better probing for gcrypt repositories using new --check option

Now can tell if a repo uses gcrypt or not, and whether it's decryptable
with the current gpg keys.

This closes the hole that undecryptable gcrypt repos could have before been
combined into the repo in encrypted mode.

Git/GCrypt.hs

@@ -32,8 +32,8 @@ isEncrypted _ = False
  - Throws an exception if an url is invalid or the repo does not use
  - gcrypt.
  -}
-encryptedRepo :: Repo -> Repo -> IO Repo
-encryptedRepo baserepo = go
+encryptedRemote :: Repo -> Repo -> IO Repo
+encryptedRemote baserepo = go
   where
   	go Repo { location = Url url }
 		| urlPrefix `isPrefixOf` u =
@@ -45,20 +45,26 @@ encryptedRepo baserepo = go
 	go _ = notencrypted
 	notencrypted = error "not a gcrypt encrypted repository"
 
-{- Checks if the git repo at a location is a gcrypt repo that
- - we can decrypt. This works by trying to fetch from the repo
- - at the location, into the baserepo.
- -
- - Returns false if the git repo is not using gcrypt, or if it is using
- - gcrypt but cannot be decrypted. We do not try to detect gcrypt
- - repos that cannot be decrypted, because gcrypt may change in the future
- - to avoid easy fingerprinting of gcrypt repos.
+data ProbeResult = Decryptable | NotDecryptable | NotEncrypted
+
+{- Checks if the git repo at a location uses gcrypt.
+ - 
+ - Rather expensive -- many need to fetch the entire repo contents.
+ - (Which is fine if the repo is going to be added as a remote..)
  -}
-probeGCryptRepo :: FilePath -> Repo -> IO Bool
-probeGCryptRepo dir baserepo = catchBoolIO $ Command.runBool
-	[ Param "fetch"
-	, Param $ urlPrefix ++ dir
-	] baserepo
+probeRepo :: String -> Repo -> IO ProbeResult
+probeRepo loc baserepo = do
+	let p = proc "git" $ toCommand $ Command.gitCommandLine
+		[ Param "remote-gcrypt"
+		, Param "--check"
+		, Param loc
+		] baserepo
+	(_, _, _, pid) <- createProcess p
+	code <- waitForProcess pid
+	return $ case code of
+		ExitSuccess -> Decryptable
+		ExitFailure 1 -> NotDecryptable
+		ExitFailure _ -> NotEncrypted
 
 type RemoteName = String
 type GCryptId = String