improve back-compat of post-receive hook

* init: Improve generated post-receive hook, so it won't fail when
  run on a system whose git-annex is too old to support git-annex post-receive
* init: Update the post-receive hook when re-run in an existing repository.

This commit was sponsored by Jack Hill on Patreon.

Annex/Hook.hs

@@ -4,7 +4,7 @@
  - not change, otherwise removing old hooks using an old version of
  - the script would fail.
  -
- - Copyright 2013-2017 Joey Hess <id@joeyh.name>
+ - Copyright 2013-2018 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -20,16 +20,23 @@ import Utility.Shell
 import qualified Data.Map as M
 
 preCommitHook :: Git.Hook
-preCommitHook = Git.Hook "pre-commit" (mkHookScript "git annex pre-commit .")
+preCommitHook = Git.Hook "pre-commit" (mkHookScript "git annex pre-commit .") []
 
 postReceiveHook :: Git.Hook
-postReceiveHook = Git.Hook "post-receive" (mkHookScript "git annex post-receive")
+postReceiveHook = Git.Hook "post-receive"
+	-- Only run git-annex post-receive when git-annex supports it,
+	-- to avoid failing if the repository with this hook is used
+	-- with an older version of git-annex.
+	(mkHookScript "if git annex post-receive --help >/dev/null 2>&1; then git annex post-receive; fi")
+	-- This is an old version of the hook script.
+	[ mkHookScript "git annex post-receive"
+	]
 
 preCommitAnnexHook :: Git.Hook
-preCommitAnnexHook = Git.Hook "pre-commit-annex" ""
+preCommitAnnexHook = Git.Hook "pre-commit-annex" "" []
 
 postUpdateAnnexHook :: Git.Hook
-postUpdateAnnexHook = Git.Hook "post-update-annex" ""
+postUpdateAnnexHook = Git.Hook "post-update-annex" "" []
 
 mkHookScript :: String -> String
 mkHookScript s = unlines

CHANGELOG

@@ -3,6 +3,9 @@ git-annex (6.20180914) UNRELEASED; urgency=medium
   [ Joey Hess ]
   * Fixes a reversion in the last release that broke interoperation with
     older versions of git-annex-shell.
+  * init: Improve generated post-receive hook, so it won't fail when
+    run on a system whose git-annex is too old to support git-annex post-receive
+  * init: Update the post-receive hook when re-run in an existing repository.
   * S3: Fix url construction bug when the publicurl has been set to an url
     that does not end with a slash.
   * --debug shows urls accessed by git-annex, like it used to do when 

Git/Hook.hs

@@ -1,6 +1,6 @@
 {- git hooks
  -
- - Copyright 2013-2015 Joey Hess <id@joeyh.name>
+ - Copyright 2013-2018 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -20,6 +20,7 @@ import Utility.FileMode
 data Hook = Hook
 	{ hookName :: FilePath
 	, hookScript :: String
+	, hookOldScripts :: [String]
 	}
 	deriving (Ord)
 
@@ -30,38 +31,47 @@ hookFile :: Hook -> Repo -> FilePath
 hookFile h r = localGitDir r </> "hooks" </> hookName h
 
 {- Writes a hook. Returns False if the hook already exists with a different
- - content. -}
+ - content. Upgrades old scripts. -}
 hookWrite :: Hook -> Repo -> IO Bool
-hookWrite h r = do
-	let f = hookFile h r
-	ifM (doesFileExist f)
-		( expectedContent h r
-		, do
-			viaTmp writeFile f (hookScript h)
-			p <- getPermissions f
-			setPermissions f $ p {executable = True}
-			return True
-		)
+hookWrite h r = ifM (doesFileExist f)
+	( expectedContent h r >>= \case
+		UnexpectedContent -> return False
+		ExpectedContent -> return True
+		OldExpectedContent -> go
+	, go
+	)
+  where
+	f = hookFile h r
+	go = do
+		viaTmp writeFile f (hookScript h)
+		p <- getPermissions f
+		setPermissions f $ p {executable = True}
+		return True
 
 {- Removes a hook. Returns False if the hook contained something else, and
  - could not be removed. -}
 hookUnWrite :: Hook -> Repo -> IO Bool
-hookUnWrite h r = do
-	let f = hookFile h r
-	ifM (doesFileExist f)
-		( ifM (expectedContent h r)
-			( do
-				removeFile f
-				return True
-			, return False
-			)
-		, return True
-		)
+hookUnWrite h r = ifM (doesFileExist f)
+	( expectedContent h r >>= \case
+		UnexpectedContent -> return False
+		_ -> do
+			removeFile f
+			return True
+	, return True
+	)
+  where
+	f = hookFile h r
 
-expectedContent :: Hook -> Repo -> IO Bool
+data ExpectedContent = UnexpectedContent | ExpectedContent | OldExpectedContent
+
+expectedContent :: Hook -> Repo -> IO ExpectedContent
 expectedContent h r = do
 	content <- readFile $ hookFile h r
-	return $ content == hookScript h
+	return $ if content == hookScript h
+		then ExpectedContent
+		else if any (content ==) (hookOldScripts h)
+			then OldExpectedContent
+			else UnexpectedContent
 
 hookExists :: Hook -> Repo -> IO Bool
 hookExists h r = do

doc/bugs/post-receive.mdwn

@@ -65,3 +65,5 @@ tai@trasa:~$
 I did not hit enter, the script just failed on me during tab-complete and exited.
 
 Thanks, I look forward to any response from the community this might get.
+
+> [[fixed|done]] --[[Joey]]

doc/bugs/post-receive/comment_1_80c2251beb6a434b364346abc1b2fc02._comment

@@ -0,0 +1,22 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2018-09-25T18:17:59Z"
+ content="""
+> When I try to use that drive with my raspberry pi, however, there are some new git hooks (or at least one) that the older version of git annex (still the latest available in the Pi's repos, 2016, which is really old :/).
+
+Ok, that sentance no verb. But I'll make a guess what you meant to say..
+The old post-receive hook installed by the new version of git-annex runs
+"git-annex post-receive", which fails on the old version of git-annex.
+
+Yes, it's fine to delete the hook in this situation.
+
+The fist version of git-annex to support that is 6.20170228.
+The latest raspbian release is tracking debian stable AFAICS, which has
+6.20170101, just slightly too old.
+
+I agree this is a backwards compatability problem that should have been avoided.
+I've made `git annex init` generate a better hook script that won't fail
+with an older git-annex version. You can re-run `git annex init` in
+your repository and it will update the hook script.
+"""]]