repeated addurl behavior reversion fix

addurl: When the file youtube-dl will download is already an annexed file,
don't download it again and fail to overwrite it, instead just do nothing,
like it used to when quvi was used.

This commit was sponsored by Anthony DeRobertis on Patreon.

Command/AddUrl.hs

@@ -272,26 +272,42 @@ downloadWeb o url urlinfo file =
 		showDestinationFile file
 		liftIO $ createDirectoryIfMissing True (parentDir file)
 		finishDownloadWith tmp webUUID url file
-	tryyoutubedl tmp = withTmpWorkDir mediakey $ \workdir ->
-		Transfer.notifyTransfer Transfer.Download url $
-			Transfer.download webUUID mediakey (AssociatedFile Nothing) Transfer.noRetry $ \_p ->
-				youtubeDl url workdir >>= \case
-					Right (Just mediafile) -> do
-						pruneTmpWorkDirBefore tmp (liftIO . nukeFile)
-						let dest = if isJust (fileOption o)
-							then file
-							else takeFileName mediafile
-						checkCanAdd dest $ do
-							showDestinationFile dest
-							addWorkTree webUUID mediaurl dest mediakey (Just mediafile)
-							return $ Just mediakey
-					Right Nothing -> normalfinish tmp
-					Left msg -> do
-						warning msg
-						return Nothing
+	tryyoutubedl tmp
+		| isJust (fileOption o) = go file
+		-- Ask youtube-dl what filename it will download
+		-- first, and check if that is already an annexed file,
+		-- to avoid unnecessary work in that case.
+		| otherwise = youtubeDlFileName' url >>= \case
+			Right dest -> ifAnnexed dest 
+				(alreadyannexed dest)
+				(dl dest)
+			Left _ -> normalfinish tmp
 	  where
+		dl dest = withTmpWorkDir mediakey $ \workdir ->
+			Transfer.notifyTransfer Transfer.Download url $
+				Transfer.download webUUID mediakey (AssociatedFile Nothing) Transfer.noRetry $ \_p ->
+					youtubeDl url workdir >>= \case
+						Right (Just mediafile) -> do
+							pruneTmpWorkDirBefore tmp (liftIO . nukeFile)
+							checkCanAdd dest $ do
+								showDestinationFile dest
+								addWorkTree webUUID mediaurl dest mediakey (Just mediafile)
+								return $ Just mediakey
+						Right Nothing -> normalfinish tmp
+						Left msg -> do
+							warning msg
+							return Nothing
 		mediaurl = setDownloader url YoutubeDownloader
 		mediakey = Backend.URL.fromUrl mediaurl Nothing
+		-- Does the already annexed file have the mediaurl
+		-- as an url? If so nothing to do.
+		alreadyannexed dest k = do
+			us <- getUrls k
+			if mediaurl `elem` us
+				then return (Just k)
+				else do
+					warning $ dest ++ " already exists; not overwriting"
+					return Nothing
 
 showDestinationFile :: FilePath -> Annex ()
 showDestinationFile file = do