add RemoteStateHandle

This solves the problem of sameas remotes trampling over per-remote
state. Used for:

* per-remote state, of course
* per-remote metadata, also of course
* per-remote content identifiers, because two remote implementations
  could in theory generate the same content identifier for two different
  peices of content

While chunk logs are per-remote data, they don't use this, because the
number and size of chunks stored is a common property across sameas
remotes.

External special remote had a complication, where it was theoretically
possible for a remote to send SETSTATE or GETSTATE during INITREMOTE or
EXPORTSUPPORTED. Since the uuid of the remote is typically generate in
Remote.setup, it would only be possible to pass a Maybe
RemoteStateHandle into it, and it would otherwise have to construct its
own. Rather than go that route, I decided to send an ERROR in this case.
It seems unlikely that any existing external special remote will be
affected. They would have to make up a git-annex key, and set state for
some reason during INITREMOTE. I can imagine such a hack, but it doesn't
seem worth complicating the code in such an ugly way to support it.

Unfortunately, both TestRemote and Annex.Import needed the Remote
to have a new field added that holds its RemoteStateHandle.

Types/Remote.hs

@@ -14,6 +14,7 @@ module Types.Remote
 	, RemoteConfig
 	, RemoteTypeA(..)
 	, RemoteA(..)
+	, RemoteStateHandle
 	, SetupStage(..)
 	, Availability(..)
 	, Verification(..)
@@ -36,6 +37,7 @@ import Types.UUID
 import Types.GitConfig
 import Types.Availability
 import Types.Creds
+import Types.RemoteState
 import Types.UrlContents
 import Types.NumCopies
 import Types.Export
@@ -61,7 +63,7 @@ data RemoteTypeA a = RemoteType
 	-- The Bool is True if automatic initialization of remotes is desired
 	, enumerate :: Bool -> a [Git.Repo]
 	-- generates a remote of this type
-	, generate :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> a (Maybe (RemoteA a))
+	, generate :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> RemoteStateHandle -> a (Maybe (RemoteA a))
 	-- initializes or enables a remote
 	, setup :: SetupStage -> Maybe UUID -> Maybe CredPair -> RemoteConfig -> RemoteGitConfig -> a (RemoteConfig, UUID)
 	-- check if a remote of this type is able to support export
@@ -151,6 +153,7 @@ data RemoteA a = Remote
 	-- its contents, without downloading the full content.
 	-- Throws an exception if the url is inaccessible.
 	, checkUrl :: Maybe (URLString -> a UrlContents)
+	, remoteStateHandle :: RemoteStateHandle
 	}
 
 instance Show (RemoteA a) where

Types/RemoteState.hs

@@ -0,0 +1,19 @@
+{- git-annex remote state handle type
+ -
+ - Copyright 2019 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module Types.RemoteState where
+
+import Types.UUID
+
+{- When per-remote state, its UUID is used to identify it.
+ -
+ - However, sameas remotes mean that two different Remote implementations
+ - can be used for the same underlying data store. To avoid them using
+ - state in conflicting ways, a different UUID needs to be used for each
+ - additional sameas remote.
+ -}
+newtype RemoteStateHandle = RemoteStateHandle UUID