBound .. maxBound]
-  where
-	check l = parseTrustLog (showTrustLog l) == l
+trustMapRaw = calcTrustMap <$> Annex.Branch.get trustLog
diff --git a/Logs/Trust/Pure.hs b/Logs/Trust/Pure.hs
new file mode 100644
index 000000000..11cfbe056
--- /dev/null
+++ b/Logs/Trust/Pure.hs
@@ -0,0 +1,36 @@
+{- git-annex trust log, pure operations
+ -
+ - Copyright 2010-2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs.Trust.Pure where
+
+import Common.Annex
+import Types.TrustLevel
+import Logs.UUIDBased
+
+calcTrustMap :: String -> TrustMap
+calcTrustMap = simpleMap . parseLog (Just . parseTrustLog)
+
+{- The trust.log used to only list trusted repos, without a field for the
+ - trust status, which is why this defaults to Trusted. -}
+parseTrustLog :: String -> TrustLevel
+parseTrustLog s = maybe Trusted parse $ headMaybe $ words s
+  where
+	parse "1" = Trusted
+	parse "0" = UnTrusted
+	parse "X" = DeadTrusted
+	parse _ = SemiTrusted
+
+showTrustLog :: TrustLevel -> String
+showTrustLog Trusted = "1"
+showTrustLog UnTrusted = "0"
+showTrustLog DeadTrusted = "X"
+showTrustLog SemiTrusted = "?"
+
+prop_parse_show_TrustLog :: Bool
+prop_parse_show_TrustLog = all check [minBound .. maxBound]
+  where
+	check l = parseTrustLog (showTrustLog l) == l
diff --git a/Logs/UUID.hs b/Logs/UUID.hs
index 2f24a388e..ef1074e78 100644
--- a/Logs/UUID.hs
+++ b/Logs/UUID.hs
@@ -28,13 +28,10 @@ import Types.UUID
 import Common.Annex
 import qualified Annex
 import qualified Annex.Branch
+import Logs
 import Logs.UUIDBased
 import qualified Annex.UUID
 
-{- Filename of uuid.log. -}
-uuidLog :: FilePath
-uuidLog = "uuid.log"
-
 {- Records a description for a uuid in the log. -}
 describeUUID :: UUID -> String -> Annex ()
 describeUUID uuid desc = do
diff --git a/Logs/Web.hs b/Logs/Web.hs
index 47ab61943..0239f2335 100644
--- a/Logs/Web.hs
+++ b/Logs/Web.hs
@@ -11,8 +11,6 @@ module Logs.Web (
 	getUrls,
 	setUrlPresent,
 	setUrlMissing,
-	urlLog,
-	urlLogKey,
 	knownUrls,
 	Downloader(..),
 	getDownloader,
@@ -22,9 +20,9 @@ module Logs.Web (
 import qualified Data.ByteString.Lazy.Char8 as L
 
 import Common.Annex
+import Logs
 import Logs.Presence
 import Logs.Location
-import Types.Key
 import qualified Annex.Branch
 import Annex.CatFile
 import qualified Git
@@ -36,35 +34,9 @@ type URLString = String
 webUUID :: UUID
 webUUID = UUID "00000000-0000-0000-0000-000000000001"
 
-urlLogExt :: String
-urlLogExt = ".log.web"
-
-urlLog :: Key -> FilePath
-urlLog key = hashDirLower key </> keyFile key ++ urlLogExt
-
-{- Converts a url log file into a key.
- - (Does not work on oldurlLogs.) -}
-urlLogKey :: FilePath -> Maybe Key
-urlLogKey file
-	| ext == urlLogExt = fileKey base
-	| otherwise = Nothing
-  where
-  	(base, ext) = splitAt (length file - extlen) file
-	extlen = length urlLogExt
-
-isUrlLog :: FilePath -> Bool
-isUrlLog file = urlLogExt `isSuffixOf` file
-
-{- Used to store the urls elsewhere. -}
-oldurlLogs :: Key -> [FilePath]
-oldurlLogs key = 
-	[ "remote/web" </> hashDirLower key </> key2file key ++ ".log"
-	, "remote/web" </> hashDirLower key </> keyFile key ++ ".log"
-	]
-
 {- Gets all urls that a key might be available from. -}
 getUrls :: Key -> Annex [URLString]
-getUrls key = go $ urlLog key : oldurlLogs key
+getUrls key = go $ urlLogFile key : oldurlLogs key
   where
 	go [] = return []
 	go (l:ls) = do
@@ -77,13 +49,13 @@ setUrlPresent :: Key -> URLString -> Annex ()
 setUrlPresent key url = do
 	us <- getUrls key
 	unless (url `elem` us) $ do
-		addLog (urlLog key) =<< logNow InfoPresent url
+		addLog (urlLogFile key) =<< logNow InfoPresent url
 		-- update location log to indicate that the web has the key
 		logChange key webUUID InfoPresent
 
 setUrlMissing :: Key -> URLString -> Annex ()
 setUrlMissing key url = do
-	addLog (urlLog key) =<< logNow InfoMissing url
+	addLog (urlLogFile key) =<< logNow InfoMissing url
 	whenM (null <$> getUrls key) $
 		logChange key webUUID InfoMissing
 
