{- management of the git-annex branch
 -
 - Copyright 2011 Joey Hess <joey@kitenet.net>
 -
 - Licensed under the GNU GPL version 3 or higher.
 -}

module Annex.Branch (
	name,
	hasOrigin,
	hasSibling,
	create,
	update,
	get,
	change,
	commit,
	files,
) where

import qualified Data.ByteString.Lazy.Char8 as L

import Common.Annex
import Annex.Exception
import Annex.BranchState
import Annex.Journal
import qualified Git
import qualified Git.Command
import qualified Git.Ref
import qualified Git.Branch
import qualified Git.UnionMerge
import qualified Git.HashObject
import qualified Git.Index
import Annex.CatFile

{- Name of the branch that is used to store git-annex's information. -}
name :: Git.Ref
name = Git.Ref "git-annex"

{- Fully qualified name of the branch. -}
fullname :: Git.Ref
fullname = Git.Ref $ "refs/heads/" ++ show name

{- Branch's name in origin. -}
originname :: Git.Ref
originname = Git.Ref $ "origin/" ++ show name

{- Does origin/git-annex exist? -}
hasOrigin :: Annex Bool
hasOrigin = inRepo $ Git.Ref.exists originname

{- Does the git-annex branch or a sibling foo/git-annex branch exist? -}
hasSibling :: Annex Bool
hasSibling = not . null <$> siblingBranches

{- List of git-annex (refs, branches), including the main one and any
 - from remotes. Duplicate refs are filtered out. -}
siblingBranches :: Annex [(Git.Ref, Git.Branch)]
siblingBranches = inRepo $ Git.Ref.matching name

{- Creates the branch, if it does not already exist. -}
create :: Annex ()
create = do
	_ <- getBranch
	return ()

{- Returns the ref of the branch, creating it first if necessary. -}
getBranch :: Annex (Git.Ref)
getBranch = maybe (hasOrigin >>= go >>= use) (return) =<< branchsha
	where
		go True = do
			inRepo $ Git.Command.run "branch"
				[Param $ show name, Param $ show originname]
			fromMaybe (error $ "failed to create " ++ show name)
				<$> branchsha
		go False = withIndex' True $ do
			inRepo $ Git.Branch.commit "branch created" fullname []
		use sha = do
			setIndexSha sha
			return sha
		branchsha = inRepo $ Git.Ref.sha fullname

{- Ensures that the branch and index are is up-to-date; should be
 - called before data is read from it. Runs only once per git-annex run.
 -
 - Before refs are merged into the index, it's important to first stage the
 - journal into the index. Otherwise, any changes in the journal would
 - later get staged, and might overwrite changes made during the merge.
 -
 - (It would be cleaner to handle the merge by updating the journal, not the
 - index, with changes from the branches.)
 -
 - The branch is fast-forwarded if possible, otherwise a merge commit is
 - made.
 -}
update :: Annex ()
update = runUpdateOnce $ do
	-- ensure branch exists, and get its current ref
	branchref <- getBranch
	-- check what needs updating before taking the lock
	dirty <- journalDirty
	(refs, branches) <- unzip <$> newerSiblings
	if (not dirty && null refs)
		then updateIndex branchref
		else withIndex $ lockJournal $ do
			when dirty stageJournal
			let merge_desc = if null branches
				then "update" 
				else "merging " ++
					unwords (map Git.Ref.describe branches) ++ 
					" into " ++ show name
			unless (null branches) $ do
				showSideAction merge_desc
				mergeIndex branches
			ff <- if dirty
				then return False
				else inRepo $ Git.Branch.fastForward fullname refs
			if ff
				then updateIndex branchref
				else commitBranch branchref merge_desc
					(nub $ fullname:refs)
			invalidateCache
	where
		newerSiblings = filterM isnewer =<< siblingBranches
		isnewer (_, b) = inRepo $ Git.Branch.changed fullname b

{- Gets the content of a file on the branch, or content from the journal, or
 - staged in the index.
 -
 - Returns an empty string if the file doesn't exist yet. -}
get :: FilePath -> Annex String
get = get' False

{- Like get, but does not merge the branch, so the info returned may not
 - reflect changes in remotes. (Changing the value this returns, and then
 - merging is always the same as using get, and then changing its value.) -}
getStale :: FilePath -> Annex String
getStale = get' True

get' :: Bool -> FilePath -> Annex String
get' staleok file = fromcache =<< getCache file
	where
		fromcache (Just content) = return content
		fromcache Nothing = fromjournal =<< getJournalFile file
		fromjournal (Just content) = cache content
		fromjournal Nothing
			| staleok = withIndex frombranch
			| otherwise = withIndexUpdate $ frombranch >>= cache
		frombranch = L.unpack <$> catFile fullname file
		cache content = do
			setCache file content
			return content

{- Applies a function to modifiy the content of a 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 :: FilePath -> (String -> String) -> Annex ()
change file a = lockJournal $ getStale file >>= return . a >>= set file

{- Records new content of a file into the journal and cache. -}
set :: FilePath -> String -> Annex ()
set file content = do
	setJournalFile file content
	setCache file content

