Fix a P2P protocol hang

When readContent got Nothing from prepSendAnnex, it did not run its
callback, and the callback is what sends the DATA reply.

sendContent checks with contentSize that the object file is present, but
that doesn't really guarantee that prepSendAnnex won't return Nothing.

So, it was possible for a P2P protocol GET to not receive a response,
and appear to hang. When what it's really doing is waiting for the next
protocol command.

This seems most likely to happen when the annex is in direct mode, and the
file being requested has been modified. It could also happen in an indirect
mode repository if genInodeCache somehow failed. Perhaps due to a race
with a drop of the content file.

Fixed by making readContent behave the way its spec said it should,
and run the callback with L.empty in this case.

Note that, it's finee for readContent to send any amount of data
to the callback, including L.empty. sendBytes deals with that
by making sure it sends exactly the specified number of bytes,
aborting the protocol if it's too short. So, when L.empty is sent,
the protocol will end up aborting.

This work is supported by the NIH-funded NICEMAN (ReproNim TR&D3) project.

CHANGELOG

@@ -1,6 +1,7 @@
 git-annex (7.20181032) UNRELEASED; urgency=medium
 
   * Increase minimum QuickCheck version.
+  * Fix a P2P protocol hang.
 
  -- Joey Hess <id@joeyh.name>  Wed, 31 Oct 2018 15:53:03 -0400
 

P2P/Annex.hs

@@ -50,18 +50,19 @@ runLocal runst runner a = case a of
 		size <- inAnnex' isJust Nothing getsize k
 		runner (next (Len <$> size))
 	ReadContent k af o sender next -> do
+		let proceed c = do
+			r <- tryNonAsync c
+			case r of
+				Left e -> return $ Left $ ProtoFailureException e
+				Right (Left e) -> return $ Left e
+				Right (Right ok) -> runner (next ok)
 		v <- tryNonAsync $ prepSendAnnex k
 		case v of
-			Right (Just (f, checkchanged)) -> do
-				v' <- tryNonAsync $
-					transfer upload k af $
-						sinkfile f o checkchanged sender
-				case v' of
-					Left e -> return $ Left $ ProtoFailureException e
-					Right (Left e) -> return $ Left e
-					Right (Right ok) -> runner (next ok)
-			-- content not available
- 			Right Nothing -> runner (next False)
+			Right (Just (f, checkchanged)) -> proceed $
+				transfer upload k af $
+					sinkfile f o checkchanged sender
+ 			Right Nothing -> proceed $
+				runner (sender mempty (return Invalid))
 			Left e -> return $ Left $ ProtoFailureException e
 	StoreContent k af o l getb validitycheck next -> do
 		-- This is the same as the retrievalSecurityPolicy of

P2P/Protocol.hs

@@ -238,8 +238,9 @@ data LocalF c
 	-- present.
 	| ReadContent Key AssociatedFile Offset (L.ByteString -> Proto Validity -> Proto Bool) (Bool -> c)
 	-- ^ Reads the content of a key and sends it to the callback.
-	-- If the content is not available, sends L.empty to the callback.
-	-- Note that the content may change while it's being sent.
+	-- May send any amount of data, including L.empty if the content is
+	-- not available. The callback must deal with that.
+	-- And the content may change while it's being sent.
 	-- The callback is passed a validity check that it can run after
 	-- sending the content to detect when this happened.
 	| StoreContent Key AssociatedFile Offset Len (Proto L.ByteString) (Proto (Maybe Validity)) (Bool -> c)