diffdriver: New git-annex command, to make git external diff drivers work with annexed files.

Closes https://github.com/datalad/datalad/issues/18

Annex/Link.hs

@@ -35,13 +35,17 @@ isAnnexLink file = maybe Nothing (fileKey . takeFileName) <$> getAnnexLinkTarget
  - content.
  -}
 getAnnexLinkTarget :: FilePath -> Annex (Maybe LinkTarget)
-getAnnexLinkTarget file = ifM (coreSymlinks <$> Annex.getGitConfig)
+getAnnexLinkTarget f = getAnnexLinkTarget' f
-	( check readSymbolicLink $
+	=<< (coreSymlinks <$> Annex.getGitConfig)
+
+{- Pass False to force looking inside file. -}
+getAnnexLinkTarget' :: FilePath -> Bool -> Annex (Maybe LinkTarget)
+getAnnexLinkTarget' file coresymlinks = if coresymlinks
+	then check readSymbolicLink $
 		return Nothing
-	, check readSymbolicLink $
+	else check readSymbolicLink $
 		check probefilecontent $
 			return Nothing
-	)
   where
 	check getlinktarget fallback = do
 		v <- liftIO $ catchMaybeIO $ getlinktarget file

CmdLine/GitAnnex.hs

@@ -84,6 +84,7 @@ import qualified Command.Indirect
 import qualified Command.Upgrade
 import qualified Command.Forget
 import qualified Command.Proxy
+import qualified Command.DiffDriver
 import qualified Command.Undo
 import qualified Command.Version
 import qualified Command.Help
@@ -178,6 +179,7 @@ cmds = concat
 	, Command.Upgrade.cmd
 	, Command.Forget.cmd
 	, Command.Proxy.cmd
+	, Command.DiffDriver.cmd
 	, Command.Undo.cmd
 	, Command.Version.cmd
 	, Command.Help.cmd

Command/DiffDriver.hs

@@ -0,0 +1,102 @@
+{- git-annex command
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Command.DiffDriver where
+
+import Common.Annex
+import Command
+import Annex.Content
+import Annex.Link
+import Git.Types
+
+cmd :: [Command]
+cmd = [dontCheck repoExists $
+	command "diffdriver" ("[-- cmd --opts]") seek
+		SectionPlumbing "external git diff driver shim"]
+
+seek :: CommandSeek
+seek = withWords start
+
+start :: [String] -> CommandStart
+start opts = do
+	let (req, differ) = parseReq opts
+	void $ liftIO . exitBool =<< liftIO . differ =<< fixupReq req
+	stop
+
+data Req 
+	= Req
+		{ rPath :: FilePath
+		, rOldFile :: FilePath
+		, rOldHex :: String
+		, rOldMode :: String
+		, rNewFile :: FilePath
+		, rNewHex :: String
+		, rNewMode ::String
+		}
+	| UnmergedReq
+		{ rPath :: FilePath
+		}
+
+type Differ = Req -> IO Bool
+
+serializeReq :: Req -> [CommandParam]
+serializeReq req@(UnmergedReq {}) = [Param $ rPath req]
+serializeReq req@(Req {}) = map Param
+	[ rPath req
+	, rOldFile req
+	, rOldHex req
+	, rOldMode req
+	, rNewFile req
+	, rNewHex req
+	, rNewMode req
+	]
+
+parseReq :: [String] -> (Req, Differ)
+parseReq opts = case separate (== "--") opts of
+	(c:ps, l) -> (mk l, externalDiffer c ps)
+	([],_) -> badopts
+  where
+	mk (path:old_file:old_hex:old_mode:new_file:new_hex:new_mode:[]) =
+		Req
+			{ rPath = path
+			, rOldFile = old_file
+			, rOldHex = old_hex
+			, rOldMode = old_mode
+			, rNewFile = new_file
+			, rNewHex = new_hex
+			, rNewMode = new_mode
+			}
+	mk (unmergedpath:[]) = UnmergedReq { rPath = unmergedpath }
+	mk _ = badopts
+
+	badopts = error $ "Unexpected input: " ++ unwords opts
+
+{- Check if either file is a symlink to a git-annex object,
+ - which git-diff will leave as a normal file containing the link text.
+ - Adjust the Req to instead point to the actual location of the annexed
+ - object (which may or may not exist). -}
+fixupReq :: Req -> Annex Req
+fixupReq req@(UnmergedReq {}) = return req
+fixupReq req@(Req {}) = 
+	check rOldFile rOldMode (\r f -> r { rOldFile = f }) req
+		>>= check rNewFile rNewMode (\r f -> r { rNewFile = f })
+  where
+	check getfile getmode setfile r = case readBlobType (getmode r) of
+		Just SymlinkBlob -> do
+			v <- getAnnexLinkTarget' (getfile r) False
+			case fileKey . takeFileName =<< v of
+				Nothing -> return r
+				Just k -> setfile r <$>
+					withObjectLoc k
+						-- indirect mode
+						return 
+						-- direct mode
+						(return . Prelude.head)
+		_ -> return r
+
+externalDiffer :: String -> [String] -> Differ
+externalDiffer c ps = \req -> boolSystem c (map Param ps ++ serializeReq req )

debian/changelog

@@ -13,6 +13,8 @@ git-annex (5.20141126) UNRELEASED; urgency=medium
   * Debian package is now maintained by Gergely Nagy.
   * Windows: Remove Alt+A keyboard shortcut, which turns out to have scope
     outside the menus.
+  * diffdriver: New git-annex command, to make git external diff drivers
+    work with annexed files.
 
  -- Joey Hess <joeyh@debian.org>  Mon, 10 Nov 2014 15:31:55 -0400
 

doc/git-annex.mdwn

@@ -989,6 +989,21 @@ subdirectories).
   Merge conflicts between two files that are not annexed will not be
   automatically resolved.
 
+* `diffdriver`
+
+  This is an external git diff driver shim. Normally, when using `git diff`
+  with an external git driver, the symlinks to annexed files are not set up
+  right, so the external git driver cannot read them in order to perform
+  smart diffing of their contents. This command works around the problem,
+  by passing the fixed up files to the real external diff driver.
+
+  To use, just configure git to use "git-annex diffdriver -- cmd params --"
+  as the external diff command, where cmd is the real external diff
+  command you want to use, and params are any extra parameters to pass
+  to it. Note the trailing "--", which is required.
+
+  For example, set `GIT_EXTERNAL_DIFF=git-annex diffdriver -- j-c-diff --`
+
 * `remotedaemon`
 
   Detects when network remotes have received git pushes and fetches from them.

doc/todo/wishlist:_git_annex_diff.mdwn

@@ -7,3 +7,7 @@ Should be relatively simple, only there would have to be a way to deal with the
 Of course you wouldn't want to diff two large files, but with git annex assistant, all files are annexed by default (right?), so this would be useful.
 
 There might already be a way to easily diff two versions of an annexed file which I'm missing -- in that case please point me to it! :)
+
+> [[done]]; rather than adding a `git annex diff`, I made git-annex be able to be used as a git diff driver command,
+> which in turn can run some third-party external diff driver that does
+> some smart handling of binary files.