optimise read and write for Keys database (untested)

Writes are optimised by queueing up multiple writes when possible.
The queue is flushed after the Annex monad action finishes. That makes it
happen on program termination, and also whenever a nested Annex monad action
finishes.

Reads are optimised by checking once (per AnnexState) if the database
exists. If the database doesn't exist yet, all reads return mempty.

Reads also cause queued writes to be flushed, so reads will always be
consistent with writes (as long as they're made inside the same Annex monad).
A future optimisation path would be to determine when that's not necessary,
which is probably most of the time, and avoid flushing unncessarily.

Design notes for this commit:

- separate reads from writes
- reuse a handle which is left open until program
  exit or until the MVar goes out of scope (and autoclosed then)
- writes are queued
  - queue is flushed periodically
  - immediate queue flush before any read
  - auto-flush queue when database handle is garbage collected
  - flush queue on exit from Annex monad
    (Note that this may happen repeatedly for a single database connection;
    or a connection may be reused for multiple Annex monad actions,
    possibly even concurrent ones.)
- if database does not exist (or is empty) the handle
  is not opened by reads; reads instead return empty results
- writes open the handle if it was not open previously

2 files changed, 55 insertions, 14 deletions

diff --git a/Database/Keys/Handle.hs b/Database/Keys/Handle.hs
new file mode 100644
index 000000000..5a5912b0b
--- /dev/null
+++ b/Database/Keys/Handle.hs
@@ -0,0 +1,55 @@
+{- Handle for the Keys database.
+ -
+ - Copyright 2015 Joey Hess <id@joeyh.name>
+ -:
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Database.Keys.Handle (
+	DbHandle,
+	newDbHandle,
+	DbState(..),
+	withDbState,
+	flushDbQueue,
+) where
+
+import qualified Database.Queue as H
+import Utility.Exception
+
+import Control.Concurrent
+import Control.Monad.IO.Class (liftIO, MonadIO)
+
+-- The MVar is always left full except when actions are run
+-- that access the database.
+newtype DbHandle = DbHandle (MVar DbState)
+
+-- The database can be closed or open, but it also may have been
+-- tried to open (for read) and didn't exist yet.
+data DbState = DbClosed | DbOpen H.DbQueue | DbEmpty
+
+newDbHandle :: IO DbHandle
+newDbHandle = DbHandle <$> newMVar DbClosed
+
+-- Runs an action on the state of the handle, which can change its state.
+-- The MVar is empty while the action runs, which blocks other users
+-- of the handle from running.
+withDbState
+	:: (MonadIO m, MonadCatch m)
+	=> DbHandle
+	-> (DbState
+	-> m (v, DbState))
+	-> m v
+withDbState (DbHandle mvar) a = do
+	st <- liftIO $ takeMVar mvar
+	go st `onException` (liftIO $ putMVar mvar st)
+  where
+	go st = do
+		(v, st') <- a st
+		liftIO $ putMVar mvar st'
+		return v
+
+flushDbQueue :: DbHandle -> IO ()
+flushDbQueue (DbHandle mvar) = go =<< readMVar mvar
+  where
+	go (DbOpen qh) = H.flushDbQueue qh
+	go _ = return ()
diff --git a/Database/Keys/Types.hs b/Database/Keys/Types.hs
deleted file mode 100644
index 3fabafcf2..000000000
--- a/Database/Keys/Types.hs
+++ /dev/null
@@ -1,14 +0,0 @@
-{- Sqlite database of information about Keys, data types.
- -
- - Copyright 2015 Joey Hess <id@joeyh.name>
- -:
- - Licensed under the GNU GPL version 3 or higher.
- -}
-
-module Database.Keys.Types (
-	DbHandle(..)
-) where
-
-import qualified Database.Queue as H
-
-newtype DbHandle = DbHandle H.DbQueue