fsck: Commit incremental fsck database after every 1000 files fscked, or every 5 minutes, whichever comes first.

Previously, commits were made every 1000 files fscked.

Also, improve docs

Database/Handle.hs

@@ -20,6 +20,7 @@ module Database.Handle (
 ) where
 
 import Utility.Exception
+import Utility.Monad
 import Messages
 
 import Database.Persist.Sqlite
@@ -33,6 +34,7 @@ import qualified Data.Text as T
 import Control.Monad.Trans.Resource (runResourceT)
 import Control.Monad.Logger (runNoLoggingT)
 import Data.List
+import Data.Time.Clock
 
 {- A DbHandle is a reference to a worker thread that communicates with
  - the database. It has a MVar which Jobs are submitted to. -}
@@ -64,7 +66,7 @@ openDb :: FilePath -> TableName -> IO DbHandle
 openDb db tablename = do
 	jobs <- newEmptyMVar
 	worker <- async (workerThread (T.pack db) tablename jobs)
-	q <- newMVar emptyDbQueue
+	q <- newMVar =<< emptyDbQueue
 	return $ DbHandle worker jobs q
 
 data Job
@@ -145,16 +147,19 @@ closeDb h@(DbHandle worker jobs _) = do
 
 type Size = Int
 
-{- A queue of actions to perform, with a count of the number of actions
- - queued. -}
-data DbQueue = DbQueue Size (SqlPersistM ())
+type LastCommitTime = UTCTime
 
-emptyDbQueue :: DbQueue
-emptyDbQueue = DbQueue 0 (return ())
+{- A queue of actions to perform, with a count of the number of actions
+ - queued, and a last commit time. -}
+data DbQueue = DbQueue Size LastCommitTime (SqlPersistM ())
+
+emptyDbQueue :: IO DbQueue
+emptyDbQueue = do
+	now <- getCurrentTime
+	return $ DbQueue 0 now (return ())
 
 {- Queues a change to be made to the database. It will be buffered
- - to be committed later, unless the queue gets larger than the specified
- - size.
+ - to be committed later, unless the commitchecker action returns true.
  -
  - (Be sure to call closeDb or flushQueueDb to ensure the change
  - gets committed.)
@@ -164,25 +169,32 @@ emptyDbQueue = DbQueue 0 (return ())
  - process, the transaction is put back in the queue. This solves
  - the sqlite multiple writer problem.
  -}
-queueDb :: DbHandle -> Size -> SqlPersistM () -> IO ()
-queueDb h@(DbHandle _ _ qvar) maxsz a = do
-	DbQueue sz qa <- takeMVar qvar
+queueDb 
+	:: DbHandle
+	-> (Size -> LastCommitTime -> IO Bool) 
+	-> SqlPersistM ()
+	-> IO ()
+queueDb h@(DbHandle _ _ qvar) commitchecker a = do
+	DbQueue sz lastcommittime qa <- takeMVar qvar
 	let !sz' = sz + 1
 	let qa' = qa >> a
-	let enqueue newsz = putMVar qvar (DbQueue newsz qa')
-	if sz' > maxsz
-		then do
+	let enqueue = putMVar qvar
+	ifM (commitchecker sz' lastcommittime)
+		( do
 			r <- commitDb h qa'
 			case r of
-				Left _ -> enqueue 0
-				Right _ -> putMVar qvar emptyDbQueue
-		else enqueue sz'
+				Left _ -> enqueue $ DbQueue sz' lastcommittime qa'
+				Right _ -> do
+					now <- getCurrentTime
+					enqueue $ DbQueue 0 now (return ())
+		, enqueue $ DbQueue sz' lastcommittime qa'
+		)
 
 {- If flushing the queue fails, this could be because there is another
  - writer to the database. Retry repeatedly for up to 10 seconds. -}
 flushQueueDb :: DbHandle -> IO ()
 flushQueueDb h@(DbHandle _ _ qvar) = do
-	DbQueue sz qa <- takeMVar qvar	
+	DbQueue sz _ qa <- takeMVar qvar	
 	when (sz > 0) $
 		robustly Nothing 100 (commitDb h qa)
   where