change retrieveExport and getKey to throw exception

retrieveExport is part of ongoing transition to make remote methods
throw exceptions, rather than silently hide them.

getKey very rarely fails, and when it does it's always for the same reason
(user configured annex.backend to url for some reason). So, this will
avoid dealing with Nothing everywhere it's used.

This commit was sponsored by Ilya Shlyakhter on Patreon.

Backend/Hash.hs

@@ -63,7 +63,7 @@ backends = concatMap (\h -> [genBackendE h, genBackend h]) hashes
 genBackend :: Hash -> Backend
 genBackend hash = Backend
 	{ backendVariety = hashKeyVariety hash (HasExt False)
-	, getKey = keyValue hash
+	, getKey = Just (keyValue hash)
 	, verifyKeyContent = Just $ checkKeyChecksum hash
 	, canUpgradeKey = Just needsUpgrade
 	, fastMigrate = Just trivialMigrate
@@ -73,7 +73,7 @@ genBackend hash = Backend
 genBackendE :: Hash -> Backend
 genBackendE hash = (genBackend hash)
 	{ backendVariety = hashKeyVariety hash (HasExt True)
-	, getKey = keyValueE hash
+	, getKey = Just (keyValueE hash)
 	}
 
 hashKeyVariety :: Hash -> HasExt -> KeyVariety
@@ -88,26 +88,26 @@ hashKeyVariety (Blake2sHash size) he = Blake2sKey size he
 hashKeyVariety (Blake2spHash size) he = Blake2spKey size he
 
 {- A key is a hash of its contents. -}
-keyValue :: Hash -> KeySource -> MeterUpdate -> Annex (Maybe Key)
+keyValue :: Hash -> KeySource -> MeterUpdate -> Annex Key
 keyValue hash source meterupdate = do
 	let file = fromRawFilePath (contentLocation source)
 	filesize <- liftIO $ getFileSize file
 	s <- hashFile hash file meterupdate
-	return $ Just $ mkKey $ \k -> k
+	return $ mkKey $ \k -> k
 		{ keyName = encodeBS s
 		, keyVariety = hashKeyVariety hash (HasExt False)
 		, keySize = Just filesize
 		}
 
 {- Extension preserving keys. -}
-keyValueE :: Hash -> KeySource -> MeterUpdate -> Annex (Maybe Key)
+keyValueE :: Hash -> KeySource -> MeterUpdate -> Annex Key
 keyValueE hash source meterupdate =
-	keyValue hash source meterupdate >>= maybe (return Nothing) addE
+	keyValue hash source meterupdate >>= addE
   where
 	addE k = do
 		maxlen <- annexMaxExtensionLength <$> Annex.getGitConfig
 		let ext = selectExtension maxlen (keyFilename source)
-		return $ Just $ alterKey k $ \d -> d
+		return $ alterKey k $ \d -> d
 			{ keyName = keyName d <> ext
 			, keyVariety = hashKeyVariety hash (HasExt True)
 			}
@@ -296,7 +296,10 @@ md5Hasher = show . md5
 testKeyBackend :: Backend
 testKeyBackend = 
 	let b = genBackendE (SHA2Hash (HashSize 256))
-	in b { getKey = \ks p -> (fmap addE) <$> getKey b ks p } 
+	    gk = case getKey b of
+		Nothing -> Nothing
+		Just f -> Just (\ks p -> addE <$> f ks p)
+	in b { getKey = gk }
   where
 	addE k = alterKey k $ \d -> d
 		{ keyName = keyName d <> longext

Backend/URL.hs

@@ -21,7 +21,7 @@ backends = [backend]
 backend :: Backend
 backend = Backend
 	{ backendVariety = URLKey
-	, getKey = \_ _ -> return Nothing
+	, getKey = Nothing
 	, verifyKeyContent = Nothing
 	, canUpgradeKey = Nothing
 	, fastMigrate = Nothing

Backend/WORM.hs

@@ -24,7 +24,7 @@ backends = [backend]
 backend :: Backend
 backend = Backend
 	{ backendVariety = WORMKey
-	, getKey = keyValue
+	, getKey = Just keyValue
 	, verifyKeyContent = Nothing
 	, canUpgradeKey = Just needsUpgrade
 	, fastMigrate = Just removeSpaces
@@ -34,14 +34,14 @@ backend = Backend
 {- The key includes the file size, modification time, and the
  - original filename relative to the top of the git repository.
  -}
-keyValue :: KeySource -> MeterUpdate -> Annex (Maybe Key)
+keyValue :: KeySource -> MeterUpdate -> Annex Key
 keyValue source _ = do
 	let f = contentLocation source
 	stat <- liftIO $ R.getFileStatus f
 	sz <- liftIO $ getFileSize' (fromRawFilePath f) stat
 	relf <- fromRawFilePath . getTopFilePath
 		<$> inRepo (toTopFilePath $ keyFilename source)
-	return $ Just $ mkKey $ \k -> k
+	return $ mkKey $ \k -> k
 		{ keyName = genKeyName relf
 		, keyVariety = WORMKey
 		, keySize = Just sz