remove unused backend machinery

The only remaining vestiage of backends is different types of keys. These
are still called "backends", mostly to avoid needing to change user interface
and configuration. But everything to do with storing keys in different
backends was gone; instead different types of remotes are used.

In the refactoring, lots of code was moved out of odd corners like
Backend.File, to closer to where it's used, like Command.Drop and
Command.Fsck. Quite a lot of dead code was removed. Several data structures
became simpler, which may result in better runtime efficiency. There should
be no user-visible changes.

25 files changed, 308 insertions, 445 deletions

diff --git a/Annex.hs b/Annex.hs
index c21cfb37c..f7e3e29f8 100644
--- a/Annex.hs
+++ b/Annex.hs
@@ -34,7 +34,6 @@ type Annex = StateT AnnexState IO
 data AnnexState = AnnexState
 	{ repo :: Git.Repo
 	, backends :: [Backend Annex]
-	, supportedBackends :: [Backend Annex]
 	, remotes :: [Remote Annex]
 	, repoqueue :: Queue
 	, quiet :: Bool
@@ -52,12 +51,11 @@ data AnnexState = AnnexState
 	, cipher :: Maybe Cipher
 	}
 
-newState :: [Backend Annex] -> Git.Repo -> AnnexState
-newState allbackends gitrepo = AnnexState
+newState :: Git.Repo -> AnnexState
+newState gitrepo = AnnexState
 	{ repo = gitrepo
 	, backends = []
 	, remotes = []
-	, supportedBackends = allbackends
 	, repoqueue = empty
 	, quiet = False
 	, force = False
@@ -75,9 +73,8 @@ newState allbackends gitrepo = AnnexState
 	}
 
 {- Create and returns an Annex state object for the specified git repo. -}
-new :: Git.Repo -> [Backend Annex] -> IO AnnexState
-new gitrepo allbackends =
-	newState allbackends `liftM` (liftIO . Git.configRead) gitrepo
+new :: Git.Repo -> IO AnnexState
+new gitrepo = newState `liftM` (liftIO . Git.configRead) gitrepo
 
 {- performs an action in the Annex monad -}
 run :: AnnexState -> Annex a -> IO (a, AnnexState)
diff --git a/Backend.hs b/Backend.hs
index b1cd4c8f0..cf976d2b8 100644
--- a/Backend.hs
+++ b/Backend.hs
@@ -1,16 +1,4 @@
-{- git-annex key-value storage backends
- -
- - git-annex uses a key-value abstraction layer to allow files contents to be
- - stored in different ways. In theory, any key-value storage system could be
- - used to store the file contents, and git-annex would then retrieve them
- - as needed and put them in `.git/annex/`.
- - 
- - When a file is annexed, a key is generated from its content and/or metadata.
- - This key can later be used to retrieve the file's content (its value). This
- - key generation must be stable for a given file content, name, and size.
- - 
- - Multiple pluggable backends are supported, and more than one can be used
- - to store different files' contents in a given repository.
+{- git-annex key/value backends
  -
  - Copyright 2010 Joey Hess <joey@kitenet.net>
  -
@@ -19,15 +7,10 @@
 
 module Backend (
 	list,
-	storeFileKey,
-	retrieveKeyFile,
-	removeKey,
-	hasKey,
-	fsckKey,
-	upgradableKey,
+	orderedList,
+	genKey,
 	lookupFile,
 	chooseBackends,
-	keyBackend,
 	lookupBackendName,
 	maybeLookupBackendName
 ) where
@@ -36,7 +19,6 @@ import Control.Monad.State (liftIO, when)
 import System.IO.Error (try)
 import System.FilePath
 import System.Posix.Files
-import System.Directory
 
 import Locations
 import qualified Git
@@ -45,12 +27,20 @@ import Types
 import Types.Key
 import qualified Types.Backend as B
 import Messages
-import Content
-import DataUnits
+
+-- When adding a new backend, import it here and add it to the list.
+import qualified Backend.WORM
+import qualified Backend.SHA
+
+list :: [Backend Annex]
+list = concat 
+	[ Backend.WORM.backends
+	, Backend.SHA.backends
+	]
 
 {- List of backends in the order to try them when storing a new key. -}
-list :: Annex [Backend Annex]
-list = do
+orderedList :: Annex [Backend Annex]
+orderedList = do
 	l <- Annex.getState Annex.backends -- list is cached here
 	if not $ null l
 		then return l
@@ -59,92 +49,49 @@ list = do
 			d <- Annex.getState Annex.forcebackend
 			handle d s
 	where
-		parseBackendList l [] = l
-		parseBackendList bs s = map (lookupBackendName bs) $ words s
+		parseBackendList [] = list
+		parseBackendList s = map lookupBackendName $ words s
 		handle Nothing s = return s
 		handle (Just "") s = return s
 		handle (Just name) s = do
-			bs <- Annex.getState Annex.supportedBackends
-			let l' = (lookupBackendName bs name):s
+			let l' = (lookupBackendName name):s
 			Annex.changeState $ \state -> state { Annex.backends = l' }
 			return l'
 		getstandard = do
-			bs <- Annex.getState Annex.supportedBackends
 			g <- Annex.gitRepo
-			return $ parseBackendList bs $
+			return $ parseBackendList $
 				Git.configGet g "annex.backends" ""
 
