unannex: Now skips files whose content is not present, rather than it being an error. This allows gradual conversion from one backend to another by running unannex followed by add in each repository.

Command/Unannex.hs

@@ -29,8 +29,12 @@ seek = [withFilesInGit start]
 {- The unannex subcommand undoes an add. -}
 start :: CommandStartString
 start file = isAnnexed file $ \(key, backend) -> do
-	showStart "unannex" file
+	ishere <- inAnnex key
-	return $ Just $ perform file key backend
+	if ishere
+		then do
+			showStart "unannex" file
+			return $ Just $ perform file key backend
+		else return Nothing
 
 perform :: FilePath -> Key -> Backend -> CommandPerform
 perform file key backend = do

debian/changelog

@@ -1,3 +1,11 @@
+git-annex (0.17) UNRELEASED; urgency=low
+
+  * unannex: Now skips files whose content is not present, rather than
+    it being an error. This allows gradual conversion from one backend
+    to another by running unannex followed by add in each repository.
+
+ -- Joey Hess <joeyh@debian.org>  Sat, 08 Jan 2011 15:04:48 -0400
+
 git-annex (0.16) unstable; urgency=low
 
   * git-annex-shell: Avoid exposing any git repo config except for the

doc/walkthrough.mdwn

@@ -277,6 +277,19 @@ add something like this to `.gitattributes`:
 
 	* annex.backend=SHA1
 
+## migrating between backends
+
+Perhaps you had been using the WORM backend, but now have configured
+git-annex to use SHA1 for new files. Your old files are still in WORM. How
+to migrate that content? A quick and dirty way is to use the unannex
+subcommand, which removes a file from git-annex's control, followed by
+a re-add of the file, to put it in the new backend.
+
+	# git annex unannex my_cool_big_file
+	unannex my_cool_big_file ok
+	# git annex add my_cool_big_file
+	add my_cool_big_file (checksum ...) ok
+
 ## unused data
 
 It's possible for data to accumulate in the annex that no files point to

test.hs

@@ -16,6 +16,7 @@ import Data.List
 import System.IO.Error
 import System.Posix.Env
 import qualified Control.Exception.Extensible as E
+import Control.Exception (throw)
 
 import qualified GitRepo as Git
 import qualified Locations
@@ -87,10 +88,11 @@ test_unannex :: Test
 test_unannex = "git-annex unannex" ~: TestList [nocopy, withcopy]
 	where
 		nocopy = "no content" ~: intmpclonerepo $ do
-			r <- git_annex "unannex" ["-q", annexedfile] 
+			annexed_notpresent annexedfile
-			not r @? "unannex incorrectly succeeded with no copy"
+			git_annex "unannex" ["-q", annexedfile] @? "unannex failed with no copy"
-			unannexed annexedfile
+			annexed_notpresent annexedfile
 		withcopy = "with content" ~: intmpcopyrepo $ do
+			annexed_present annexedfile
 			git_annex "unannex" ["-q", annexedfile] @? "unannex failed"
 			unannexed annexedfile
 			git_annex "unannex" ["-q", annexedfile] @? "unannex failed on non-annexed file"
@@ -280,7 +282,7 @@ indir dir a = do
 		(E.try (a)::IO (Either E.SomeException ()))
 	case r of
 		Right () -> return ()
-		Left e -> error $ show e
+		Left e -> throw e
 
 setuprepo :: FilePath -> IO FilePath
 setuprepo dir = do