avoid reposize database locking overhead when not needed

Only when the preferred content expression being matched uses balanced
preferred content is this overhead needed.

It might be possible to eliminate the locking entirely. Eg, check the
live changes before and after the action and re-run if they are not
stable. For now, this is good enough, it avoids existing preferred
content getting slow. If balanced preferred content turns out to be too
slow to check, that could be tried later.

Annex/FileMatcher.hs

@@ -90,7 +90,7 @@ checkMatcher matcher mkey afile lu notpresent notconfigured d
 
 checkMatcher' :: FileMatcher Annex -> MatchInfo -> LiveUpdate -> AssumeNotPresent -> Annex Bool
 checkMatcher' (matcher, (MatcherDesc matcherdesc)) mi lu notpresent =
-	checkLiveUpdate lu go
+	checkLiveUpdate lu matcher go
   where
 	go = do
 		(matches, desc) <- runWriterT $ matchMrun' matcher $ \op ->
@@ -281,6 +281,7 @@ call desc (Right sub) = Right $ Operation $ MatchFiles
 	, matchNeedsFileContent = any matchNeedsFileContent sub
 	, matchNeedsKey = any matchNeedsKey sub
 	, matchNeedsLocationLog = any matchNeedsLocationLog sub
+	, matchNeedsLiveRepoSize = any matchNeedsLiveRepoSize sub
 	, matchDesc = matchDescSimple desc
 	}
 call _ (Left err) = Left err

Annex/RepoSize/LiveUpdate.hs

@@ -13,6 +13,8 @@ import Annex.Common
 import Logs.Presence.Pure
 import qualified Database.RepoSize as Db
 import Annex.UUID
+import Types.FileMatcher
+import qualified Utility.Matcher as Matcher
 
 import Control.Concurrent
 import System.Process
@@ -95,9 +97,16 @@ needLiveUpdate lu = liftIO $ void $ tryPutMVar (liveUpdateNeeded lu) ()
 -- This serializes calls to the action, so that if the action
 -- queries getLiveRepoSizes it will not race with another such action
 -- that may also be starting a live update.
-checkLiveUpdate :: LiveUpdate -> Annex Bool -> Annex Bool
-checkLiveUpdate NoLiveUpdate a = a
-checkLiveUpdate lu a = Db.lockDbWhile (const go) go
+checkLiveUpdate
+	:: LiveUpdate
+	-> Matcher.Matcher (MatchFiles Annex)
+	-> Annex Bool
+	-> Annex Bool
+checkLiveUpdate NoLiveUpdate _ a = a
+checkLiveUpdate lu matcher a
+	| Matcher.introspect matchNeedsLiveRepoSize matcher =
+		Db.lockDbWhile (const go) go
+	| otherwise = a
   where
 	go = do
 		r <- a