-{- Looks up a backend in a list. May fail if unknown. -}
-lookupBackendName :: [Backend Annex] -> String -> Backend Annex
-lookupBackendName bs s = maybe unknown id $ maybeLookupBackendName bs s
-	where
-		unknown = error $ "unknown backend " ++ s
-maybeLookupBackendName :: [Backend Annex] -> String -> Maybe (Backend Annex)
-maybeLookupBackendName bs s =
-	if 1 /= length matches
-		then Nothing
-		else Just $ head matches
-	where matches = filter (\b -> s == B.name b) bs
-
-{- Attempts to store a file in one of the backends. -}
-storeFileKey :: FilePath -> Maybe (Backend Annex) -> Annex (Maybe (Key, Backend Annex))
-storeFileKey file trybackend = do
-	bs <- list
+{- Generates a key for a file, trying each backend in turn until one
+ - accepts it. -}
+genKey :: FilePath -> Maybe (Backend Annex) -> Annex (Maybe (Key, Backend Annex))
+genKey file trybackend = do
+	bs <- orderedList
 	let bs' = maybe bs (:bs) trybackend
-	storeFileKey' bs' file
-storeFileKey' :: [Backend Annex] -> FilePath -> Annex (Maybe (Key, Backend Annex))
-storeFileKey' [] _ = return Nothing
-storeFileKey' (b:bs) file = maybe nextbackend store =<< (B.getKey b) file
-	where
-		nextbackend = storeFileKey' bs file
-		store key = do
-			stored <- (B.storeFileKey b) file key
-			if (not stored)
-				then nextbackend
-				else return $ Just (key, b)
-
-{- Attempts to retrieve an key from one of the backends, saving it to
- - a specified location. -}
-retrieveKeyFile :: Backend Annex -> Key -> FilePath -> Annex Bool
-retrieveKeyFile backend key dest = (B.retrieveKeyFile backend) key dest
-
-{- Removes a key from a backend. -}
-removeKey :: Backend Annex -> Key -> Maybe Int -> Annex Bool
-removeKey backend key numcopies = (B.removeKey backend) key numcopies
-
-{- Checks if a key is present in its backend. -}
-hasKey :: Key -> Annex Bool
-hasKey key = do
-	backend <- keyBackend key
-	(B.hasKey backend) key
-
-{- Checks a key for problems. -}
-fsckKey :: Backend Annex -> Key -> Maybe FilePath -> Maybe Int -> Annex Bool
-fsckKey backend key file numcopies = do
-	size_ok <- checkKeySize key
-	backend_ok <-(B.fsckKey backend) key file numcopies
-	return $ size_ok && backend_ok
-
-{- Checks if a key is upgradable to a newer representation. -}
-upgradableKey :: Backend Annex -> Key -> Annex Bool
-upgradableKey backend key = (B.upgradableKey backend) key
+	genKey' bs' file
+genKey' :: [Backend Annex] -> FilePath -> Annex (Maybe (Key, Backend Annex))
+genKey' [] _ = return Nothing
+genKey' (b:bs) file = do
+	r <- (B.getKey b) file
+	case r of
+		Nothing -> genKey' bs file
+		Just k -> return $ Just (k, b)
 
 {- Looks up the key and backend corresponding to an annexed file,
  - by examining what the file symlinks to. -}
 lookupFile :: FilePath -> Annex (Maybe (Key, Backend Annex))
 lookupFile file = do
-	bs <- Annex.getState Annex.supportedBackends
 	tl <- liftIO $ try getsymlink
 	case tl of
 		Left _ -> return Nothing
-		Right l -> makekey bs l
+		Right l -> makekey l
 	where
 		getsymlink = do
 			l <- readSymbolicLink file
 			return $ takeFileName l
-		makekey bs l = maybe (return Nothing) (makeret bs l) (fileKey l)
-		makeret bs l k =
-			case maybeLookupBackendName bs bname of
+		makekey l = maybe (return Nothing) (makeret l) (fileKey l)
+		makeret l k =
+			case maybeLookupBackendName bname of
 					Just backend -> return $ Just (k, backend)
 					Nothing -> do
 						when (isLinkToAnnex l) $
@@ -164,37 +111,20 @@ chooseBackends fs = do
 	forced <- Annex.getState Annex.forcebackend
 	if forced /= Nothing
 		then do
-			l <- list
+			l <- orderedList
 			return $ map (\f -> (f, Just $ head l)) fs
 		else do
-			bs <- Annex.getState Annex.supportedBackends
 			pairs <- liftIO $ Git.checkAttr g "annex.backend" fs
-			return $ map (\(f,b) -> (f, maybeLookupBackendName bs b)) pairs
-
-{- Returns the backend to use for a key. -}
-keyBackend :: Key -> Annex (Backend Annex)
-keyBackend key = do
-	bs <- Annex.getState Annex.supportedBackends
-	return $ lookupBackendName bs $ keyBackendName key
+			return $ map (\(f,b) -> (f, maybeLookupBackendName b)) pairs
 
