Tahoe: Avoid verifying hash after download, since tahoe does sufficient verification itself

See my comment in the next commit for some details about why
Verified needs a hash with preimage resistance. As far as tahoe goes,
it's fully cryptographically secure.

I think that bup could also return Verified. However, the Retriever
interface does not currenly support that.

CHANGELOG

@@ -22,6 +22,8 @@ git-annex (8.20210128) UNRELEASED; urgency=medium
   * Fix build on openbsd.
     Thanks, James Cook for the patch.
   * Include libkqueue.h file needed to build the assistant on BSDs.
+  * Tahoe: Avoid verifying hash after download, since tahoe does sufficient
+    verification itself.
 
  -- Joey Hess <id@joeyh.name>  Thu, 28 Jan 2021 12:34:32 -0400
 

Remote/BitTorrent.hs

@@ -99,6 +99,9 @@ gen r _ rc gc rs = do
 downloadKey :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Verification
 downloadKey key _file dest p = do
 	get . map (torrentUrlNum . fst . getDownloader) =<< getBitTorrentUrls key
+	-- While bittorrent verifies the hash in the torrent file,
+	-- the torrent file itself is downloaded without verification,
+	-- so the overall download is not verified.
 	return UnVerified
   where
 	get [] = giveup "could not download torrent"

Remote/Tahoe.hs

@@ -147,7 +147,9 @@ store rs hdl k _f _p = sendAnnex k noop $ \src ->
 retrieve :: RemoteStateHandle -> TahoeHandle -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Verification
 retrieve rs hdl k _f d _p = do
 	go =<< getCapability rs k
-	return UnVerified
+	-- Tahoe verifies the content it retrieves using cryptographically
+	-- secure methods.
+	return Verified
   where
 	go Nothing = giveup "tahoe capability is not known"
 	go (Just cap) = unlessM (liftIO $ requestTahoe hdl "get" [Param cap, File d]) $

Types/Remote.hs

@@ -197,7 +197,9 @@ data Verification
 	-- ok, so if verification is disabled, don't verify it
 	| Verified
 	-- ^ Content was verified during transfer, so don't verify it
-	-- again.
+	-- again. The verification does not need to use a
+	-- cryptographically secure hash, but the hash does need to
+	-- have preimage resistance.
 	| MustVerify
 	-- ^ Content likely to have been altered during transfer,
 	-- verify even if verification is normally disabled