webapp: support adding existing gcrypt special remotes from removable drives

When adding a removable drive, it's now detected if the drive contains
a gcrypt special remote, and that's all handled nicely. This includes
fetching the git-annex branch from the gcrypt repo in order to find
out how to set up the special remote.

Note that gcrypt repos that are not git-annex special remotes are not
supported. It will attempt to detect such a gcrypt repo and refuse
to use it. (But this is hard to do any may fail; see
https://github.com/blake2-ppc/git-remote-gcrypt/issues/6)

The problem with supporting regular gcrypt repos is that we don't know
what the gcrypt.participants setting is intended to be for the repo.
So even if we can decrypt it, if we push changes to it they might not be
visible to other participants.

Anyway, encrypted sneakernet (or mailnet) is now fully possible with the
git-annex assistant! Assuming that the gpg key distribution is handled
somehow, which the assistant doesn't yet help with.

This commit was sponsored by Navishkar Rao.

Assistant/WebApp/Gpg.hs

@@ -12,6 +12,12 @@ module Assistant.WebApp.Gpg where
 import Assistant.WebApp.Common
 import Utility.Gpg
 import Utility.UserInfo
+import qualified Git.Command
+import qualified Git.Remote
+import qualified Annex.Branch
+import qualified Git.GCrypt
+import Assistant.MakeRemote
+import Logs.Remote
 
 import qualified Data.Map as M
 
@@ -45,3 +51,25 @@ withNewSecretKey use = do
 	case results of
 		[] -> error "Failed to generate gpg key!"
 		(key:_) -> use key
+
+{- Tries to find the name used in remote.log for a gcrypt repository
+ - with a given uuid.
+ -
+ - The gcrypt remote may not be on that is listed in the local remote.log
+ - (or the info may be out of date), so this actually fetches the git-annex
+ - branch from the gcrypt remote and merges it in, and then looks up
+ - the name.
+ -}
+getGCryptRemoteName :: UUID -> String -> Annex (Maybe Git.Remote.RemoteName)
+getGCryptRemoteName u repoloc = do
+	tmpremote <- uniqueRemoteName "tmpgcryptremote" 0 <$> gitRepo
+	void $ inRepo $ Git.Command.runBool
+		[Params "remote add", Param tmpremote, Param $ Git.GCrypt.urlPrefix ++ repoloc]
+	mname <- ifM (inRepo $ Git.Command.runBool [Param "fetch", Param tmpremote])
+		( do
+			void $ Annex.Branch.forceUpdate
+			(M.lookup "name" <=< M.lookup u) <$> readRemoteLog
+		, return Nothing
+		)
+	void $ inRepo $ Git.Remote.remove tmpremote
+	return mname