check if object is modified before starting to send it

Fix bug that caused some transfers to incorrectly fail with "content
changed while it was being sent", when the content was not changed.

While I don't know how to reproduce the problem that several people
reported, it is presumably due to the inode cache somehow being stale.
So check isUnmodified', and if it's not modified, include the file's
current inode cache in the set to accept, when checking for modification
after the transfer.

That seems like the right thing to do for another reason: The failure
says the file changed while it was being sent, but if the object file was
changed before the transfer started, that's wrong. So it needs to check
before allowing the transfer at all if the file is modified.

(Other calls to sameInodeCache or elemInodeCaches, when operating on inode
caches from the database, could also be problimatic if the inode cache is
somehow getting stale. This does not address such problems.)

Sponsored-by: Dartmouth College's Datalad project

Annex/Content/Presence.hs

@@ -1,6 +1,6 @@
 {- git-annex object content presence
  -
- - Copyright 2010-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -15,6 +15,7 @@ module Annex.Content.Presence (
 	objectFileExists,
 	withObjectLoc,
 	isUnmodified,
+	isUnmodified',
 	isUnmodifiedCheap,
 	verifyKeyContent,
 	VerifyConfig(..),
@@ -145,11 +146,17 @@ withObjectLoc key a = a =<< calcRepo (gitAnnexLocation key)
  - The cheaper way is to see if the InodeCache for the key matches the
  - file. -}
 isUnmodified :: Key -> RawFilePath -> Annex Bool
-isUnmodified key f = go =<< geti
+isUnmodified key f = 
+	withTSDelta (liftIO . genInodeCache f) >>= \case
+		Just fc -> do
+			ic <- Database.Keys.getInodeCaches key
+			isUnmodified' key f fc ic
+		Nothing -> return False
+
+isUnmodified' :: Key -> RawFilePath -> InodeCache -> [InodeCache] -> Annex Bool
+isUnmodified' key f fc ic = isUnmodifiedCheap'' fc ic <||> expensivecheck
   where
-	go Nothing = return False
-	go (Just fc) = isUnmodifiedCheap' key fc <||> expensivecheck fc
-	expensivecheck fc = ifM (verifyKeyContent RetrievalAllKeysSecure AlwaysVerify UnVerified key f)
+	expensivecheck = ifM (verifyKeyContent RetrievalAllKeysSecure AlwaysVerify UnVerified key f)
 		( do
 			-- The file could have been modified while it was
 			-- being verified. Detect that.
@@ -177,8 +184,11 @@ isUnmodifiedCheap key f = maybe (return False) (isUnmodifiedCheap' key)
 	=<< withTSDelta (liftIO . genInodeCache f)
 
 isUnmodifiedCheap' :: Key -> InodeCache -> Annex Bool
-isUnmodifiedCheap' key fc = 
-	anyM (compareInodeCaches fc) =<< Database.Keys.getInodeCaches key
+isUnmodifiedCheap' key fc = isUnmodifiedCheap'' fc
+	=<< Database.Keys.getInodeCaches key
+
+isUnmodifiedCheap'' :: InodeCache -> [InodeCache] -> Annex Bool
+isUnmodifiedCheap'' fc ic = anyM (compareInodeCaches fc) ic
 
 {- Verifies that a file is the expected content of a key.
  -