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/Helper/Special.hs

@@ -11,6 +11,8 @@ module Remote.Helper.Special (
 	Preparer,
 	Storer,
 	Retriever,
+	Remover,
+	CheckPresent,
 	simplyPrepare,
 	ContentSource,
 	checkPrepare,
@@ -21,6 +23,8 @@ module Remote.Helper.Special (
 	byteRetriever,
 	storeKeyDummy,
 	retreiveKeyFileDummy,
+	removeKeyDummy,
+	checkPresentDummy,
 	SpecialRemoteCfg(..),
 	specialRemoteCfg,
 	specialRemote,
@@ -36,6 +40,7 @@ import Config.Cost
 import Utility.Metered
 import Remote.Helper.Chunked as X
 import Remote.Helper.Encryptable as X hiding (encryptableRemote)
+import Remote.Helper.Messages
 import Annex.Content
 import Annex.Exception
 import qualified Git
@@ -114,16 +119,27 @@ byteRetriever :: (Key -> (L.ByteString -> Annex Bool) -> Annex Bool) -> Retrieve
 byteRetriever a k _m callback = a k (callback . ByteContent)
 
 {- The base Remote that is provided to specialRemote needs to have
- - storeKey and retreiveKeyFile methods, but they are never
- - actually used (since specialRemote replaces them).
+ - storeKey, retreiveKeyFile, removeKey, and checkPresent methods,
+ - but they are never actually used (since specialRemote replaces them).
  - Here are some dummy ones.
  -}
 storeKeyDummy :: Key -> AssociatedFile -> MeterUpdate -> Annex Bool
 storeKeyDummy _ _ _ = return False
 retreiveKeyFileDummy :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Bool
 retreiveKeyFileDummy _ _ _ _ = return False
+removeKeyDummy :: Key -> Annex Bool
+removeKeyDummy _ = return False
+checkPresentDummy :: Key -> Annex Bool
+checkPresentDummy _ = error "missing checkPresent implementation"
 
-type RemoteModifier = RemoteConfig -> Preparer Storer -> Preparer Retriever -> Remote -> Remote
+type RemoteModifier
+	= RemoteConfig
+	-> Preparer Storer
+	-> Preparer Retriever
+	-> Preparer Remover
+	-> Preparer CheckPresent
+	-> Remote
+	-> Remote
 
 data SpecialRemoteCfg = SpecialRemoteCfg
 	{ chunkConfig :: ChunkConfig
@@ -139,13 +155,14 @@ specialRemote :: RemoteModifier
 specialRemote c = specialRemote' (specialRemoteCfg c) c
 
 specialRemote' :: SpecialRemoteCfg -> RemoteModifier
-specialRemote' cfg c preparestorer prepareretriever baser = encr
+specialRemote' cfg c preparestorer prepareretriever prepareremover preparecheckpresent baser = encr
   where
 	encr = baser
 		{ storeKey = \k _f p -> cip >>= storeKeyGen k p
 		, retrieveKeyFile = \k _f d p -> cip >>= retrieveKeyFileGen k d p
 		, retrieveKeyFileCheap = \k d -> cip >>= maybe
 			(retrieveKeyFileCheap baser k d)
+			-- retrieval of encrypted keys is never cheap
 			(\_ -> return False)
 		, removeKey = \k -> cip >>= removeKeyGen k
 		, checkPresent = \k -> cip >>= checkPresentGen k
@@ -160,8 +177,7 @@ specialRemote' cfg c preparestorer prepareretriever baser = encr
 	safely a = catchNonAsyncAnnex a (\e -> warning (show e) >> return False)
 
 	-- chunk, then encrypt, then feed to the storer
-	storeKeyGen k p enc =
-		safely $ preparestorer k $ safely . go
+	storeKeyGen k p enc = safely $ preparestorer k $ safely . go
 	  where
 		go (Just storer) = sendAnnex k rollback $ \src ->
 			displayprogress p k $ \p' ->
@@ -178,7 +194,7 @@ specialRemote' cfg c preparestorer prepareretriever baser = encr
 				readBytes $ \encb ->
 					storer (enck k) (ByteContent encb) p
 
-	-- call retriever to get chunks; decrypt them; stream to dest file
+	-- call retrieve-r to get chunks; decrypt them; stream to dest file
 	retrieveKeyFileGen k dest p enc =
 		safely $ prepareretriever k $ safely . go
 	  where
@@ -188,15 +204,17 @@ specialRemote' cfg c preparestorer prepareretriever baser = encr
 		go Nothing = return False
 		enck = maybe id snd enc
 
-	removeKeyGen k enc = removeChunks remover (uuid baser) chunkconfig enck k
+	removeKeyGen k enc = safely $ prepareremover k $ safely . go
 	  where
+		go (Just remover) = removeChunks remover (uuid baser) chunkconfig enck k
+		go Nothing = return False
 		enck = maybe id snd enc
-		remover = removeKey baser
 
-	checkPresentGen k enc = checkPresentChunks checker (uuid baser) chunkconfig enck k
+	checkPresentGen k enc = preparecheckpresent k go
 	  where
+	  	go (Just checker) = checkPresentChunks checker (uuid baser) chunkconfig enck k
+		go Nothing = cantCheck baser
 		enck = maybe id snd enc
-		checker = checkPresent baser
 
 	chunkconfig = chunkConfig cfg