fix a race

Another process may stage journalled files before the lock is
taken, so need to get the list of journalled files afterwards.

It's unfortunate this means getting the directory contents twice,
but it seems better to do that than sometimes take the lock
unnecessarily.

Annex/Branch.hs

@@ -111,33 +111,32 @@ create = unlessM hasBranch $ do
 
 {- Stages the journal, and commits staged changes to the branch. -}
 commit :: String -> Annex ()
-commit message = do
-	fs <- getJournalFiles
-	when (not $ null fs) $ lockJournal $ do
-		stageJournalFiles fs
-		g <- gitRepo
-		withIndex $ liftIO $ Git.commit g message fullname [fullname]
+commit message = whenM journalDirty $ lockJournal $ do
+	stageJournalFiles
+	g <- gitRepo
+	withIndex $ liftIO $ Git.commit g message fullname [fullname]
 
 {- Ensures that the branch is up-to-date; should be called before
- - data is read from it. Runs only once per git-annex run. -}
+ - data is read from it. Runs only once per git-annex run.
+ -
+ - Before refs are merged into the index, it's
+ - important to first stage the journal into the
+ - index. Otherwise, any changes in the journal
+ - would later get staged, and might overwrite
+ - changes made during the merge.
+ -
+ - It would be cleaner to handle the merge by
+ - updating the journal, not the index, with changes
+ - from the branches.
+ -}
 update :: Annex ()
 update = onceonly $ do
 	-- check what needs updating before taking the lock
-	fs <- getJournalFiles
+	dirty <- journalDirty
 	c <- filterM changedbranch =<< siblingBranches
 	let (refs, branches) = unzip c
-	unless (null fs && null refs) $ withIndex $ lockJournal $ do
-		{- Before refs are merged into the index, it's
-		 - important to first stage the journal into the
-		 - index. Otherwise, any changes in the journal
-		 - would later get staged, and might overwrite
-		 - changes made during the merge.
-		 -
-		 - It would be cleaner to handle the merge by
-		 - updating the journal, not the index, with changes
-		 - from the branches.
-		 -}
-		unless (null fs) $ stageJournalFiles fs
+	unless (not dirty && null refs) $ withIndex $ lockJournal $ do
+		when dirty $ stageJournalFiles
 		g <- gitRepo
 		unless (null branches) $ do
 			showSideAction $ "merging " ++
@@ -279,8 +278,9 @@ getJournalFiles = do
 	return $ filter (`notElem` [".", ".."]) fs
 
 {- Stages the specified journalfiles. -}
-stageJournalFiles :: [FilePath] -> Annex ()
-stageJournalFiles fs = do
+stageJournalFiles :: Annex ()
+stageJournalFiles = do
+	fs <- getJournalFiles
 	g <- gitRepo
 	withIndex $ liftIO $ do
 		let dir = gitAnnexJournalDir g
@@ -305,6 +305,10 @@ stageJournalFiles fs = do
 		index_lines shas = map genline . zip shas
 		genline (sha, file) = Git.UnionMerge.update_index_line sha file
 
+{- Checks if there are changes in the journal. -}
+journalDirty :: Annex Bool
+journalDirty = not . null <$> getJournalFiles
+
 {- Produces a filename to use in the journal for a file on the branch.
  -
  - The journal typically won't have a lot of files in it, so the hashing