diff --git a/Test.hs b/Test.hs
index 3eb330c22..ec70c4ecb 100644
--- a/Test.hs
+++ b/Test.hs
@@ -33,6 +33,7 @@ import qualified Types.KeySource
 import qualified Types.Backend
 import qualified Types.TrustLevel
 import qualified Types
+import qualified Logs
 import qualified Logs.UUIDBased
 import qualified Logs.Trust
 import qualified Logs.Remote
@@ -115,6 +116,7 @@ quickcheck =
 	, check "prop_idempotent_key_encode" Types.Key.prop_idempotent_key_encode
 	, check "prop_idempotent_shellEscape" Utility.SafeCommand.prop_idempotent_shellEscape
 	, check "prop_idempotent_shellEscape_multiword" Utility.SafeCommand.prop_idempotent_shellEscape_multiword
+	, check "prop_logs_sane" Logs.prop_logs_sane
 	, check "prop_idempotent_configEscape" Logs.Remote.prop_idempotent_configEscape
 	, check "prop_parse_show_Config" Logs.Remote.prop_parse_show_Config
 	, check "prop_parentDir_basics" Utility.Path.prop_parentDir_basics
diff --git a/Upgrade/V2.hs b/Upgrade/V2.hs
index b5de6c8c0..42419b8ab 100644
--- a/Upgrade/V2.hs
+++ b/Upgrade/V2.hs
@@ -12,9 +12,9 @@ import qualified Git
 import qualified Git.Command
 import qualified Git.Ref
 import qualified Annex.Branch
-import Logs.Location
 import Annex.Content
 import Utility.Tmp
+import Logs
 
 olddir :: Git.Repo -> FilePath
 olddir g
@@ -47,7 +47,7 @@ upgrade = do
 
 	e <- liftIO $ doesDirectoryExist old
 	when e $ do
-		mapM_ (\(k, f) -> inject f $ logFile k) =<< locationLogs
+		mapM_ (\(k, f) -> inject f $ locationLogFile k) =<< locationLogs
 		mapM_ (\f -> inject f f) =<< logFiles old
 
 	saveState False
@@ -73,7 +73,7 @@ locationLogs = do
   where
 	tryDirContents d = catchDefaultIO [] $ dirContents d
 	islogfile f = maybe Nothing (\k -> Just (k, f)) $
-			logFileKey $ takeFileName f
+			locationLogFileKey f
 
 inject :: FilePath -> FilePath -> Annex ()
 inject source dest = do
diff --git a/Utility/Misc.hs b/Utility/Misc.hs
index 804a9e487..48ce4c929 100644
--- a/Utility/Misc.hs
+++ b/Utility/Misc.hs
@@ -91,6 +91,12 @@ massReplace vs = go [] vs
 			go (replacement:acc) vs (drop (length val) s)
 		| otherwise = go acc rest s
 
+{- First item in the list that is not Nothing. -}
+firstJust :: Eq a => [Maybe a] -> Maybe a
+firstJust ms = case dropWhile (== Nothing) ms of
+	[] -> Nothing
+	(md:_) -> md
+
 {- Given two orderings, returns the second if the first is EQ and returns
  - the first otherwise.
  -
diff --git a/debian/changelog b/debian/changelog
index 59fdeff36..f880fbcf7 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,12 +1,17 @@
 git-annex (4.20130828) UNRELEASED; urgency=low
 
+  * forget: New command, causes git-annex branch history to be forgotten
+    in a way that will spread to other clones of the repository.
+    (As long as they're running this version or newer of git-annex.)
+  * forget --drop-dead: Completely removes mentions of repositories that
+    have been marked as dead from the git-annex branch.
+  * sync, assistant: Force push of the git-annex branch. Necessary
+    to ensure it gets pushed to remotes after being rewritten by forget.
   * importfeed: Also ignore transient problems with downloading content
     from feeds.
   * Honor core.sharedrepository when receiving and adding files in direct
     mode.
 
- -- Joey Hess <joeyh@debian.org>  Tue, 03 Sep 2013 14:31:45 -0400
-
 git-annex (4.20130827) unstable; urgency=low
 
   * Youtube support! (And 53 other video hosts). When quvi is installed,
diff --git a/doc/git-annex.mdwn b/doc/git-annex.mdwn
index 7cac9087d..269588add 100644
--- a/doc/git-annex.mdwn
+++ b/doc/git-annex.mdwn
@@ -479,6 +479,23 @@ subdirectories).
 
   Upgrades the repository to current layout.
 
+* forget
+
+  Causes the git-annex branch to be rewritten, throwing away historical
+  data about past locations of files. The resulting branch will use less
+  space, but `git annex log` will not be able to show where
+  files used to be located.
+  
+  To also prune references to repositories that have been marked as dead,
+  specify --drop-dead.
+
+  When this rewritten branch is merged into other clones of
+  the repository, git-annex will automatically perform the same rewriting
+  to their local git-annex branches. So the forgetfulness will automatically
+  propigate out from its starting point until all repositories running
+  git-annex have forgotten their old history. (You may need to force
+  git to push the branch to any git repositories not running git-annex.
+
 # QUERY COMMANDS
 
 * version