-{- The size of the data for a key is checked against the size encoded in
- - the key's metadata, if available. -}
-checkKeySize :: Key -> Annex Bool
-checkKeySize key = do
-	g <- Annex.gitRepo
-	let file = gitAnnexLocation g key
-	present <- liftIO $ doesFileExist file
-	case (present, keySize key) of
-		(_, Nothing) -> return True
-		(False, _) -> return True
-		(True, Just size) -> do
-			stat <- liftIO $ getFileStatus file
-			let size' = fromIntegral (fileSize stat)
-			if size == size'
-				then return True
-				else do
-					dest <- moveBad key
-					warning $ "Bad file size (" ++
-						compareSizes storageUnits True size size' ++ 
-						"); moved to " ++ dest
-					return False
+{- Looks up a backend by name. May fail if unknown. -}
+lookupBackendName :: String -> Backend Annex
+lookupBackendName s = maybe unknown id $ maybeLookupBackendName s
+	where
+		unknown = error $ "unknown backend " ++ s
+maybeLookupBackendName :: String -> Maybe (Backend Annex)
+maybeLookupBackendName s =
+	if 1 /= length matches
+		then Nothing
+		else Just $ head matches
+	where matches = filter (\b -> s == B.name b) list
diff --git a/Backend/File.hs b/Backend/File.hs
deleted file mode 100644
index 174da4e6d..000000000
--- a/Backend/File.hs
+++ /dev/null
@@ -1,220 +0,0 @@
-{- git-annex pseudo-backend
- -
- - This backend does not really do any independant data storage,
- - it relies on the file contents in .git/annex/ in this repo,
- - and other accessible repos.
- -
- - This is an abstract backend; name, getKey and fsckKey have to be implemented
- - to complete it.
- -
- - Copyright 2010 Joey Hess <joey@kitenet.net>
- -
- - Licensed under the GNU GPL version 3 or higher.
- -}
-
-module Backend.File (backend, checkKey) where
-
-import Data.List
-import Data.String.Utils
-
-import Types.Backend
-import LocationLog
-import qualified Remote
-import qualified Git
-import Content
-import qualified Annex
-import Types
-import UUID
-import Messages
-import Trust
-import Types.Key
-
-backend :: Backend Annex
-backend = Backend {
-	name = mustProvide,
-	getKey = mustProvide,
-	storeFileKey = dummyStore,
-	retrieveKeyFile = copyKeyFile,
-	removeKey = checkRemoveKey,
-	hasKey = inAnnex,
-	fsckKey = checkKeyOnly,
-	upgradableKey = checkUpgradableKey
-}
-
-mustProvide :: a
-mustProvide = error "must provide this field"
-
-{- Storing a key is a no-op. -}
-dummyStore :: FilePath -> Key -> Annex Bool
-dummyStore _ _ = return True
-
-{- Try to find a copy of the file in one of the remotes,
- - and copy it to here. -}
-copyKeyFile :: Key -> FilePath -> Annex Bool
-copyKeyFile key file = do
-	remotes <- Remote.keyPossibilities key
-	if null remotes
-		then do
-			showNote "not available"
-			showLocations key []
-			return False
-		else trycopy remotes remotes
-	where
-		trycopy full [] = do
-			showTriedRemotes full
-			showLocations key []
-			return False
-		trycopy full (r:rs) = do
-			probablythere <- probablyPresent r
-			if probablythere
-				then docopy r (trycopy full rs)
-				else trycopy full rs
-		-- This check is to avoid an ugly message if a remote is a
-		-- drive that is not mounted.
-		probablyPresent r =
-			if Remote.hasKeyCheap r
-				then do
-					res <- Remote.hasKey r key
-					case res of
-						Right b -> return b
-						Left _ -> return False
-				else return True
-		docopy r continue = do
-			showNote $ "from " ++ Remote.name r ++ "..."
-			copied <- Remote.retrieveKeyFile r key file
-			if copied
-				then return True
-				else continue
-
-{- Checks remotes to verify that enough copies of a key exist to allow
- - for a key to be safely removed (with no data loss), and fails with an
- - error if not. -}
-checkRemoveKey :: Key -> Maybe Int -> Annex Bool
-checkRemoveKey key numcopiesM = do
-	force <- Annex.getState Annex.force
-	if force || numcopiesM == Just 0
-		then return True
-		else do
-			(remotes, trusteduuids) <- Remote.keyPossibilitiesTrusted key
-			untrusteduuids <- trustGet UnTrusted
-			let tocheck = Remote.remotesWithoutUUID remotes (trusteduuids++untrusteduuids)
-			numcopies <- getNumCopies numcopiesM
-			findcopies numcopies trusteduuids tocheck []
-	where
-		findcopies need have [] bad
-			| length have >= need = return True
-			| otherwise = notEnoughCopies need have bad
-		findcopies need have (r:rs) bad
-			| length have >= need = return True
-			| otherwise = do
-				let u = Remote.uuid r
-				let dup = u `elem` have
-				haskey <- Remote.hasKey r key
-				case (dup, haskey) of
-					(False, Right True)	-> findcopies need (u:have) rs bad
-					(False, Left _)		-> findcopies need have rs (r:bad)
-					_			-> findcopies need have rs bad
-		notEnoughCopies need have bad = do
-			unsafe
-			showLongNote $
-				"Could only verify the existence of " ++
-				show (length have) ++ " out of " ++ show need ++ 
-				" necessary copies"
-			showTriedRemotes bad
-			showLocations key have
-			hint
-			return False
-		unsafe = showNote "unsafe"
-		hint = showLongNote "(Use --force to override this check, or adjust annex.numcopies.)"
-
-showLocations :: Key -> [UUID] -> Annex ()
-showLocations key exclude = do
-	g <- Annex.gitRepo
-	u <- getUUID g
-	uuids <- keyLocations key
-	untrusteduuids <- trustGet UnTrusted
-	let uuidswanted = filteruuids uuids (u:exclude++untrusteduuids) 
-	let uuidsskipped = filteruuids uuids (u:exclude++uuidswanted)
-	ppuuidswanted <- Remote.prettyPrintUUIDs uuidswanted
-	ppuuidsskipped <- Remote.prettyPrintUUIDs uuidsskipped
-	showLongNote $ message ppuuidswanted ppuuidsskipped
-	where
-		filteruuids list x = filter (`notElem` x) list
-		message [] [] = "No other repository is known to contain the file."
-		message rs [] = "Try making some of these repositories available:\n" ++ rs
-		message [] us = "Also these untrusted repositories may contain the file:\n" ++ us
-		message rs us = message rs [] ++ message [] us
-
-showTriedRemotes :: [Remote.Remote Annex] -> Annex ()
-showTriedRemotes [] = return ()	
-showTriedRemotes remotes =
-	showLongNote $ "Unable to access these remotes: " ++
-		(join ", " $ map Remote.name remotes)
-
-{- If a value is specified, it is used; otherwise the default is looked up
- - in git config. forcenumcopies overrides everything. -}
-getNumCopies :: Maybe Int -> Annex Int
-getNumCopies v = 
-	Annex.getState Annex.forcenumcopies >>= maybe (use v) (return . id)
-	where
-		use (Just n) = return n
-		use Nothing = do
-			g <- Annex.gitRepo
-			return $ read $ Git.configGet g config "1"
-		config = "annex.numcopies"
-
-{- Ideally, all keys have file size metadata. Old keys may not. -}
-checkUpgradableKey :: Key -> Annex Bool
-checkUpgradableKey key
-	| keySize key == Nothing = return True
-	| otherwise = return False
-
-{- This is used to check that numcopies is satisfied for the key on fsck.
- - This trusts data in the the location log, and so can check all keys, even
- - those with data not present in the current annex.
- -
- - The passed action is first run to allow backends deriving this one
- - to do their own checks.
- -}
-checkKey :: (Key -> Annex Bool) -> Key -> Maybe FilePath -> Maybe Int -> Annex Bool
-checkKey a key file numcopies = do
-	a_ok <- a key
-	copies_ok <- checkKeyNumCopies key file numcopies
-	return $ a_ok && copies_ok
-
-checkKeyOnly :: Key -> Maybe FilePath -> Maybe Int -> Annex Bool
-checkKeyOnly = checkKey (\_ -> return True)
-
-checkKeyNumCopies :: Key -> Maybe FilePath -> Maybe Int -> Annex Bool
-checkKeyNumCopies key file numcopies = do
-	needed <- getNumCopies numcopies
-	locations <- keyLocations key
-	untrusted <- trustGet UnTrusted
-	let untrustedlocations = intersect untrusted locations
-	let safelocations = filter (`notElem` untrusted) locations
-	let present = length safelocations
-	if present < needed
-		then do
-			ppuuids <- Remote.prettyPrintUUIDs untrustedlocations
-			warning $ missingNote (filename file key) present needed ppuuids
-			return False
-		else return True
-	where
-		filename Nothing k = show k
-		filename (Just f) _ = f
-
-missingNote :: String -> Int -> Int -> String -> String
-missingNote file 0 _ [] = 
-		"** No known copies exist of " ++ file
-missingNote file 0 _ untrusted =
-		"Only these untrusted locations may have copies of " ++ file ++
-		"\n" ++ untrusted ++
-		"Back it up to trusted locations with git-annex copy."
-missingNote file present needed [] =
-		"Only " ++ show present ++ " of " ++ show needed ++ 
-		" trustworthy copies exist of " ++ file ++
-		"\nBack it up with git-annex copy."
-missingNote file present needed untrusted = 
-		missingNote file present needed [] ++
-		"\nThe following untrusted locations may also have copies: " ++
-		"\n" ++ untrusted
diff --git a/Backend/SHA.hs b/Backend/SHA.hs
index 8930e4b93..bd6e411a0 100644
--- a/Backend/SHA.hs
+++ b/Backend/SHA.hs
@@ -16,7 +16,6 @@ import Data.Maybe
 import System.Posix.Files
 import System.FilePath
 
