avoid sync/assistant drop from appendonly

Make git-annex sync and the assistant skip trying to drop from appendonly
remotes since it's just going to fail.

git-annex drop and similar commands will still try to drop from
appendonly, so the user will see failure messages when they try to do
that. To do otherwise would be confusing since the user has explicitly
asked for a drop with those commands.

This commit was supported by the NSF-funded DataLad project.

Annex/Drop.hs

@@ -11,7 +11,7 @@ import Annex.Common
 import qualified Annex
 import Logs.Trust
 import Annex.NumCopies
-import Types.Remote (uuid)
+import Types.Remote (uuid, appendonly)
 import qualified Remote
 import qualified Command.Drop
 import Command
@@ -29,6 +29,9 @@ type Reason = String
 {- Drop a key from local and/or remote when allowed by the preferred content
  - and numcopies settings.
  -
+ - Skips trying to drop from remotes that are appendonly, since those drops
+ - would presumably fail.
+ -
  - The UUIDs are ones where the content is believed to be present.
  - The Remote list can include other remotes that do not have the content;
  - only ones that match the UUIDs will be dropped from.
@@ -58,9 +61,10 @@ handleDropsFrom locs rs reason fromhere key afile preverified runner = do
 		AssociatedFile (Just f) -> nub (f : l)
 		AssociatedFile Nothing -> l
 	n <- getcopies fs
+	let rs' = filter (not . appendonly) rs
 	void $ if fromhere && checkcopies n Nothing
-		then go fs rs n >>= dropl fs
+		then go fs rs' n >>= dropl fs
-		else go fs rs n
+		else go fs rs' n
   where
 	getcopies fs = do
 		(untrusted, have) <- trustPartition UnTrusted locs

Command/Export.hs

@@ -285,10 +285,10 @@ cleanupUnexport r ea db eks loc = do
 			removeExportedLocation db (asKey ek) loc
 		flushDbQueue db
 
-	-- A readonly remote can support removeExportLocation to remove
+	-- An appendonly remote can support removeExportLocation to remove
 	-- the file from the exported tree, but still retain the content
 	-- and allow retrieving it.
-	unless (Remote.readonly) $ do
+	unless (appendonly r) $ do
 		remaininglocs <- liftIO $ 
 			concat <$> forM eks (\ek -> getExportedLocation db (asKey ek))
 		when (null remaininglocs) $