start implementing hidden git-annex repositories

This adds a separate journal, which does not currently get committed to
an index, but is planned to be committed to .git/annex/index-private.

Changes that are regarding a UUID that is private will get written to
this journal, and so will not be published into the git-annex branch.

All log writing should have been made to indicate the UUID it's
regarding, though I've not verified this yet.

Currently, no UUIDs are treated as private yet, a way to configure that
is needed.

The implementation is careful to not add any additional IO work when
privateUUIDsKnown is False. It will skip looking at the private journal
at all. So this should be free, or nearly so, unless the feature is
used. When it is used, all branch reads will be about twice as expensive.

It is very lucky -- or very prudent design -- that Annex.Branch.change
and maybeChange are the only ways to change a file on the branch,
and Annex.Branch.set is only internal use. That let Annex.Branch.get
always yield any private information that has been recorded, without
the risk that Annex.Branch.set might be called, with a non-private UUID,
and end up leaking the private information into the git-annex branch.

And, this relies on the way git-annex union merges the git-annex branch.
When reading a file, there can be a public and a private version, and
they are just concacenated together. That will be handled the same as if
there were two diverged git-annex branches that got union merged.

Annex/Journal.hs

@@ -4,7 +4,7 @@
  - git-annex branch. Among other things, it ensures that if git-annex is
  - interrupted, its recorded data is not lost.
  -
- - Copyright 2011-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -39,6 +39,24 @@ instance Journalable Builder where
 	writeJournalHandle = hPutBuilder
 	journalableByteString = toLazyByteString
 
+{- When a file in the git-annex branch is changed, this indicates what
+ - repository UUID (or in some cases, UUIDs) a change is regarding.
+ -
+ - Using this lets changes regarding private UUIDs be written to the
+ - private index, rather than to the main branch index, so it does
+ - not get exposed to other remotes.
+ -}
+data RegardingUUID = RegardingUUID [UUID]
+
+regardingPrivateUUID :: RegardingUUID -> Bool
+regardingPrivateUUID _ = False -- TODO
+
+-- Are any private UUIDs known to exist? If so, extra work has to be done,
+-- to check for information separately recorded for them, outside the usual
+-- locations.
+privateUUIDsKnown :: Bool
+privateUUIDsKnown = False -- TODO
+
 {- Records content for a file in the branch to the journal.
  -
  - Using the journal, rather than immediatly staging content to the index
@@ -48,20 +66,25 @@ instance Journalable Builder where
  - getJournalFileStale to always return a consistent journal file
  - content, although possibly not the most current one.
  -}
-setJournalFile :: Journalable content => JournalLocked -> RawFilePath -> content -> Annex ()
-setJournalFile _jl file content = withOtherTmp $ \tmp -> do
-	createAnnexDirectory =<< fromRepo gitAnnexJournalDir
+setJournalFile :: Journalable content => JournalLocked -> RegardingUUID -> RawFilePath -> content -> Annex ()
+setJournalFile _jl ru file content = withOtherTmp $ \tmp -> do
+	jd <- fromRepo $ if regardingPrivateUUID ru
+		then gitAnnexPrivateJournalDir
+		else gitAnnexJournalDir
+	createAnnexDirectory jd
 	-- journal file is written atomically
-	jfile <- fromRepo (journalFile file)
-	let tmpfile = fromRawFilePath (tmp P.</> P.takeFileName jfile)
+	let jfile = journalFile file
+	let tmpfile = fromRawFilePath (tmp P.</> jfile)
 	liftIO $ do
 		withFile tmpfile WriteMode $ \h -> writeJournalHandle h content
-		moveFile tmpfile (fromRawFilePath jfile)
+		moveFile tmpfile (fromRawFilePath (jd P.</> jfile))
 
 {- Gets any journalled content for a file in the branch. -}
-getJournalFile :: JournalLocked -> RawFilePath -> Annex (Maybe L.ByteString)
+getJournalFile :: JournalLocked -> GetPrivate -> RawFilePath -> Annex (Maybe L.ByteString)
 getJournalFile _jl = getJournalFileStale
 
