git-annex/Assistant/TransferSlots.hs

{- git-annex assistant transfer slots
 -
 - Copyright 2012 Joey Hess <joey@kitenet.net>
 -
 - Licensed under the GNU GPL version 3 or higher.
 -}

module Assistant.TransferSlots where

import Assistant.Common
import Utility.ThreadScheduler
import Assistant.Types.TransferSlots
import Assistant.DaemonStatus
import Assistant.TransferrerPool
import Assistant.Types.TransferrerPool
import Logs.Transfer

import qualified Control.Exception as E
import Control.Concurrent
import qualified Control.Concurrent.MSemN as MSemN

type TransferGenerator = Assistant (Maybe (Transfer, TransferInfo, Transferrer -> Assistant ()))

{- Waits until a transfer slot becomes available, then runs a
 - TransferGenerator, and then runs the transfer action in its own thread. 
 -}
inTransferSlot :: FilePath -> TransferGenerator -> Assistant ()
inTransferSlot program gen = do
	flip MSemN.wait 1 <<~ transferSlots
	runTransferThread program =<< gen

{- Runs a TransferGenerator, and its transfer action,
 - without waiting for a slot to become available. -}
inImmediateTransferSlot :: FilePath -> TransferGenerator -> Assistant ()
inImmediateTransferSlot program gen = do
	flip MSemN.signal (-1) <<~ transferSlots
	runTransferThread program =<< gen

{- Runs a transfer action, in an already allocated transfer slot.
 - Once it finishes, frees the transfer slot.
 -
 - Note that the action is subject to being killed when the transfer
 - is canceled or paused.
 -
 - A PauseTransfer exception is handled by letting the action be killed,
 - then pausing the thread until a ResumeTransfer exception is raised,
 - then rerunning the action.
 -}
runTransferThread :: FilePath -> Maybe (Transfer, TransferInfo, Transferrer -> Assistant ()) -> Assistant ()
runTransferThread _ Nothing = flip MSemN.signal 1 <<~ transferSlots
runTransferThread program (Just (t, info, a)) = do
	d <- getAssistant id
	aio <- asIO1 a
	tid <- liftIO $ forkIO $ runTransferThread' program d aio
	updateTransferInfo t $ info { transferTid = Just tid }

runTransferThread' :: FilePath -> AssistantData -> (Transferrer -> IO ()) -> IO ()
runTransferThread' program d run = go
  where
	go = catchPauseResume $
		withTransferrer program (transferrerPool d)
			run
	pause = catchPauseResume $
		runEvery (Seconds 86400) noop
	{- Note: This must use E.try, rather than E.catch.
	 - When E.catch is used, and has called go in its exception
	 - handler, Control.Concurrent.throwTo will block sometimes
	 - when signaling. Using E.try avoids the problem. -}
	catchPauseResume a' = do
		r <- E.try a' :: IO (Either E.SomeException ())
		case r of
			Left e -> case E.fromException e of
				Just PauseTransfer -> pause
				Just ResumeTransfer -> go
				_ -> done
			_ -> done
	done = runAssistant d $ 
		flip MSemN.signal 1 <<~ transferSlots