fork off git-annex copy for transfers

This doesn't quite work, because canceling a transfer sends a signal
to git-annex, but not to rsync (etc).

Looked at making git-annex run in its own process group, which could then
be killed, and would kill child processes. But, rsync checks if it's
process group is the foreground process group and doesn't show progress if
not, and when git has run git-annex, if git-annex makes a new process
group, that is not the case. Also, if git has run git-annex, ctrl-c
wouldn't be propigated to it if it made a new process group.

So this seems like a blind alley, but recording it here just in case.

Assistant/Threads/Transferrer.hs

@@ -14,7 +14,6 @@ import Assistant.TransferQueue
 import Assistant.TransferSlots
 import Assistant.Alert
 import Logs.Transfer
-import Logs.Presence
 import Logs.Location
 import Annex.Content
 import qualified Remote
@@ -41,7 +40,7 @@ transfererThread st dstatus transferqueue slots = go
 				( do
 					debug thisThread [ "Transferring:" , show t ]
 					notifyTransfer dstatus
-					transferThread st dstatus slots t info
+					transferThread dstatus slots t info
 				, do
 					debug thisThread [ "Skipping unnecessary transfer:" , show t ]
 					-- getNextTransfer added t to the
@@ -71,22 +70,22 @@ shouldTransfer t info
 	where
 		key = transferKey t
 
-{- A transfer is run in a separate thread, with a *copy* of the Annex
- - state. This is necessary to avoid blocking the rest of the assistant
- - on the transfer completing, and also to allow multiple transfers to run
- - at once. This requires GHC's threaded runtime to work!
+{- A sepeate git-annex process is forked off to run a transfer.
+ - This allows killing the process if the user decides to cancel the
+ - transfer.
  -
- - The copy of state  means that the transfer processes are responsible
- - for doing any necessary shutdown cleanups, and that the parent
- - thread's cache must be invalidated once a transfer completes, as
- - changes may have been made to the git-annex branch.
- -}
-transferThread :: ThreadState -> DaemonStatusHandle -> TransferSlots -> Transfer -> TransferInfo -> IO ()
-transferThread st dstatus slots t info = case (transferRemote info, associatedFile info) of
+ - A thread is forked off to run the process, and the thread
+ - occupys one of the transfer slots. If all slots are in use, this will
+ - block until one becomes available. The thread's id is also recorded in
+ - the transfer info; the thread will also be killed when a transfer is
+ - stopped, to avoid it displaying any alert about the transfer having
+ - failed. -}
+transferThread :: DaemonStatusHandle -> TransferSlots -> Transfer -> TransferInfo -> IO ()
+transferThread dstatus slots t info = case (transferRemote info, associatedFile info) of
 	(Nothing, _) -> noop
 	(_, Nothing) -> noop
 	(Just remote, Just file) -> do
-		tid <- inTransferSlot slots st $
+		tid <- inTransferSlot slots $
 			transferprocess remote file
 		now <- getCurrentTime
 		adjustTransfers dstatus $
@@ -97,24 +96,15 @@ transferThread st dstatus slots t info = case (transferRemote info, associatedFi
 	where
 		direction = transferDirection t
 		isdownload = direction == Download
-		tofrom
-			| isdownload = "from"
-			| otherwise = "to"
-		key = transferKey t
 
-		transferprocess remote file = do
-			showStart "copy" file
-			showAction $ tofrom ++ " " ++ Remote.name remote
-			ok <- runTransfer t (Just file) $
-				if isdownload
-					then getViaTmp key $
-						Remote.retrieveKeyFile remote key (Just file)
-					else do
-						ok <- Remote.storeKey remote key $ Just file
-						when ok $
-							Remote.logStatus remote key InfoPresent
-						return ok
-			showEndResult ok
-			liftIO $ addAlert dstatus $
+		transferprocess remote file = void $ do
+			ok <- boolSystem "git-annex"
+				[ Param "copy"
+				, Param "--fast"
+				, Param $ if isdownload then "--from" else "--to"
+				, Param $ Remote.name remote
+				, File file
+				]
+			addAlert dstatus $
 				makeAlertFiller ok $ 
 					transferFileAlert direction file