remove empty tmp workdir on failure

No point in keeping an empty tmp workdir around.

The associated tmp object file is retained even if empty, didn't want to
deal with any possible races with something else downloading to that
file at the same time this would check if it's empty. Anyhow, temp
object files are normally retained, and this will get cleaned out the
same way those do, by dropunused.

Annex/Content.hs

@@ -1060,8 +1060,12 @@ pruneTmpWorkDirBefore f action = do
 {- Runs an action, passing it a temporary work directory where
  - it can write files while receiving the content of a key.
  -
+ - Preserves the invariant that the workdir never exists without the
+ - content file, by creating an empty content file first.
+ -
  - On exception, or when the action returns Nothing,
- - the temporary work directory is left, so resumes can use it.
+ - the temporary work directory is retained (unless
+ - empty), so anything in it can be used on resume.
  -}
 withTmpWorkDir :: Key -> (FilePath -> Annex (Maybe a)) -> Annex (Maybe a)
 withTmpWorkDir key action = do
@@ -1078,7 +1082,7 @@ withTmpWorkDir key action = do
 	res <- action tmpdir
 	case res of
 		Just _ -> liftIO $ removeDirectoryRecursive tmpdir
-		Nothing -> noop
+		Nothing -> liftIO $ void $ tryIO $ removeDirectory tmpdir
 	return res
 
 {- Finds items in the first, smaller list, that are not