avoid partial functions

This is horrible old code and ghc has started to warn about head and
tail. Rewrote it to avoid all partial functions except !! and guarded
uses of !! with length checks.

Upgrade/V1.hs

@@ -1,6 +1,6 @@
 {- git-annex v1 -> v2 upgrade support
  -
- - Copyright 2011 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2024 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -18,6 +18,7 @@ import qualified Data.ByteString.Short as S (toShort, fromShort)
 import qualified Data.ByteString.Lazy as L
 import qualified System.FilePath.ByteString as P
 import System.PosixCompat.Files (isRegularFile)
+import Text.Read
 
 import Annex.Common
 import Types.Upgrade
@@ -149,13 +150,16 @@ oldlog2key l
 -- WORM backend keys: "WORM:mtime:size:filename"
 -- all the rest: "backend:key"
 --
--- If the file looks like "WORM:XXX-...", then it was created by mixing
+-- If the file looks like "WORM:FOO-...", then it was created by mixing
 -- v2 and v1; that infelicity is worked around by treating the value
 -- as the v2 key that it is.
 readKey1 :: String -> Key
-readKey1 v
-	| mixup = fromJust $ deserializeKey $ intercalate ":" $ Prelude.tail bits
-	| otherwise = mkKey $ \d -> d
+readKey1 = fromMaybe (giveup "unable to parse v0 key") . readKey1'
+
+readKey1' :: String -> Maybe Key
+readKey1' v
+	| mixup = deserializeKey $ intercalate ":" $ drop 1 bits
+	| otherwise = Just $ mkKey $ \d -> d
 		{ keyName = S.toShort (encodeBS n)
 		, keyVariety = parseKeyVariety (encodeBS b)
 		, keySize = s
@@ -166,13 +170,13 @@ readKey1 v
 	b = Prelude.head bits
 	n = intercalate ":" $ drop (if wormy then 3 else 1) bits
 	t = if wormy
-		then Just (Prelude.read (bits !! 1) :: EpochTime)
+		then readMaybe (bits !! 1) :: Maybe EpochTime
 		else Nothing
-	s = if wormy
-		then Just (Prelude.read (bits !! 2) :: Integer)
+	s = if wormy && length bits > 2
+		then readMaybe (bits !! 2) :: Maybe Integer
 		else Nothing
-	wormy = Prelude.head bits == "WORM"
-	mixup = wormy && isUpper (Prelude.head $ bits !! 1)
+	wormy = length bits > 1 && headMaybe bits == Just "WORM"
+	mixup = wormy && fromMaybe False (isUpper <$> (headMaybe $ bits !! 1))
 
 showKey1 :: Key -> String
 showKey1 k = intercalate ":" $ filter (not . null)