code to update a git-annex branch

There is no suitable git hook to run code when pulling changes that
might need to be merged into the git-annex branch. The post-merge hook
is only run when changes are merged into HEAD, and it's possible,
and indeed likely that many pulls will only have changes in git-annex,
but not in HEAD, and not trigger it.

So, git-annex will have to take care to update the branch before reading
from it, to make sure it has merged in current info from remotes. Happily,
this can be done quite inexpensively, just a git-show-ref to list
branches, and a minimalized git-log to see if there are unmerged changes
on the branches. To further speed up, it will be done only once per
git-annex run, max.

Annex.hs

@@ -39,6 +39,7 @@ data AnnexState = AnnexState
 	, quiet :: Bool
 	, force :: Bool
 	, fast :: Bool
+	, updated :: Bool
 	, forcebackend :: Maybe String
 	, forcenumcopies :: Maybe Int
 	, defaultkey :: Maybe String
@@ -59,6 +60,7 @@ newState allbackends gitrepo = AnnexState
 	, quiet = False
 	, force = False
 	, fast = False
+	, updated = False
 	, forcebackend = Nothing
 	, forcenumcopies = Nothing
 	, defaultkey = Nothing

Branch.hs

@@ -0,0 +1,50 @@
+{- git-annex branch management
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Branch where
+
+import Control.Monad (unless)
+import Control.Monad.State (liftIO)
+
+import GitUnionMerge
+import GitRepo as Git
+import qualified Annex
+import Utility
+import Types
+import Messages
+
+name :: String
+name = "git-annex"
+
+fullname :: String
+fullname = "refs/heads/" ++ name
+
+{- Ensures that the branch is up-to-date; should be called before
+ - data is read from it. Runs only once per git-annex run. -}
+update :: Annex ()
+update = do
+	updated <- Annex.getState Annex.updated
+	unless updated $ do
+		g <- Annex.gitRepo
+		refs <- liftIO $ Git.pipeRead g [Param "show-ref", Param name]
+		mapM_ updateRef $ map (last . words) (lines refs)
+		Annex.changeState $ \s -> s { Annex.updated = True }
+
+{- Ensures that a given ref has been merged into the local git-annex branch. -}
+updateRef :: String -> Annex ()
+updateRef ref
+	| ref == fullname = return ()
+	| otherwise = do
+		g <- Annex.gitRepo
+		diffs <- liftIO $ Git.pipeRead g [
+			Param "log",
+			Param (name++".."++ref),
+			Params "--oneline -n1"
+			]
+		unless (null diffs) $ do
+			showSideAction "merging " ++ ref ++ " into " ++ name ++ "..."
+			liftIO $ unionMerge g fullname ref fullname