run Preparer to get Remover and CheckPresent actions

This will allow special remotes to eg, open a http connection and reuse it,
while checking if chunks are present, or removing chunks.

S3 and WebDAV both need this to support chunks with reasonable speed.

Note that a special remote might want to cache a http connection across
multiple requests. A simple case of this is that CheckPresent is typically
called before Store or Remove. A remote using this interface can certianly
use a Preparer that eg, uses a MVar to cache a http connection.

However, it's up to the remote to then deal with things like stale or
stalled http connections when eg, doing a series of downloads from a remote
and other places. There could be long delays between calls to a remote,
which could lead to eg, http connection stalls; the machine might even
move to a new network, etc.

It might be nice to improve this interface later to allow
the simple case without needing to handle the full complex case.
One way to do it would be to have a `Transaction SpecialRemote cache`,
where SpecialRemote contains methods for Storer, Retriever, Remover, and
CheckPresent, that all expect to be passed a `cache`.

Remote/S3.hs

@@ -47,6 +47,8 @@ gen r u c gc = new <$> remoteCost gc expensiveRemoteCost
 	new cst = Just $ specialRemote c
 		(prepareStore this)
 		(prepareRetrieve this)
+		(simplyPrepare $ remove this c)
+		(simplyPrepare $ checkKey this)
 		this
 	  where
 		this = Remote {
@@ -55,9 +57,9 @@ gen r u c gc = new <$> remoteCost gc expensiveRemoteCost
 			name = Git.repoDescribe r,
 			storeKey = storeKeyDummy,
 			retrieveKeyFile = retreiveKeyFileDummy,
-			retrieveKeyFileCheap = retrieveCheap this,
-			removeKey = remove this c,
-			checkPresent = checkKey this,
+			retrieveKeyFileCheap = retrieveCheap,
+			removeKey = removeKeyDummy,
+			checkPresent = checkPresentDummy,
 			checkPresentCheap = False,
 			whereisKey = Nothing,
 			remoteFsck = Nothing,
@@ -150,13 +152,13 @@ prepareRetrieve r = resourcePrepare (const $ s3Action r False) $ \(conn, bucket)
 		liftIO (getObject conn $ bucketKey r bucket k)
 			>>= either s3Error (sink . obj_data)
 
-retrieveCheap :: Remote -> Key -> FilePath -> Annex Bool
-retrieveCheap _ _ _ = return False
+retrieveCheap :: Key -> FilePath -> Annex Bool
+retrieveCheap _ _ = return False
 
 {- Internet Archive doesn't easily allow removing content.
  - While it may remove the file, there are generally other files
  - derived from it that it does not remove. -}
-remove :: Remote -> RemoteConfig -> Key -> Annex Bool
+remove :: Remote -> RemoteConfig -> Remover
 remove r c k
 	| isIA c = do
 		warning "Cannot remove content from the Internet Archive"
@@ -167,7 +169,7 @@ remove' :: Remote -> Key -> Annex Bool
 remove' r k = s3Action r False $ \(conn, bucket) ->
 	s3Bool =<< liftIO (deleteObject conn $ bucketKey r bucket k)
 
-checkKey :: Remote -> Key -> Annex Bool
+checkKey :: Remote -> CheckPresent
 checkKey r k = s3Action r noconn $ \(conn, bucket) -> do
 	showAction $ "checking " ++ name r
 	res <- liftIO $ getObjectInfo conn $ bucketKey r bucket k