{- Stages the journal, and commits staged changes to the branch. -}
commit :: String -> Annex ()
commit message = whenM journalDirty $ lockJournal $ do
	stageJournal
	ref <- getBranch
	withIndex $ commitBranch ref message [fullname]

{- Commits the staged changes in the index to the branch.
 - 
 - Ensures that the branch's index file is first updated to the state
 - of the brannch at branchref, before running the commit action. This
 - is needed because the branch may have had changes pushed to it, that
 - are not yet reflected in the index.
 -
 - Also safely handles a race that can occur if a change is being pushed
 - into the branch at the same time. When the race happens, the commit will
 - be made on top of the newly pushed change, but without the index file
 - being updated to include it. The result is that the newly pushed
 - change is reverted. This race is detected and another commit made
 - to fix it.
 - 
 - The branchref value can have been obtained using getBranch at any
 - previous point, though getting it a long time ago makes the race
 - more likely to occur.
 -}
commitBranch :: Git.Ref -> String -> [Git.Ref] -> Annex ()
commitBranch branchref message parents = do
	updateIndex branchref
	committedref <- inRepo $ Git.Branch.commit message fullname parents
	setIndexSha committedref
	parentrefs <- commitparents <$> catObject committedref
	when (racedetected branchref parentrefs) $
		fixrace committedref parentrefs
	where
		-- look for "parent ref" lines and return the refs
		commitparents = map (Git.Ref . snd) . filter isparent .
			map (toassoc . L.unpack) . L.lines
		toassoc = separate (== ' ')
		isparent (k,_) = k == "parent"
		
 		{- The race can be detected by checking the commit's
		 - parent, which will be the newly pushed branch,
		 - instead of the expected ref that the index was updated to. -}
		racedetected expectedref parentrefs
			| expectedref `elem` parentrefs = False -- good parent
			| otherwise = True -- race!
		
		{- To recover from the race, union merge the lost refs
		 - into the index, and recommit on top of the bad commit. -}
		fixrace committedref lostrefs = do
			mergeIndex lostrefs
			commitBranch committedref racemessage [committedref]
		
		racemessage = message ++ " (recovery from race)"

{- Lists all files on the branch. There may be duplicates in the list. -}
files :: Annex [FilePath]
files = withIndexUpdate $ do
	bfiles <- inRepo $ Git.Command.pipeNullSplit
		[Params "ls-tree --name-only -r -z", Param $ show fullname]
	jfiles <- getJournalledFiles
	return $ jfiles ++ bfiles


{- Populates the branch's index file with the current branch contents.
 - 
 - This is only done when the index doesn't yet exist, and the index 
 - is used to build up changes to be commited to the branch, and merge
 - in changes from other branches.
 -}
genIndex :: Git.Repo -> IO ()
genIndex g = Git.UnionMerge.stream_update_index g
	[Git.UnionMerge.ls_tree fullname g]

{- Merges the specified branches into the index.
 - Any changes staged in the index will be preserved. -}
mergeIndex :: [Git.Ref] -> Annex ()
mergeIndex branches = do
	h <- catFileHandle
	inRepo $ \g -> Git.UnionMerge.merge_index h g branches

{- Runs an action using the branch's index file. -}
withIndex :: Annex a -> Annex a
withIndex = withIndex' False
withIndex' :: Bool -> Annex a -> Annex a
withIndex' bootstrapping a = do
	f <- fromRepo gitAnnexIndex
	bracketIO (Git.Index.override f) id $ do
		unlessM (liftIO $ doesFileExist f) $ do
			unless bootstrapping create
			liftIO $ createDirectoryIfMissing True $ takeDirectory f
			unless bootstrapping $ inRepo genIndex
		a

{- Runs an action using the branch's index file, first making sure that
 - the branch and index are up-to-date. -}
withIndexUpdate :: Annex a -> Annex a
withIndexUpdate a = update >> withIndex a

{- Updates the branch's index to reflect the current contents of the branch.
 - Any changes staged in the index will be preserved.
 -
 - Compares the ref stored in the lock file with the current
 - ref of the branch to see if an update is needed.
 -}
updateIndex :: Git.Ref -> Annex ()
updateIndex branchref = do
	lock <- fromRepo gitAnnexIndexLock
	lockref <- Git.Ref . firstLine <$>
		liftIO (catchDefaultIO (readFileStrict lock) "")
	when (lockref /= branchref) $ do
		withIndex $ mergeIndex [fullname]
		setIndexSha branchref

{- Record that the branch's index has been updated to correspond to a
 - given ref of the branch. -}
setIndexSha :: Git.Ref -> Annex ()
setIndexSha ref = do
        lock <- fromRepo gitAnnexIndexLock
	liftIO $ writeFile lock $ show ref ++ "\n"

{- Stages the journal into the index. -}
stageJournal :: Annex ()
stageJournal = do
	fs <- getJournalFiles
	g <- gitRepo
	withIndex $ liftIO $ do
		let dir = gitAnnexJournalDir g
		let paths = map (dir </>) fs
		(shas, cleanup) <- Git.HashObject.hashFiles paths g
		Git.UnionMerge.update_index g $
			index_lines shas (map fileJournal fs)
		cleanup
		mapM_ removeFile paths
	where
		index_lines shas = map genline . zip shas
		genline (sha, file) = Git.UnionMerge.update_index_line sha file