-import qualified Backend.File
 import Messages
 import qualified Annex
 import Locations
@@ -42,10 +41,10 @@ genBackend size
 	| shaCommand size == Nothing = Nothing
 	| otherwise = Just b
 	where
-		b = Backend.File.backend 
+		b = Types.Backend.Backend
 			{ name = shaName size
 			, getKey = keyValue size
-			, fsckKey = Backend.File.checkKey $ checkKeyChecksum size
+			, fsckKey = checkKeyChecksum size
 			}
 
 genBackendE :: SHASize -> Maybe (Backend Annex)
diff --git a/Backend/WORM.hs b/Backend/WORM.hs
index dc2e48adc..036d0564c 100644
--- a/Backend/WORM.hs
+++ b/Backend/WORM.hs
@@ -11,7 +11,6 @@ import Control.Monad.State
 import System.FilePath
 import System.Posix.Files
 
-import qualified Backend.File
 import Types.Backend
 import Types
 import Types.Key
@@ -20,9 +19,10 @@ backends :: [Backend Annex]
 backends = [backend]
 
 backend :: Backend Annex
-backend = Backend.File.backend {
+backend = Types.Backend.Backend {
 	name = "WORM",
-	getKey = keyValue
+	getKey = keyValue,
+	fsckKey = const (return True)
 }
 
 {- The key includes the file size, modification time, and the
diff --git a/BackendList.hs b/BackendList.hs
deleted file mode 100644
index e4e1d76fe..000000000
--- a/BackendList.hs
+++ /dev/null
@@ -1,19 +0,0 @@
-{- git-annex backend list
- -
- - Copyright 2010 Joey Hess <joey@kitenet.net>
- -
- - Licensed under the GNU GPL version 3 or higher.
- -}
-
-module BackendList (allBackends) where
-
--- When adding a new backend, import it here and add it to the list.
-import qualified Backend.WORM
-import qualified Backend.SHA
-import Types
-
-allBackends :: [Backend Annex]
-allBackends = concat 
-	[ Backend.WORM.backends
-	, Backend.SHA.backends
-	]
diff --git a/CmdLine.hs b/CmdLine.hs
index 46b980fbc..b807046df 100644
--- a/CmdLine.hs
+++ b/CmdLine.hs
@@ -22,7 +22,6 @@ import qualified Git
 import Content
 import Types
 import Command
-import BackendList
 import Version
 import Options
 import Messages
@@ -32,7 +31,7 @@ import UUID
 dispatch :: [String] -> [Command] -> [Option] -> String -> Git.Repo -> IO ()
 dispatch args cmds options header gitrepo = do
 	setupConsole
-	state <- Annex.new gitrepo allBackends
+	state <- Annex.new gitrepo
 	(actions, state') <- Annex.run state $ parseCmd args header cmds options
 	tryRun state' $ [startup] ++ actions ++ [shutdown]
 
diff --git a/Command/Add.hs b/Command/Add.hs
index 6a1ffb5da..2831e1b35 100644
--- a/Command/Add.hs
+++ b/Command/Add.hs
@@ -42,8 +42,8 @@ start pair@(file, _) = notAnnexed file $ do
 
 perform :: BackendFile -> CommandPerform
 perform (file, backend) = do
-	stored <- Backend.storeFileKey file backend
-	case stored of
+	k <- Backend.genKey file backend
+	case k of
 		Nothing -> stop
 		Just (key, _) -> do
 			moveAnnex key file
diff --git a/Command/AddUrl.hs b/Command/AddUrl.hs
index ebf0810ba..e80fe9621 100644
--- a/Command/AddUrl.hs
+++ b/Command/AddUrl.hs
@@ -51,8 +51,8 @@ perform url file = do
 	if ok
 		then do
 			[(_, backend)] <- Backend.chooseBackends [file]
-			stored <- Backend.storeFileKey tmp backend
-			case stored of
+			k <- Backend.genKey tmp backend
+			case k of
 				Nothing -> stop
 				Just (key, _) -> do
 					moveAnnex key tmp
diff --git a/Command/Drop.hs b/Command/Drop.hs
index bd4740741..14f098349 100644
--- a/Command/Drop.hs
+++ b/Command/Drop.hs
@@ -8,12 +8,15 @@
 module Command.Drop where
 
 import Command
-import qualified Backend
+import qualified Remote
+import qualified Annex
 import LocationLog
 import Types
 import Content
 import Messages
 import Utility
+import Trust
+import Config
 
 command :: [Command]
 command = [repoCommand "drop" paramPath seek
@@ -25,19 +28,19 @@ seek = [withAttrFilesInGit "annex.numcopies" start]
 {- Indicates a file's content is not wanted anymore, and should be removed
  - if it's safe to do so. -}
 start :: CommandStartAttrFile
-start (file, attr) = isAnnexed file $ \(key, backend) -> do
-	inbackend <- Backend.hasKey key
-	if inbackend
+start (file, attr) = isAnnexed file $ \(key, _) -> do
+	present <- inAnnex key
+	if present
 		then do
 			showStart "drop" file
-			next $ perform key backend numcopies
+			next $ perform key numcopies
 		else stop
 	where
 		numcopies = readMaybe attr :: Maybe Int
 
