resume properly when copying a file to/from a local git remote is interrupted

Probably this fixes a reversion, but I don't know what version broke it.

This does use withOtherTmp for a temp file that could be quite large.
Though albeit a reflink copy that will not actually take up any space
as long as the file it was copied from still exists. So if the copy cow
succeeds but git-annex is interrupted just before that temp file gets
renamed into the usual .git/annex/tmp/ location, there is a risk that
the other temp directory ends up cluttered with a larger temp file than
later. It will eventually be cleaned up, and the changes of this being
a problem are small, so this seems like an acceptable thing to do.

Sponsored-by: Shae Erisson on Patreon

Annex/CopyFile.hs

@@ -15,6 +15,8 @@ import Utility.CopyFile
 import Utility.FileMode
 import Utility.Touch
 import Utility.Hash (IncrementalVerifier(..))
+import Annex.Tmp
+import Utility.Tmp
 
 import Control.Concurrent
 import qualified Data.ByteString as S
@@ -28,23 +30,34 @@ newCopyCoWTried :: IO CopyCoWTried
 newCopyCoWTried = CopyCoWTried <$> newEmptyMVar
 
 {- Copies a file is copy-on-write is supported. Otherwise, returns False. -}
-tryCopyCoW :: CopyCoWTried -> FilePath -> FilePath -> MeterUpdate -> IO Bool
+tryCopyCoW :: CopyCoWTried -> FilePath -> FilePath -> MeterUpdate -> Annex Bool
 tryCopyCoW (CopyCoWTried copycowtried) src dest meterupdate =
 	-- If multiple threads reach this at the same time, they
 	-- will both try CoW, which is acceptable.
-	ifM (isEmptyMVar copycowtried)
+	ifM (liftIO $ isEmptyMVar copycowtried)
 		( do
 			ok <- docopycow
-			void $ tryPutMVar copycowtried ok
+			void $ liftIO $ tryPutMVar copycowtried ok
 			return ok
-		, ifM (readMVar copycowtried)
+		, ifM (liftIO $ readMVar copycowtried)
 			( docopycow
 			, return False
 			)
 		)
   where
-	docopycow = watchFileSize dest meterupdate $
-		copyCoW CopyTimeStamps src dest
+	-- copyCow needs a destination file that does not exist,
+	-- but the dest file might already. So use it with another
+	-- temp file, and if it succeeds, rename it into place. If it fails,
+	-- the dest file is left as-is, to support resuming.
+	docopycow = withOtherTmp $ \othertmp -> liftIO $
+		withTmpFileIn (fromRawFilePath othertmp) (takeFileName dest) $ \tmpdest _h -> do
+			copied <- watchFileSize tmpdest meterupdate $
+				copyCoW CopyTimeStamps src tmpdest
+			if copied
+				then liftIO $ catchBoolIO $ do
+					rename tmpdest dest
+					return True
+				else return False
 
 data CopyMethod = CopiedCoW | Copied
 
@@ -70,7 +83,7 @@ fileCopier :: CopyCoWTried -> FilePath -> FilePath -> MeterUpdate -> Maybe Incre
 fileCopier _ src dest meterupdate iv = docopy
 #else
 fileCopier copycowtried src dest meterupdate iv =
-	ifM (liftIO $ tryCopyCoW copycowtried src dest meterupdate)
+	ifM (tryCopyCoW copycowtried src dest meterupdate)
 		( do
 			liftIO $ maybe noop unableIncremental iv
 			return CopiedCoW