fix git-annex repair false positive

Avoid treating refs/annex/last-index or other refs that are not commit
objects as evidence of repository corruption.

The repair code checks to find bad refs by trying to run `git log` on
them, and assumes that no output means something is broken.  But git log
on a tree object is empty.

This was worth fixing generally, not as a special case, since it's certainly
possible that other things store tree or other objects in refs.

Sponsored-by: Max Thoursie on Patreon

CHANGELOG

@@ -14,6 +14,8 @@ git-annex (10.20220323) UNRELEASED; urgency=medium
   * rsync 3.2.4 broke backwards-compatability by preventing exposing
     filenames to the shell. Made the rsync and gcrypt special remotes
     detect this and disable shellescape. Closes: #1010397
+  * repair: Avoid treating refs/annex/last-index or other refs that
+    are not commit objects as evidence of repository corruption.
 
  -- Joey Hess <id@joeyh.name>  Mon, 28 Mar 2022 14:46:10 -0400
 

Git/Repair.hs

@@ -325,7 +325,11 @@ findUncorruptedCommit missing goodcommits branch r = do
  - the commit. Also adds to a set of commit shas that have been verified to
  - be good, which can be passed into subsequent calls to avoid
  - redundant work when eg, chasing down branches to find the first
- - uncorrupted commit. -}
+ - uncorrupted commit.
+ -
+ - When the sha is not a commit but some other git object, returns
+ - true, but does not add it to the set.
+ -}
 verifyCommit :: MissingObjects -> GoodCommits -> Sha -> Repo -> IO (Bool, GoodCommits)
 verifyCommit missing goodcommits commit r
 	| checkGoodCommit commit goodcommits = return (True, goodcommits)
@@ -337,16 +341,23 @@ verifyCommit missing goodcommits commit r
 			, Param (fromRef commit)
 			] r
 		let committrees = map (parse . decodeBL) ls
-		if any isNothing committrees || null committrees
-			then do
-				void cleanup
-				return (False, goodcommits)
-			else do
-				let cts = catMaybes committrees
-				ifM (cleanup <&&> check cts)
-					( return (True, addGoodCommits (map fst cts) goodcommits)
-					, return (False, goodcommits)
-					)
+		-- git log on an object that is not a commit will
+		-- succeed without any output
+		if null committrees
+			then ifM cleanup
+				( return (True, goodcommits)
+				, return (False, goodcommits)
+				)
+			else if any isNothing committrees
+				then do
+					void cleanup
+					return (False, goodcommits)
+				else do
+					let cts = catMaybes committrees
+					ifM (cleanup <&&> check cts)
+						( return (True, addGoodCommits (map fst cts) goodcommits)
+						, return (False, goodcommits)
+						)
   where
 	parse l = case words l of
 		(commitsha:treesha:[]) -> (,)

doc/bugs/git_annex_repair_failed__58___missing_objects.mdwn

@@ -45,4 +45,4 @@ The bug is very easy to reproduce. This happens in a bigger repository of mine,
 
 Using it daily, with small-sized repositories.
 
-
+> [[fixed|done]] --[[Joey]] 

doc/bugs/git_annex_repair_failed__58___missing_objects/comment_3_317bee33fe675a5abb6fe65540a33d25._comment

@@ -0,0 +1,14 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 3"""
+ date="2022-05-04T15:06:31Z"
+ content="""
+Thank you, your "Reproducible Example" worked for me to reproduce it!
+Great work finding this and tracking down an easy way to reproduce it.
+
+So the root of the problem is refs/annex/last-index,
+which contains a tree object, not a commit object. That is unusual for a ref,
+but git-annex has a good reason to record a tree's ref there. 
+
+Fixed
+"""]]