-perform :: Key -> Backend Annex -> Maybe Int -> CommandPerform
-perform key backend numcopies = do
-	success <- Backend.removeKey backend key numcopies
+perform :: Key -> Maybe Int -> CommandPerform
+perform key numcopies = do
+	success <- dropKey key numcopies
 	if success
 		then next $ cleanup key
 		else stop
@@ -47,3 +50,44 @@ cleanup key = do
 	whenM (inAnnex key) $ removeAnnex key
 	logStatus key InfoMissing
 	return True
+
+{- Checks remotes to verify that enough copies of a key exist to allow
+ - for a key to be safely removed (with no data loss), and fails with an
+ - error if not. -}
+dropKey :: Key -> Maybe Int -> Annex Bool
+dropKey key numcopiesM = do
+	force <- Annex.getState Annex.force
+	if force || numcopiesM == Just 0
+		then return True
+		else do
+			(remotes, trusteduuids) <- Remote.keyPossibilitiesTrusted key
+			untrusteduuids <- trustGet UnTrusted
+			let tocheck = Remote.remotesWithoutUUID remotes (trusteduuids++untrusteduuids)
+			numcopies <- getNumCopies numcopiesM
+			findcopies numcopies trusteduuids tocheck []
+	where
+		findcopies need have [] bad
+			| length have >= need = return True
+			| otherwise = notEnoughCopies need have bad
+		findcopies need have (r:rs) bad
+			| length have >= need = return True
+			| otherwise = do
+				let u = Remote.uuid r
+				let dup = u `elem` have
+				haskey <- Remote.hasKey r key
+				case (dup, haskey) of
+					(False, Right True)	-> findcopies need (u:have) rs bad
+					(False, Left _)		-> findcopies need have rs (r:bad)
+					_			-> findcopies need have rs bad
+		notEnoughCopies need have bad = do
+			unsafe
+			showLongNote $
+				"Could only verify the existence of " ++
+				show (length have) ++ " out of " ++ show need ++ 
+				" necessary copies"
+			Remote.showTriedRemotes bad
+			Remote.showLocations key have
+			hint
+			return False
+		unsafe = showNote "unsafe"
+		hint = showLongNote "(Use --force to override this check, or adjust annex.numcopies.)"
diff --git a/Command/DropUnused.hs b/Command/DropUnused.hs
index 2125abdc3..55007c1f7 100644
--- a/Command/DropUnused.hs
+++ b/Command/DropUnused.hs
@@ -21,7 +21,6 @@ import qualified Command.Drop
 import qualified Command.Move
 import qualified Remote
 import qualified Git
-import Backend
 import Types.Key
 import Utility
 
@@ -64,9 +63,7 @@ perform key = maybe droplocal dropremote =<< Annex.getState Annex.fromremote
 			r <- Remote.byName name
 			showNote $ "from " ++ Remote.name r ++ "..."
 			next $ Command.Move.fromCleanup r True key
-		droplocal = do
-			backend <- keyBackend key
-			Command.Drop.perform key backend (Just 0) -- force drop
+		droplocal = Command.Drop.perform key (Just 0) -- force drop
 
 performOther :: (Git.Repo -> Key -> FilePath) -> Key -> CommandPerform
 performOther filespec key = do
diff --git a/Command/FromKey.hs b/Command/FromKey.hs
index 34816d657..fb9ab0775 100644
--- a/Command/FromKey.hs
+++ b/Command/FromKey.hs
@@ -15,7 +15,6 @@ import Control.Monad (unless)
 import Command
 import qualified AnnexQueue
 import Utility
-import qualified Backend
 import Content
 import Messages
 import Types.Key
@@ -30,7 +29,7 @@ seek = [withFilesMissing start]
 start :: CommandStartString
 start file = notBareRepo $ do
 	key <- cmdlineKey
-	inbackend <- Backend.hasKey key
+	inbackend <- inAnnex key
 	unless inbackend $ error $
 		"key ("++keyName key++") is not present in backend"
 	showStart "fromkey" file
diff --git a/Command/Fsck.hs b/Command/Fsck.hs
index 988cfd28d..446d25a44 100644
--- a/Command/Fsck.hs
+++ b/Command/Fsck.hs
@@ -9,10 +9,15 @@ module Command.Fsck where
 
 import Control.Monad (when)
 import Control.Monad.State (liftIO)
+import System.Directory
+import Data.List
+import System.Posix.Files
 
 import Command
-import qualified Backend
 import qualified Annex
+import qualified Remote
+import qualified Types.Backend
+import qualified Types.Key
 import UUID
 import Types
 import Messages
@@ -20,6 +25,9 @@ import Utility
 import Content
 import LocationLog
 import Locations
+import Trust
+import DataUnits
+import Config
 
