log migration trees to git-annex branch

This will allow distributed migration: Start a migration in one clone of
a repo, and then update other clones.

commitMigration is a bit of a bear.. There is some inversion of control
that needs some TMVars. Also streamLogFile's finalizer does not handle
recording the trees, so an interrupt at just the wrong time can cause
migration.log to be emptied but the git-annex branch not updated.

Sponsored-by: Graham Spencer on Patreon

Utility/CoProcess.hs

@@ -19,6 +19,7 @@ module Utility.CoProcess (
 import Common
 
 import Control.Concurrent.MVar
+import Control.Monad.IO.Class (MonadIO)
 
 type CoProcessHandle = MVar CoProcessState
 
@@ -65,11 +66,11 @@ stop ch = do
 {- To handle a restartable process, any IO exception thrown by the send and
  - receive actions are assumed to mean communication with the process
  - failed, and the failed action is re-run with a new process. -}
-query :: CoProcessHandle -> (Handle -> IO a) -> (Handle -> IO b) -> IO b
+query :: (MonadIO m, MonadCatch m) => CoProcessHandle -> (Handle -> m a) -> (Handle -> m b) -> m b
 query ch send receive = do
-	s <- readMVar ch
+	s <- liftIO $ readMVar ch
 	restartable s (send $ coProcessTo s) $ const $
-		restartable s (hFlush $ coProcessTo s) $ const $
+		restartable s (liftIO $ hFlush $ coProcessTo s) $ const $
 			restartable s (receive $ coProcessFrom s)
 				return
   where
@@ -78,12 +79,12 @@ query ch send receive = do
 			maybe restart cont =<< catchMaybeIO a
 		| otherwise = cont =<< a
 	restart = do
-		s <- takeMVar ch
-		void $ catchMaybeIO $ do
+		s <- liftIO $ takeMVar ch
+		void $ liftIO $ catchMaybeIO $ do
 			hClose $ coProcessTo s
 			hClose $ coProcessFrom s
-		void $ waitForProcess $ coProcessPid s
-		s' <- start' $ (coProcessSpec s)
+		void $ liftIO $ waitForProcess $ coProcessPid s
+		s' <- liftIO $ start' $ (coProcessSpec s)
 			{ coProcessNumRestarts = coProcessNumRestarts (coProcessSpec s) - 1 }
-		putMVar ch s'
+		liftIO $ putMVar ch s'
 		query ch send receive