fix up Read instance incompatability caused by recent commit

9c4650358c changed the Read instance for
Key.

I've checked all uses of that instance (by removing it and seeing what
breaks), and they're all limited to the webapp, except one.
That is GitAnnexDistribution's Read instance.

So, 9c4650358c would have broken upgrades
of git-annex from downloads.kitenet.net. Once the .info files there got
updated for a new release, old releases would have failed to parse them
and never upgraded.

To fix this, I found a way to make the .info files that contain
GitAnnexDistribution values be readable by the old version of git-annex.

This commit was sponsored by Ewen McNeill.

Types/Distribution.hs

@@ -1,16 +1,22 @@
 {- Data type for a distribution of git-annex
  -
- - Copyright 2013 Joey Hess <id@joeyh.name>
+ - Copyright 2013, 2017 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
 module Types.Distribution where
 
+import Utility.PartialPrelude
 import Types.Key
+import Key
 import Data.Time.Clock
 import Git.Config (isTrue, boolConfig)
 
+import Data.String.Utils
+
+type GitAnnexVersion = String
+
 data GitAnnexDistribution = GitAnnexDistribution
 	{ distributionUrl :: String
 	, distributionKey :: Key
@@ -20,7 +26,39 @@ data GitAnnexDistribution = GitAnnexDistribution
 	}
 	deriving (Read, Show, Eq)
 
-type GitAnnexVersion = String
+{- The first line of the info file is in the format old versions of
+ - git-annex expect to read a GitAnnexDistribution.
+ - The remainder of the file is in the new format.
+ - This works because old versions of git-annex used readish to parse
+ - the file, and that ignores the second line.
+ -}
+formatInfoFile :: GitAnnexDistribution -> String
+formatInfoFile d = replace "keyVariant = " "keyBackendName = " (show d) ++
+	"\n" ++ formatGitAnnexDistribution d
+
+parseInfoFile :: String -> Maybe GitAnnexDistribution
+parseInfoFile s = case lines s of
+	(_oldformat:rest) -> parseGitAnnexDistribution (unlines rest)
+	_ -> Nothing
+
+formatGitAnnexDistribution :: GitAnnexDistribution -> String
+formatGitAnnexDistribution d = unlines
+	[ distributionUrl d
+	, key2file (distributionKey d)
+	, distributionVersion d
+	, show (distributionReleasedate d)
+	, maybe "" show (distributionUrgentUpgrade d)
+	]
+
+parseGitAnnexDistribution :: String -> Maybe GitAnnexDistribution
+parseGitAnnexDistribution s = case lines s of
+	(u:k:v:d:uu:_) -> GitAnnexDistribution
+		<$> pure u
+		<*> file2key k
+		<*> pure v
+		<*> readish d
+		<*> pure (readish uu)
+	_ -> Nothing
 
 data AutoUpgrade = AskUpgrade | AutoUpgrade | NoAutoUpgrade
 	deriving (Eq)