include borg archive name in tree, use empty ContentIdentifier

It's unusual to use a ContentIdentifier that is not semi-unique
for different contents. Note that in importKeys, it checks if a content
identifier is one that's known before, to avoid downloading the same
content twice. But that's done in a code path not used for borg repos,
because they are thirdpartypopulated.

Remote/Borg.hs

@@ -26,10 +26,14 @@ import Text.Read
 import Control.Exception (evaluate)
 import Control.DeepSeq
 import qualified Data.Map as M
+import qualified Data.ByteString as S
 import qualified Data.ByteString.Lazy as L
+import qualified System.FilePath.ByteString as P
 
 type BorgRepo = String
 
+type BorgArchiveName = S.ByteString
+
 remote :: RemoteType
 remote = RemoteType
 	{ typename = "borg"
@@ -123,15 +127,8 @@ borgLocal = notElem ':'
 
 -- TODO avoid rescanning archives that have already been scanned
 --
--- XXX importableHistory should probably not be populated. git-annex
--- only stores and uses the most recent imported tree, not the whole history,
--- I think. So a key that's only in a previous archive would not have
--- a known ImportLocation when retrieving it.
--- Instead, maybe need to include the archive names at the top of the
--- importlocation? (Then would not need them in the ContentIdentifier.)
---
 -- XXX the tree generated by using this does not seem to get grafted into
--- the git-annex branch, so would be subject to being lost to GC>
+-- the git-annex branch, so would be subject to being lost to GC.
 -- Is this a general problem affecting importtree too?
 listImportableContentsM :: BorgRepo -> Annex (Maybe (ImportableContents (ContentIdentifier, ByteSize)))
 listImportableContentsM borgrepo = prompt $ do
@@ -140,7 +137,7 @@ listImportableContentsM borgrepo = prompt $ do
 			let archive' = borgrepo ++ "::" ++ decodeBS' archive
 			in withborglist archive' "{size}{NUL}{path}{NUL}" $
 				liftIO . evaluate . force . parsefilelist archive
-	return (Just (mkimportablecontents (reverse ls)))
+	return (Just (mkimportablecontents ls))
   where
 	withborglist what format a = do
 		let p = (proc "borg" ["list", what, "--format", format])
@@ -158,26 +155,49 @@ listImportableContentsM borgrepo = prompt $ do
 	parsefilelist archive (bsz:f:rest) = case readMaybe (fromRawFilePath bsz) of
 		Nothing -> parsefilelist archive rest
 		Just sz ->
-			let loc = ThirdPartyPopulated.mkThirdPartyImportLocation f
+			let loc = genImportLocation archive f
 			-- This does a little unncessary work to parse the 
 			-- key, which is then thrown away. But, it lets the
 			-- file list be shrank down to only the ones that are
 			-- importable keys, so avoids needing to buffer all
 			-- the rest of the files in memory.
 			in case ThirdPartyPopulated.importKey' loc sz of
-				Just k -> (loc, (ContentIdentifier archive, sz))
+				Just k -> (loc, (borgContentIdentifier, sz))
 					: parsefilelist archive rest
 				Nothing -> parsefilelist archive rest
 	parsefilelist _ _ = []
 
-	mkimportablecontents [] = ImportableContents
+	-- importableHistory is not used for retrieval, so is not
-		{ importableContents = []
+	-- populated with old archives. Instead, a tree of archives
+	-- is constructed, by genImportLocation including the archive
+	-- name in the ImportLocation.
+	mkimportablecontents (l) = ImportableContents
+		{ importableContents = concat l
 		, importableHistory = []
 		}
-	mkimportablecontents (v:vs) = ImportableContents
+	
-		{ importableContents = v
+-- Borg does not allow / in the name of an archive, so the archive
-		, importableHistory = [mkimportablecontents vs]
+-- name will always be the first directory in the ImportLocation.
-		}
+--
+-- Paths in a borg archive are always relative, not absolute, so the use of
+-- </> to combine the archive name with the path will always work.
+genImportLocation :: BorgArchiveName -> RawFilePath -> ImportLocation
+genImportLocation archivename p  = 
+	ThirdPartyPopulated.mkThirdPartyImportLocation $
+		archivename P.</> p
+
+extractImportLocation :: ImportLocation -> (BorgArchiveName, RawFilePath)
+extractImportLocation loc = go $ P.splitDirectories $
+	ThirdPartyPopulated.fromThirdPartyImportLocation loc
+  where
+	go (archivename:rest) = (archivename, P.joinPath rest)
+	go _ = giveup $ "Unable to parse import location " ++ fromRawFilePath (fromImportLocation loc)
+
+-- We do not need a ContentIdentifier in order to retrieve a file from
+-- borg; the ImportLocation contains all that's needed. So, this is left
+-- empty.
+borgContentIdentifier :: ContentIdentifier
+borgContentIdentifier = ContentIdentifier mempty
 
 retrieveExportWithContentIdentifierM :: BorgRepo -> ImportLocation -> ContentIdentifier -> FilePath -> Annex Key -> MeterUpdate -> Annex Key
 retrieveExportWithContentIdentifierM borgrepo loc cid dest k p = error "TODO"