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this journal, and so will not be published into the git-annex branch.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/Branch.hs

@@ -1,6 +1,6 @@
 {- management of the git-annex branch
  -
- - Copyright 2011-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -20,6 +20,7 @@ module Annex.Branch (
 	updateTo,
 	get,
 	getHistorical,
+	RegardingUUID(..),
 	change,
 	maybeChange,
 	commitMessage,
@@ -172,7 +173,7 @@ updateTo' :: [(Git.Sha, Git.Branch)] -> Annex UpdateMade
 updateTo' pairs = do
 	-- ensure branch exists, and get its current ref
 	branchref <- getBranch
-	dirty <- journalDirty
+	dirty <- journalDirty gitAnnexJournalDir
 	ignoredrefs <- getIgnoredRefs
 	let unignoredrefs = excludeset ignoredrefs pairs
 	tomerge <- if null unignoredrefs
@@ -265,9 +266,12 @@ get file = getCache file >>= \case
  - (Changing the value this returns, and then merging is always the
  - same as using get, and then changing its value.) -}
 getLocal :: RawFilePath -> Annex L.ByteString
-getLocal file = do
+getLocal = getLocal' (GetPrivate True)
+
+getLocal' :: GetPrivate -> RawFilePath -> Annex L.ByteString
+getLocal' getprivate file = do
 	fastDebug "Annex.Branch" ("read " ++ fromRawFilePath file)
-	go =<< getJournalFileStale file
+	go =<< getJournalFileStale getprivate file
   where
 	go (Just journalcontent) = return journalcontent
 	go Nothing = getRef fullname file
@@ -297,24 +301,36 @@ getRef ref file = withIndex $ catFile ref file
  - Note that this does not cause the branch to be merged, it only
  - modifes the current content of the file on the branch.
  -}
-change :: Journalable content => RawFilePath -> (L.ByteString -> content) -> Annex ()
-change file f = lockJournal $ \jl -> f <$> getLocal file >>= set jl file
+change :: Journalable content => RegardingUUID -> RawFilePath -> (L.ByteString -> content) -> Annex ()
+change ru file f = lockJournal $ \jl -> f <$> getToChange ru file >>= set jl ru file
 
 {- Applies a function which can modify the content of a file, or not. -}
-maybeChange :: Journalable content => RawFilePath -> (L.ByteString -> Maybe content) -> Annex ()
-maybeChange file f = lockJournal $ \jl -> do
-	v <- getLocal file
+maybeChange :: Journalable content => RegardingUUID -> RawFilePath -> (L.ByteString -> Maybe content) -> Annex ()
+maybeChange ru file f = lockJournal $ \jl -> do
+	v <- getToChange ru file
 	case f v of
 		Just jv ->
 			let b = journalableByteString jv
-			in when (v /= b) $ set jl file b
+			in when (v /= b) $ set jl ru file b
 		_ -> noop
 
-{- Records new content of a file into the journal -}
-set :: Journalable content => JournalLocked -> RawFilePath -> content -> Annex ()
-set jl f c = do
+{- Only get private information when the RegardingUUID is itself private. -}
+getToChange :: RegardingUUID -> RawFilePath -> Annex L.ByteString
+getToChange = getLocal' . GetPrivate . regardingPrivateUUID
+
+{- Records new content of a file into the journal.
+ -
+ - This is not exported; all changes have to be made via change. This
+ - ensures that information that was written to the branch is not
+ - overwritten. Also, it avoids a get followed by a set without taking into
+ - account whether private information was gotten from the private
+ - git-annex index, and should not be written to the public git-annex
+ - branch.
+ -}
+set :: Journalable content => JournalLocked -> RegardingUUID -> RawFilePath -> content -> Annex ()
+set jl ru f c = do
 	journalChanged
-	setJournalFile jl f c
+	setJournalFile jl ru f c
 	fastDebug "Annex.Branch" ("set " ++ fromRawFilePath f)
 	-- Could cache the new content, but it would involve
 	-- evaluating a Journalable Builder twice, which is not very
@@ -329,7 +345,7 @@ commitMessage = fromMaybe "update" . annexCommitMessage <$> Annex.getGitConfig
 
 {- Stages the journal, and commits staged changes to the branch. -}
 commit :: String -> Annex ()
-commit = whenM journalDirty . forceCommit
+commit = whenM (journalDirty gitAnnexJournalDir) . forceCommit
 
 {- Commits the current index to the branch even without any journalled
  - changes. -}
@@ -407,11 +423,14 @@ files :: Annex ([RawFilePath], IO Bool)
 files = do
 	_  <- update
 	(bfs, cleanup) <- branchFiles
-	-- ++ forces the content of the first list to be buffered in memory,
-	-- so use getJournalledFilesStale which should be much smaller most
-	-- of the time. branchFiles will stream as the list is consumed.
-	l <- (++)
-		<$> getJournalledFilesStale
+	-- ++ forces the content of all but the last list to be buffered in
+	-- memory, so use getJournalledFilesStale which should be much smaller
+	-- most of the time. branchFiles will stream as the list is consumed.
+	l <- (\a b c -> a ++ b ++ c)
+		<$> (if privateUUIDsKnown 
+			then getJournalledFilesStale gitAnnexPrivateJournalDir
+			else pure [])
+		<*> (getJournalledFilesStale gitAnnexJournalDir)
 		<*> pure bfs
 	return (l, cleanup)
 
@@ -520,7 +539,7 @@ stageJournal jl commitindex = withIndex $ withOtherTmp $ \tmpdir -> do
 	let dir = gitAnnexJournalDir g
 	(jlogf, jlogh) <- openjlog (fromRawFilePath tmpdir)
 	h <- hashObjectHandle
-	withJournalHandle $ \jh ->
+	withJournalHandle gitAnnexJournalDir $ \jh ->
 		Git.UpdateIndex.streamUpdateIndex g
 			[genstream dir h jh jlogh]
 	commitindex