addurl, importfeed: Honor annex.diskreserve as long as the size of the url can be checked.

This adds a http HEAD before the download is done. That was already the
case when the assistant was running, and it seems worth it to avoid filling
up the whole disk, like happened to my server today.

Annex/Content.hs

@@ -15,6 +15,7 @@ module Annex.Content (
 	getViaTmp,
 	getViaTmpChecked,
 	getViaTmpUnchecked,
+	prepGetViaTmpChecked,
 	withTmp,
 	checkDiskSpace,
 	moveAnnex,
@@ -158,20 +159,31 @@ getViaTmpUnchecked :: Key -> (FilePath -> Annex Bool) -> Annex Bool
 getViaTmpUnchecked = finishGetViaTmp (return True)
 
 getViaTmpChecked :: Annex Bool -> Key -> (FilePath -> Annex Bool) -> Annex Bool
-getViaTmpChecked check key action = do
+getViaTmpChecked check key action = 
+	prepGetViaTmpChecked key $
+		finishGetViaTmp check key action
+
+{- Prepares to download a key via a tmp file, and checks that there is
+ - enough free disk space.
+ -
+ - When the temp file already exists, count the space it is using as
+ - free, since the download will overwrite it or resume.
+ -
+ - Wen there's enough free space, runs the download action.
+ -}
+prepGetViaTmpChecked :: Key -> Annex Bool -> Annex Bool
+prepGetViaTmpChecked key getkey = do
 	tmp <- fromRepo $ gitAnnexTmpLocation key
 
-	-- Check that there is enough free disk space.
-	-- When the temp file already exists, count the space
-	-- it is using as free.
 	e <- liftIO $ doesFileExist tmp
 	alreadythere <- if e
 		then fromIntegral . fileSize <$> liftIO (getFileStatus tmp)
 		else return 0
 	ifM (checkDiskSpace Nothing key alreadythere)
 		( do
+			-- The tmp file may not have been left writable
 			when e $ thawContent tmp
-			finishGetViaTmp check key action
+			getkey
 		, return False
 		)
 
@@ -210,6 +222,7 @@ checkDiskSpace destination key alreadythere = do
 	reserve <- annexDiskReserve <$> Annex.getGitConfig
 	free <- liftIO . getDiskFree =<< dir
 	force <- Annex.getState Annex.force
+	liftIO $ print (free, keySize key)
 	case (free, keySize key) of
 		(Just have, Just need) -> do
 			let ok = (need + reserve <= have + alreadythere) || force