+data GetPrivate = GetPrivate Bool
+
 {- Without locking, this is not guaranteed to be the most recent
  - version of the file in the journal, so should not be used as a basis for
  - changes.
@@ -73,42 +96,55 @@ getJournalFile _jl = getJournalFileStale
  - concurrency or other issues with a lazy read, and the minor loss of
  - laziness doesn't matter much, as the files are not very large.
  -}
-getJournalFileStale :: RawFilePath -> Annex (Maybe L.ByteString)
-getJournalFileStale file = inRepo $ \g -> catchMaybeIO $
-	L.fromStrict <$> S.readFile (fromRawFilePath $ journalFile file g)
+getJournalFileStale :: GetPrivate -> RawFilePath -> Annex (Maybe L.ByteString)
+getJournalFileStale (GetPrivate getprivate) file = inRepo $ \g -> 
+	if getprivate
+		then do
+			x <- getfrom (gitAnnexJournalDir g)
+			y <- getfrom (gitAnnexPrivateJournalDir g)
+			-- This concacenation is the same as happens in a
+			-- merge of two git-annex branches.
+			return (x <> y)
+		else getfrom (gitAnnexJournalDir g)
+  where
+	jfile = journalFile file
+	getfrom d = catchMaybeIO $
+		L.fromStrict <$> S.readFile (fromRawFilePath (d P.</> jfile))
 
-{- List of existing journal files, but without locking, may miss new ones
- - just being added, or may have false positives if the journal is staged
- - as it is run. -}
-getJournalledFilesStale :: Annex [RawFilePath]
-getJournalledFilesStale = do
+{- List of existing journal files in a journal directory, but without locking,
+ - may miss new ones just being added, or may have false positives if the
+ - journal is staged as it is run. -}
+getJournalledFilesStale :: (Git.Repo -> RawFilePath) -> Annex [RawFilePath]
+getJournalledFilesStale getjournaldir = do
 	g <- gitRepo
 	fs <- liftIO $ catchDefaultIO [] $
-		getDirectoryContents $ fromRawFilePath $ gitAnnexJournalDir g
+		getDirectoryContents $ fromRawFilePath (getjournaldir g)
 	return $ filter (`notElem` [".", ".."]) $
 		map (fileJournal . toRawFilePath) fs
 
-withJournalHandle :: (DirectoryHandle -> IO a) -> Annex a
-withJournalHandle a = do
-	d <- fromRawFilePath <$> fromRepo gitAnnexJournalDir
+{- Directory handle open on a journal directory. -}
+withJournalHandle :: (Git.Repo -> RawFilePath) -> (DirectoryHandle -> IO a) -> Annex a
+withJournalHandle getjournaldir a = do
+	d <- fromRawFilePath <$> fromRepo getjournaldir
 	bracketIO (openDirectory d) closeDirectory (liftIO . a)
 
 {- Checks if there are changes in the journal. -}
-journalDirty :: Annex Bool
-journalDirty = do
-	d <- fromRawFilePath <$> fromRepo gitAnnexJournalDir
+journalDirty :: (Git.Repo -> RawFilePath) -> Annex Bool
+journalDirty getjournaldir = do
+	d <- fromRawFilePath <$> fromRepo getjournaldir
 	liftIO $ 
 		(not <$> isDirectoryEmpty d)
 			`catchIO` (const $ doesDirectoryExist d)
 
 {- Produces a filename to use in the journal for a file on the branch.
+ - The filename does not include the journal directory.
  -
  - The journal typically won't have a lot of files in it, so the hashing
  - used in the branch is not necessary, and all the files are put directly
  - in the journal directory.
  -}
-journalFile :: RawFilePath -> Git.Repo -> RawFilePath
-journalFile file repo = gitAnnexJournalDir repo P.</> S.concatMap mangle file
+journalFile :: RawFilePath -> RawFilePath
+journalFile file = S.concatMap mangle file
   where
 	mangle c
 		| P.isPathSeparator c = S.singleton underscore