 command :: [Command]
 command = [repoCommand "fsck" (paramOptional $ paramRepeating paramPath) seek
@@ -40,7 +48,7 @@ perform key file backend numcopies = do
 	-- the location log is checked first, so that if it has bad data
 	-- that gets corrected
 	locationlogok <- verifyLocationLog key file
-	backendok <- Backend.fsckKey backend key (Just file) numcopies
+	backendok <- fsckKey backend key (Just file) numcopies
 	if locationlogok && backendok
 		then next $ return True
 		else stop
@@ -80,3 +88,68 @@ verifyLocationLog key file = do
 		fix g u s = do
 			showNote "fixing location log"
 			logChange g key u s
+
+{- Checks a key for problems. -}
+fsckKey :: Backend Annex -> Key -> Maybe FilePath -> Maybe Int -> Annex Bool
+fsckKey backend key file numcopies = do
+	size_ok <- checkKeySize key
+	copies_ok <- checkKeyNumCopies key file numcopies
+	backend_ok <-(Types.Backend.fsckKey backend) key
+	return $ size_ok && copies_ok && backend_ok
+
+{- The size of the data for a key is checked against the size encoded in
+ - the key's metadata, if available. -}
+checkKeySize :: Key -> Annex Bool
+checkKeySize key = do
+	g <- Annex.gitRepo
+	let file = gitAnnexLocation g key
+	present <- liftIO $ doesFileExist file
+	case (present, Types.Key.keySize key) of
+		(_, Nothing) -> return True
+		(False, _) -> return True
+		(True, Just size) -> do
+			stat <- liftIO $ getFileStatus file
+			let size' = fromIntegral (fileSize stat)
+			if size == size'
+				then return True
+				else do
+					dest <- moveBad key
+					warning $ "Bad file size (" ++
+						compareSizes storageUnits True size size' ++ 
+						"); moved to " ++ dest
+					return False
+
+
+checkKeyNumCopies :: Key -> Maybe FilePath -> Maybe Int -> Annex Bool
+checkKeyNumCopies key file numcopies = do
+	needed <- getNumCopies numcopies
+	locations <- keyLocations key
+	untrusted <- trustGet UnTrusted
+	let untrustedlocations = intersect untrusted locations
+	let safelocations = filter (`notElem` untrusted) locations
+	let present = length safelocations
+	if present < needed
+		then do
+			ppuuids <- Remote.prettyPrintUUIDs untrustedlocations
+			warning $ missingNote (filename file key) present needed ppuuids
+			return False
+		else return True
+	where
+		filename Nothing k = show k
+		filename (Just f) _ = f
+
+missingNote :: String -> Int -> Int -> String -> String
+missingNote file 0 _ [] = 
+		"** No known copies exist of " ++ file
+missingNote file 0 _ untrusted =
+		"Only these untrusted locations may have copies of " ++ file ++
+		"\n" ++ untrusted ++
+		"Back it up to trusted locations with git-annex copy."
+missingNote file present needed [] =
+		"Only " ++ show present ++ " of " ++ show needed ++ 
+		" trustworthy copies exist of " ++ file ++
+		"\nBack it up with git-annex copy."
+missingNote file present needed untrusted = 
+		missingNote file present needed [] ++
+		"\nThe following untrusted locations may also have copies: " ++
+		"\n" ++ untrusted
diff --git a/Command/Get.hs b/Command/Get.hs
index 50dc009fe..cc780cb6a 100644
--- a/Command/Get.hs
+++ b/Command/Get.hs
@@ -8,7 +8,6 @@
 module Command.Get where
 
 import Command
-import qualified Backend
 import qualified Annex
 import qualified Remote
 import Types
@@ -24,7 +23,7 @@ seek :: [CommandSeek]
 seek = [withFilesInGit start]
 
 start :: CommandStartString
-start file = isAnnexed file $ \(key, backend) -> do
+start file = isAnnexed file $ \(key, _) -> do
 	inannex <- inAnnex key
 	if inannex
 		then stop
@@ -32,14 +31,52 @@ start file = isAnnexed file $ \(key, backend) -> do
 			showStart "get" file
 			from <- Annex.getState Annex.fromremote
 			case from of
-				Nothing -> next $ perform key backend
+				Nothing -> next $ perform key
 				Just name -> do
 					src <- Remote.byName name
 					next $ Command.Move.fromPerform src False key
 
-perform :: Key -> Backend Annex -> CommandPerform
-perform key backend = do
-	ok <- getViaTmp key (Backend.retrieveKeyFile backend key)
+perform :: Key -> CommandPerform
+perform key = do
+	ok <- getViaTmp key (getKeyFile key)
 	if ok
 		then next $ return True -- no cleanup needed
 		else stop
+
+{- Try to find a copy of the file in one of the remotes,
+ - and copy it to here. -}
+getKeyFile :: Key -> FilePath -> Annex Bool
+getKeyFile key file = do
+	remotes <- Remote.keyPossibilities key
+	if null remotes
+		then do
+			showNote "not available"
+			Remote.showLocations key []
+			return False
+		else trycopy remotes remotes
+	where
+		trycopy full [] = do
+			Remote.showTriedRemotes full
+			Remote.showLocations key []
+			return False
+		trycopy full (r:rs) = do
+			probablythere <- probablyPresent r
+			if probablythere
+				then docopy r (trycopy full rs)
+				else trycopy full rs
+		-- This check is to avoid an ugly message if a remote is a
+		-- drive that is not mounted.
+		probablyPresent r =
+			if Remote.hasKeyCheap r
+				then do
+					res <- Remote.hasKey r key
+					case res of
+						Right b -> return b
+						Left _ -> return False
+				else return True
+		docopy r continue = do
+			showNote $ "from " ++ Remote.name r ++ "..."
+			copied <- Remote.retrieveKeyFile r key file
+			if copied
+				then return True
+				else continue
diff --git a/Command/Migrate.hs b/Command/Migrate.hs
index 09ff6df7d..495bf9fb6 100644
--- a/Command/Migrate.hs
+++ b/Command/Migrate.hs
@@ -15,6 +15,7 @@ import System.FilePath
 import Command
 import qualified Annex
 import qualified Backend
+import qualified Types.Key
 import Locations
 import Types
 import Content
@@ -32,18 +33,20 @@ start :: CommandStartBackendFile
 start (file, b) = isAnnexed file $ \(key, oldbackend) -> do
 	exists <- inAnnex key
 	newbackend <- choosebackend b
-	upgradable <- Backend.upgradableKey oldbackend key
-	if (newbackend /= oldbackend || upgradable) && exists
+	if (newbackend /= oldbackend || upgradableKey key) && exists
 		then do
 			showStart "migrate" file
 			next $ perform file key newbackend
 		else stop
 	where
-		choosebackend Nothing = do
-			backends <- Backend.list
-			return $ head backends
+		choosebackend Nothing = return . head =<< Backend.orderedList
 		choosebackend (Just backend) = return backend
 
