deal with old repositories with non-encrypted creds

See 2f3c3aa01f for backstory about how a repo
could be in this state.

When decryption fails, the repo must be using non-encrypted creds. Note
that creds are encrypted/decrypted using the encryption cipher which is
stored in the repo, so the decryption cannot fail due to missing gpg keys
etc. (For !shared encryptiom, the cipher is iteself encrypted using some
gpg key(s), and the decryption of the cipher happens earlier, so not
affected by this change.

Print a warning message for !shared repos, and continue on using the
cipher. Wrote a page explaining what users hit by this bug should do.

This commit was sponsored by Samuel Tardieu.

Creds.hs

@@ -23,7 +23,7 @@ import Annex.Perms
 import Utility.FileMode
 import Crypto
 import Types.Remote (RemoteConfig, RemoteConfigKey)
-import Remote.Helper.Encryptable (remoteCipher, embedCreds, EncryptionIsSetup)
+import Remote.Helper.Encryptable (remoteCipher, remoteCipher', embedCreds, EncryptionIsSetup)
 import Utility.Env (getEnv)
 
 import qualified Data.ByteString.Lazy.Char8 as L
@@ -90,20 +90,32 @@ getRemoteCredPair c storage = maybe fromcache (return . Just) =<< fromenv
 	fromcache = maybe fromconfig (return . Just) =<< readCacheCredPair storage
 	fromconfig = case credPairRemoteKey storage of
 		Just key -> do
-			mcipher <- remoteCipher c
+			mcipher <- remoteCipher' c
 			case (M.lookup key c, mcipher) of
 				(Nothing, _) -> return Nothing
-				(Just enccreds, Just cipher) -> do
-					creds <- liftIO $ decrypt cipher
-						(feedBytes $ L.pack $ fromB64 enccreds)
-						(readBytes $ return . L.unpack)
-					fromcreds creds
+				(Just enccreds, Just (cipher, storablecipher)) ->
+					fromenccreds enccreds cipher storablecipher
 				(Just bcreds, Nothing) ->
 					fromcreds $ fromB64 bcreds
 		Nothing -> return Nothing
+	fromenccreds enccreds cipher storablecipher = do
+		mcreds <- liftIO $ catchMaybeIO $ decrypt cipher
+			(feedBytes $ L.pack $ fromB64 enccreds)
+			(readBytes $ return . L.unpack)
+		case mcreds of
+			Just creds -> fromcreds creds
+			Nothing -> do
+				-- Work around un-encrypted creds storage
+				-- bug in old S3 and glacier remotes.
+				-- Not a problem for shared cipher.
+				case storablecipher of
+					SharedCipher {} -> showLongNote "gpg error above was caused by an old git-annex bug in credentials storage. Working around it.."
+					_ -> warning "*** Insecure credentials storage detected for this remote! See https://git-annex.branchable.com/upgrades/insecure_embedded_creds/"
+				fromcreds $ fromB64 enccreds
 	fromcreds creds = case decodeCredPair creds of
 		Just credpair -> do
 			writeCacheCredPair credpair storage
+
 			return $ Just credpair
 		_ -> error "bad creds"