fsck --from remote -J concurrency bug

fsck --from remote: Fix a concurrency bug that could make it incorrectly
detect that content in the remote is corrupt, and remove it, resulting in
data loss.

CHANGELOG

@@ -1,5 +1,8 @@
 git-annex (7.20200205) UNRELEASED; urgency=medium
 
+  * fsck --from remote: Fix a concurrency bug that could make it incorrectly
+    detect that content in the remote is corrupt, and remove it, resulting in
+    data loss.
   * When git-annex is built with a ssh that does not support ssh connection
     caching, default annex.sshcaching to false, but let the user override it.
   * Improve warning messages further when ssh connection caching cannot

Command/Fsck.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2010-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2020 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -161,6 +161,11 @@ performRemote key afile backend numcopies remote =
 		]
 	ai = mkActionItem (key, afile)
 	withtmp a = do
+		-- Put it in the gitAnnexTmpObjectDir since that's on a
+		-- filesystem where object temp files are normally
+		-- stored. The pid prevents multiple fsck processes
+		-- contending over the same file. (Multiple threads cannot,
+		-- because OnlyActionOn is used.)
 		pid <- liftIO getPID
 		t <- fromRepo gitAnnexTmpObjectDir
 		createAnnexDirectory t
@@ -541,7 +546,7 @@ badContentRemote remote localcopy key = do
 
 runFsck :: Incremental -> ActionItem -> Key -> Annex Bool -> CommandStart
 runFsck inc ai key a = stopUnless (needFsck inc key) $
-	starting "fsck" ai $ do
+	starting "fsck" (OnlyActionOn key ai) $ do
 		ok <- a
 		when ok $
 			recordFsckTime inc key

doc/bugs/Parallel_fsck_on_files_with_same_content_in_bup_remote_can_fail.mdwn

@@ -145,3 +145,5 @@ whereis file2 (1 copy)
   	5d9b0df2-000b-4273-bc4a-fb3b9d8319bd -- testrepo [here]
 ok
 """]]
+
+> [[fixed|done]] --[[Joey]]

doc/bugs/Parallel_fsck_on_files_with_same_content_in_bup_remote_can_fail/comment_1_8cc0d742cd59046b038879f4823c9639._comment

@@ -0,0 +1,11 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2020-02-14T18:49:07Z"
+ content="""
+Ugh, I think this could potentially result in data loss. Not when using bup, 
+but other special remotes.
+
+I've fixed it in git and will think about moving the date of the next
+release up.
+"""]]