+{- Checks if a key is upgradable to a newer representation. -}
+{- Ideally, all keys have file size metadata. Old keys may not. -}
+upgradableKey :: Key -> Bool
+upgradableKey key = Types.Key.keySize key == Nothing
+
 perform :: FilePath -> Key -> Backend Annex -> CommandPerform
 perform file oldkey newbackend = do
 	g <- Annex.gitRepo
@@ -55,9 +58,9 @@ perform file oldkey newbackend = do
 	let src = gitAnnexLocation g oldkey
 	let tmpfile = gitAnnexTmpDir g </> takeFileName file
 	liftIO $ createLink src tmpfile
-	stored <- Backend.storeFileKey tmpfile $ Just newbackend
+	k <- Backend.genKey tmpfile $ Just newbackend
 	liftIO $ cleantmp tmpfile
-	case stored of
+	case k of
 		Nothing -> stop
 		Just (newkey, _) -> do
 			ok <- getViaTmpUnchecked newkey $ \t -> do
diff --git a/Command/Status.hs b/Command/Status.hs
index 53589030b..2448f65a4 100644
--- a/Command/Status.hs
+++ b/Command/Status.hs
@@ -25,6 +25,7 @@ import DataUnits
 import Content
 import Types.Key
 import Locations
+import Backend
 
 -- a named computation that produces a statistic
 type Stat = StatState (Maybe (String, StatState String))
@@ -95,9 +96,8 @@ showStat s = calc =<< s
 		calc Nothing = return ()
 
 supported_backends :: Stat
-supported_backends = stat "supported backends" $
-	lift (Annex.getState Annex.supportedBackends) >>=
-		return . unwords . (map B.name)
+supported_backends = stat "supported backends" $ 
+	return $ unwords $ map B.name Backend.list
 
 supported_remote_types :: Stat
 supported_remote_types = stat "supported remote types" $
diff --git a/Command/Unannex.hs b/Command/Unannex.hs
index f0c1b27c6..f22503ee0 100644
--- a/Command/Unannex.hs
+++ b/Command/Unannex.hs
@@ -13,10 +13,10 @@ import System.Directory
 import System.Posix.Files
 
 import Command
+import qualified Command.Drop
 import qualified Annex
 import qualified AnnexQueue
 import Utility
-import qualified Backend
 import LocationLog
 import Types
 import Content
@@ -33,7 +33,7 @@ seek = [withFilesInGit start]
 
 {- The unannex subcommand undoes an add. -}
 start :: CommandStartString
-start file = isAnnexed file $ \(key, backend) -> do
+start file = isAnnexed file $ \(key, _) -> do
 	ishere <- inAnnex key
 	if ishere
 		then do
@@ -46,13 +46,12 @@ start file = isAnnexed file $ \(key, backend) -> do
 				Annex.changeState $ \s -> s { Annex.force = True }
 
 			showStart "unannex" file
-			next $ perform file key backend
+			next $ perform file key
 		else stop
 
-perform :: FilePath -> Key -> Backend Annex -> CommandPerform
-perform file key backend = do
-	-- force backend to always remove
-	ok <- Backend.removeKey backend key (Just 0)
+perform :: FilePath -> Key -> CommandPerform
+perform file key = do
+	ok <- Command.Drop.dropKey key (Just 0) -- always remove
 	if ok
 		then next $ cleanup file key
 		else stop
diff --git a/Command/Unlock.hs b/Command/Unlock.hs
index ca8b62502..8a897c365 100644
--- a/Command/Unlock.hs
+++ b/Command/Unlock.hs
@@ -12,7 +12,6 @@ import System.Directory hiding (copyFile)
 
 import Command
 import qualified Annex
-import qualified Backend
 import Types
 import Messages
 import Locations
@@ -38,7 +37,7 @@ start file = isAnnexed file $ \(key, _) -> do
 
 perform :: FilePath -> Key -> CommandPerform
 perform dest key = do
-	unlessM (Backend.hasKey key) $ error "content not present"
+	unlessM (inAnnex key) $ error "content not present"
 	
 	checkDiskSpace key
 
diff --git a/Config.hs b/Config.hs
index 1016845e7..9cbf2d52f 100644
--- a/Config.hs
+++ b/Config.hs
@@ -86,3 +86,16 @@ remoteNotIgnored r = do
 		match a = do
 			n <- Annex.getState a
 			return $ n == Git.repoRemoteName r
+
+{- If a value is specified, it is used; otherwise the default is looked up
+ - in git config. forcenumcopies overrides everything. -}
+getNumCopies :: Maybe Int -> Annex Int
+getNumCopies v = 
+	Annex.getState Annex.forcenumcopies >>= maybe (use v) (return . id)
+	where
+		use (Just n) = return n
+		use Nothing = do
+			g <- Annex.gitRepo
+			return $ read $ Git.configGet g config "1"
+		config = "annex.numcopies"
+
diff --git a/LocationLog.hs b/LocationLog.hs
index eb48b7916..28b423e2f 100644
--- a/LocationLog.hs
+++ b/LocationLog.hs
@@ -19,7 +19,7 @@ module LocationLog (
 	keyLocations,
 	loggedKeys,
 	logFile,
-	logFileKey	
+	logFileKey
 ) where
 
 import System.FilePath
diff --git a/Remote.hs b/Remote.hs
index 28c2e39cd..623b85733 100644
--- a/Remote.hs
+++ b/Remote.hs
@@ -14,10 +14,10 @@ module Remote (
 	removeKey,
 	hasKey,
 	hasKeyCheap,
+
 	keyPossibilities,
 	keyPossibilitiesTrusted,
 	forceTrust,
-
 	remoteTypes,
 	genList,
 	byName,
@@ -25,6 +25,8 @@ module Remote (
 	remotesWithUUID,
 	remotesWithoutUUID,
 	prettyPrintUUIDs,
+	showTriedRemotes,
+	showLocations,
 
 	remoteLog,
 	readRemoteLog,
@@ -40,6 +42,7 @@ import Data.List
 import qualified Data.Map as M
 import Data.Maybe
 import Data.Char
+import Data.String.Utils
 
 import qualified Branch
 import Types
@@ -49,6 +52,7 @@ import qualified Annex
 import Config
 import Trust
 import LocationLog
+import Messages
 
 import qualified Remote.Git
 import qualified Remote.S3
@@ -181,9 +185,34 @@ keyPossibilities' withtrusted key = do
 
 	return (sort validremotes, validtrusteduuids)
 
