add exporter thread to assistant

This is similar to the pusher thread, but a separate thread because git
pushes can be done in parallel with exports, and updating a big export
should not prevent other git pushes going out in the meantime.

The exportThread only runs at most every 30 seconds, since updating an
export is more expensive than pushing. This may need to be tuned.

Added a separate channel for export commits; the committer records a
commit in that channel.

Also, reconnectRemotes records a dummy commit, to make the exporter
thread wake up and make sure all exports are up-to-date. So,
connecting a drive with a directory special remote export will
immediately update it, and getting online will automatically
update S3 and WebDAV exports.

The transfer queue is not involved in exports. Instead, failed
exports are retried much like failed pushes.

This commit was sponsored by Ewen McNeill.

1 files changed, 78 insertions, 0 deletions

diff --git a/Assistant/Threads/Exporter.hs b/Assistant/Threads/Exporter.hs
new file mode 100644
index 000000000..747e919da
--- /dev/null
+++ b/Assistant/Threads/Exporter.hs
@@ -0,0 +1,78 @@
+{- git-annex assistant export updating thread
+ -
+ - Copyright 2017 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Assistant.Threads.Exporter where
+
+import Assistant.Common
+import Assistant.Commits
+import Assistant.Pushes
+import Assistant.DaemonStatus
+import Annex.Concurrent
+import Utility.ThreadScheduler
+import qualified Annex
+import qualified Remote
+import qualified Types.Remote as Remote
+import qualified Command.Sync
+
+import Control.Concurrent.Async
+import Data.Time.Clock
+import qualified Data.Map as M
+
+{- This thread retries exports that failed before. -}
+exportRetryThread :: NamedThread
+exportRetryThread = namedThread "ExportRetrier" $ runEvery (Seconds halfhour) <~> do
+	-- We already waited half an hour, now wait until there are failed
+	-- exports to retry.
+	toexport <- getFailedPushesBefore (fromIntegral halfhour) 
+		=<< getAssistant failedExportMap
+	unless (null toexport) $ do
+		debug ["retrying", show (length toexport), "failed exports"]
+		void $ exportToRemotes toexport
+  where
+	halfhour = 1800
+
+{- This thread updates exports soon after git commits are made. -}
+exportThread :: NamedThread
+exportThread = namedThread "Exporter" $ runEvery (Seconds 30) <~> do
+	-- We already waited two seconds as a simple rate limiter.
+	-- Next, wait until at least one commit has been made
+	void getExportCommits
+	-- Now see if now's a good time to push.
+	void $ exportToRemotes =<< exportTargets
+
+{- We want to avoid exporting to remotes that are marked readonly.
+ -
+ - Also, avoid exporting to local remotes we can easily tell are not available,
+ - to avoid ugly messages when a removable drive is not attached.
+ -}
+exportTargets :: Assistant [Remote]
+exportTargets = liftIO . filterM (Remote.checkAvailable True)
+	=<< candidates <$> getDaemonStatus
+  where
+	candidates = filter (not . Remote.readonly) . exportRemotes
+
+exportToRemotes :: [Remote] -> Assistant ()
+exportToRemotes rs = do
+	-- This is a long-duration action which runs in the Annex monad,
+	-- so don't just liftAnnex to run it; fork the Annex state.
+	runner <- liftAnnex $ forkState $
+		forM rs $ \r -> do
+			Annex.changeState $ \st -> st { Annex.errcounter = 0 }
+			start <- liftIO getCurrentTime
+			void $ Command.Sync.seekExportContent rs
+			-- Look at command error counter to see if the export
+			-- didn't work.
+			failed <- (> 0) <$> Annex.getState Annex.errcounter
+			Annex.changeState $ \st -> st { Annex.errcounter = 0 }
+			return $ if failed
+				then Just (r, start)
+				else Nothing
+	failed <- catMaybes
+		<$> (liftAnnex =<< liftIO . wait =<< liftIO (async runner))
+	unless (null failed) $ do
+		v <- getAssistant failedExportMap
+		changeFailedPushMap v $ M.union $ M.fromList failed