cache one more log file for metadata

My worry was that a preferred content expression that matches on metadata
would have removed the location log from cache, causing an expensive
re-read when a Seek action later checked the location log.

Especially when the --all optimisation in the previous commit
pre-cached the location log.

This also means that the --all optimisation could cache the metadata log
too, if it wanted too, but not currently done.

The cache is a list, with the most recently accessed file first. That
optimises it for the common case of reading the same file twice, eg a
get, examine, followed by set reads it twice. And sync --content reads the
location log 3 times in a row commonly.

But, as a list, it should not be made to be too long. I thought about
expanding it to 5 items, but that seemed unlikely to be a win commonly
enough to outweigh the extra time spent checking the cache.

Clearly there could be some further benchmarking and tuning here.

Annex/BranchState.hs

@@ -12,6 +12,7 @@ module Annex.BranchState where
 import Annex.Common
 import Types.BranchState
 import qualified Annex
+import Logs
 
 import qualified Data.ByteString.Lazy as L
 
@@ -88,22 +89,19 @@ enableInteractiveBranchAccess = changeState $
 
 setCache :: RawFilePath -> L.ByteString -> Annex ()
 setCache file content = changeState $ \s -> s
-	{ cachedFile = Just file
-	, cachedContent = content
-	}
+	{ cachedFileContents = add (cachedFileContents s) }
+  where
+	add l
+		| length l < logFilesToCache = (file, content) : l
+		| otherwise = (file, content) : Prelude.init l
 
 getCache :: RawFilePath -> Annex (Maybe L.ByteString)
-getCache file = go <$> getState
+getCache file = (\st -> go (cachedFileContents st) st) <$> getState
   where
-	go state
-		| cachedFile state == Just file 
-			&& not (needInteractiveAccess state) =
-				Just (cachedContent state)
-		| otherwise = Nothing
+	go [] _ = Nothing
+	go ((f,c):rest) state
+		| f == file && not (needInteractiveAccess state) = Just c
+		| otherwise = go rest state
 
 invalidateCache :: Annex ()
-invalidateCache = changeState $ \s -> s
-	{ cachedFile = Nothing
-	, cachedContent = mempty
-	}
-
+invalidateCache = changeState $ \s -> s { cachedFileContents = [] }

Logs.hs

@@ -38,6 +38,26 @@ getLogVariety config f
 	| isMetaDataLog f || f `elem` otherLogs = Just OtherLog
 	| otherwise = PresenceLog <$> firstJust (presenceLogs config f)
 
+{- Typical number of log files that may be read while processing a single
+ - key. This is used to size a cache.
+ -
+ - The location log is generally read, and the metadata log is read when
+ - matching a preferred content expression that matches on metadata,
+ - or when using metadata options.
+ -
+ - When using a remote, the url log, chunk log, remote state log, remote
+ - metadata log, and remote content identifier log might each be used,
+ - but probably at most 3 out of the 6. However, caching too much slows
+ - down all operations because the cache is a linear list, so the cache
+ - is not currently sized to include these.
+ -
+ - The result is that when seeking for files to operate on,
+ - the location log will stay in the cache if the metadata log is also
+ - read.
+ -}
+logFilesToCache :: Int
+logFilesToCache = 2
+
 {- All the old-format uuid-based logs stored in the top of the git-annex branch. -}
 topLevelOldUUIDBasedLogs :: [RawFilePath]
 topLevelOldUUIDBasedLogs =
@@ -59,7 +79,6 @@ topLevelNewUUIDBasedLogs =
 	[ exportLog
 	]
 
-
 {- All the ways to get a key from a presence log file -}
 presenceLogs :: GitConfig -> RawFilePath -> [Maybe Key]
 presenceLogs config f =

Types/BranchState.hs

@@ -19,10 +19,8 @@ data BranchState = BranchState
 	, journalIgnorable :: Bool
 	-- ^ can reading the journal be skipped, while still getting
 	-- sufficiently up-to-date information from the branch?
-	, cachedFile :: Maybe RawFilePath
-	-- ^ a file recently read from the branch
-	, cachedContent :: L.ByteString
-	-- ^ content of the cachedFile
+	, cachedFileContents :: [(RawFilePath, L.ByteString)]
+	-- ^ contents of a few files recently read from the branch
 	, needInteractiveAccess :: Bool
 	-- ^ do new changes written to the journal or branch by another
 	-- process need to be noticed while the current process is running?
@@ -31,4 +29,4 @@ data BranchState = BranchState
 	}
 
 startBranchState :: BranchState
-startBranchState = BranchState False False False Nothing mempty False
+startBranchState = BranchState False False False [] False