don't copy old date metadata when adding new version of a file

When adding a new version of a file, and annex.genmetadata is enabled,
don't copy the data metadata from the old version of the file, instead use
the mtime of the file. Rationalle being that the user has requested to
generate metadata and so would expect to get the new mtime into metadata.

Also, avoid warning about copying metadata when all the old metadata is
date metadata. Which was rather the harder part.

This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.

Annex/MetaData.hs

@@ -22,7 +22,6 @@ import Annex.CatFile
 import Utility.Glob
 
 import qualified Data.Set as S
-import qualified Data.Map as M
 import Data.Time.Calendar
 import Data.Time.Clock
 import Data.Time.Clock.POSIX
@@ -41,28 +40,43 @@ genMetaData :: Key -> FilePath -> FileStatus -> Annex ()
 genMetaData key file status = do
 	catKeyFileHEAD file >>= \case
 		Nothing -> noop
-		Just oldkey -> 
-			whenM (copyMetaData oldkey key)
+		Just oldkey ->
+			-- Have to copy first, before adding any
+			-- more metadata, because copyMetaData does not
+			-- preserve any metadata already on key.
+			whenM (copyMetaData oldkey key <&&> (not <$> onlydatemeta oldkey)) $
 				warncopied
 	whenM (annexGenMetaData <$> Annex.getGitConfig) $ do
-		curr <- getCurrentMetaData key
-		addMetaData key (dateMetaData mtime curr)
+		old <- getCurrentMetaData key
+		addMetaData key (dateMetaData mtime old)
   where
 	mtime = posixSecondsToUTCTime $ realToFrac $ modificationTime status
 	warncopied = warning $ 
 		"Copied metadata from old version of " ++ file ++ " to new version. " ++ 
 		"If you don't want this copied metadata, run: git annex metadata --remove-all " ++ file
+	-- If the only fields copied were date metadata, and they'll
+	-- be overwritten with the current mtime, no need to warn about
+	-- copying.
+	onlydatemeta oldkey = ifM (annexGenMetaData <$> Annex.getGitConfig)
+		( null . filter (not . isDateMetaField . fst) . fromMetaData 
+			<$> getCurrentMetaData oldkey
+		, return False
+		)
 
 {- Generates metadata for a file's date stamp.
- - Does not overwrite any existing metadata values. -}
+ -
+ - Any date fields in the old metadata will be overwritten.
+ - 
+ - Note that the returned MetaData does not contain all the input MetaData,
+ - only changes to add the date fields. -}
 dateMetaData :: UTCTime -> MetaData -> MetaData
-dateMetaData mtime old = MetaData $ M.fromList $ filter isnew
-	[ (yearMetaField, S.singleton $ toMetaValue $ show y)
-	, (monthMetaField, S.singleton $ toMetaValue $ show m)
-	, (dayMetaField, S.singleton $ toMetaValue $ show d)
-	]
+dateMetaData mtime old = modMeta old $
+	(SetMeta yearMetaField $ S.singleton $ toMetaValue $ show y)
+		`ComposeModMeta`
+	(SetMeta monthMetaField $ S.singleton $ toMetaValue $ show m)
+		`ComposeModMeta`
+	(SetMeta dayMetaField $ S.singleton $ toMetaValue $ show d)
   where
-	isnew (f, _) = S.null (currentMetaDataValues f old)
 	(y, m, d) = toGregorian $ utctDay mtime
 
 {- Parses field=value, field+=value, field-=value, field?=value -}