+{- Displays known locations of a key. -}
+showLocations :: Key -> [UUID] -> Annex ()
+showLocations key exclude = do
+	g <- Annex.gitRepo
+	u <- getUUID g
+	uuids <- keyLocations key
+	untrusteduuids <- trustGet UnTrusted
+	let uuidswanted = filteruuids uuids (u:exclude++untrusteduuids) 
+	let uuidsskipped = filteruuids uuids (u:exclude++uuidswanted)
+	ppuuidswanted <- Remote.prettyPrintUUIDs uuidswanted
+	ppuuidsskipped <- Remote.prettyPrintUUIDs uuidsskipped
+	showLongNote $ message ppuuidswanted ppuuidsskipped
+	where
+		filteruuids l x = filter (`notElem` x) l
+		message [] [] = "No other repository is known to contain the file."
+		message rs [] = "Try making some of these repositories available:\n" ++ rs
+		message [] us = "Also these untrusted repositories may contain the file:\n" ++ us
+		message rs us = message rs [] ++ message [] us
+
+showTriedRemotes :: [Remote Annex] -> Annex ()
+showTriedRemotes [] = return ()	
+showTriedRemotes remotes =
+	showLongNote $ "Unable to access these remotes: " ++
+		(join ", " $ map name remotes)
+
 forceTrust :: TrustLevel -> String -> Annex ()
 forceTrust level remotename = do
-	r <- Remote.nameToUUID remotename
+	r <- nameToUUID remotename
 	Annex.changeState $ \s ->
 		s { Annex.forcetrust = (r, level):Annex.forcetrust s }
 
diff --git a/Remote/Git.hs b/Remote/Git.hs
index 471417e34..b4006d7fd 100644
--- a/Remote/Git.hs
+++ b/Remote/Git.hs
@@ -112,7 +112,7 @@ inAnnex r key = if Git.repoIsUrl r
 		checklocal = do
 			-- run a local check inexpensively,
 			-- by making an Annex monad using the remote
-			a <- Annex.new r []
+			a <- Annex.new r
 			Annex.eval a (Content.inAnnex key)
 		checkremote = do
 			showNote ("checking " ++ Git.repoDescribe r ++ "...")
@@ -142,7 +142,7 @@ copyToRemote r key
 		let keysrc = gitAnnexLocation g key
 		-- run copy from perspective of remote
 		liftIO $ do
-			a <- Annex.new r []
+			a <- Annex.new r
 			Annex.eval a $ do
 				ok <- Content.getViaTmp key $
 					rsyncOrCopyFile r keysrc
diff --git a/Types/Backend.hs b/Types/Backend.hs
index 8100eaf28..f86d0845c 100644
--- a/Types/Backend.hs
+++ b/Types/Backend.hs
@@ -16,22 +16,8 @@ data Backend a = Backend {
 	name :: String,
 	-- converts a filename to a key
 	getKey :: FilePath -> a (Maybe Key),
-	-- stores a file's contents to a key
-	storeFileKey :: FilePath -> Key -> a Bool,
-	-- retrieves a key's contents to a file
-	retrieveKeyFile :: Key -> FilePath -> a Bool,
-	-- removes a key, optionally checking that enough copies are stored
-	-- elsewhere
-	removeKey :: Key -> Maybe Int -> a Bool,
-	-- checks if a backend is storing the content of a key
-	hasKey :: Key -> a Bool,
 	-- called during fsck to check a key
-	-- (second parameter may be the filename associated with it)
-	-- (third parameter may be the number of copies that there should
-	-- be of the key)
-	fsckKey :: Key -> Maybe FilePath -> Maybe Int -> a Bool,
-	-- Is a newer repesentation possible for a key?
-	upgradableKey :: Key -> a Bool
+	fsckKey :: Key -> a Bool
 }
 
 instance Show (Backend a) where
diff --git a/Upgrade/V1.hs b/Upgrade/V1.hs
index 39b8e47c5..c0bbeebaf 100644
--- a/Upgrade/V1.hs
+++ b/Upgrade/V1.hs
@@ -191,17 +191,16 @@ logFile1 repo key = Upgrade.V2.gitStateDir repo ++ keyFile1 key ++ ".log"
 
 lookupFile1 :: FilePath -> Annex (Maybe (Key, Backend Annex))
 lookupFile1 file = do
-	bs <- Annex.getState Annex.supportedBackends
 	tl <- liftIO $ try getsymlink
 	case tl of
 		Left _ -> return Nothing
-		Right l -> makekey bs l
+		Right l -> makekey l
 	where
 		getsymlink = do
 			l <- readSymbolicLink file
 			return $ takeFileName l
-		makekey bs l = do
-			case maybeLookupBackendName bs bname of
+		makekey l = do
+			case maybeLookupBackendName bname of
 				Nothing -> do
 					unless (null kname || null bname ||
 					        not (isLinkToAnnex l)) $
diff --git a/test.hs b/test.hs
index 44a792f14..76ffe4047 100644
--- a/test.hs
+++ b/test.hs
@@ -25,7 +25,6 @@ import System.Path (recurseDir)
 import System.IO.HVFS (SystemFS(..))
 
 import qualified Annex
-import qualified BackendList
 import qualified Backend
 import qualified Git
 import qualified Locations
@@ -483,7 +482,7 @@ annexeval :: Types.Annex a -> IO a
 annexeval a = do
 	g <- Git.repoFromCwd
 	g' <- Git.configRead g
-	s <- Annex.new g' BackendList.allBackends
+	s <- Annex.new g'
 	Annex.eval s a
 
 innewrepo :: Assertion -> Assertion
@@ -684,4 +683,4 @@ backendWORM :: Types.Backend Types.Annex
 backendWORM = backend_ "WORM"
 
 backend_ :: String -> Types.Backend Types.Annex
-backend_ name = Backend.lookupBackendName BackendList.allBackends name
+backend_ name = Backend.lookupBackendName name