git's handing of relative GIT_INDEX_FILE is more insane than I thought; always make absolute

This is actually worse than I thought; when git is being run with a
detached work tree, GIT_INDEX_FILE is treated as a path relative to CWD,
instead of the normal behavior of relative the top of the work tree.

This seems to make it basically impossible for any program that wants to
use GIT_INDEX_FILE to use anything other than an absolute path to it; there
are too many configurations to keep straight that can change how git
interprets what should be a simple relative path to a file.

(I have complained to the git developers.)

Annex/GitOverlay.hs

@@ -19,7 +19,7 @@ import qualified Annex
 {- Runs an action using a different git index file. -}
 withIndexFile :: FilePath -> Annex a -> Annex a
 withIndexFile f a = do
-	f' <- inRepo $ indexEnvVal f
+	f' <- liftIO $ indexEnvVal f
 	withAltRepo
 		(\g -> addGitEnv g indexEnv f')
 		(\g g' -> g' { gitEnv = gitEnv g })

Git/Index.hs

@@ -9,19 +9,24 @@ module Git.Index where
 
 import Common
 import Git
-import Git.FilePath
 import Utility.Env
 
 indexEnv :: String
 indexEnv = "GIT_INDEX_FILE"
 
-{- When relative, GIT_INDEX_FILE is interpreted by git as being 
+{- Gets value to set GIT_INDEX_FILE to. Input should be absolute path,
+ - or relative to the CWD.
+ -
+ - When relative, GIT_INDEX_FILE is interpreted by git as being 
  - relative to the top of the work tree of the git repository,
- - not to the CWD. -}
-indexEnvVal :: FilePath -> Repo -> IO String
-indexEnvVal index r
-	| isAbsolute index = return index
-	| otherwise = getTopFilePath <$> toTopFilePath index r
+ - not to the CWD. Worse, other environment variables (GIT_WORK_TREE)
+ - or git options (--work-tree) or configuration (core.worktree)
+ - can change what the relative path is interpreted relative to.
+ -
+ - So, an absolute path is the only safe option for this to return.
+ -}
+indexEnvVal :: FilePath -> IO String
+indexEnvVal = absPath
 
 {- Forces git to use the specified index file.
  -
@@ -31,9 +36,9 @@ indexEnvVal index r
  - Warning: Not thread safe.
  -}
 override :: FilePath -> Repo -> IO (IO ())
-override index r = do
+override index _r = do
 	res <- getEnv var
-	val <- indexEnvVal index r
+	val <- indexEnvVal index
 	setEnv var val True
 	return $ reset res
   where