rename from annexobjects location on export

(When possible, of course it may not be there, or it may get renamed from
there for another exported file first. Or the remote may not support
renames.)

This will avoids redundant uploads.

An example case where this is important: Proxying to a exporttree remote,
a file is uploaded to it but is not yet in an exported tree. When the
exported tree is pushed, the remote needs to be updated by exporting to
it. In this case, the proxy doesn't have a copy of the file, so it would
need to download it from annexobjects before uploading it to the final
location. With this optimisation, it can just rename it.

However: If a key is used twice in an exported tree, it seems a proxy
will need to download and reupload anyway. Unless a copy operation is
added to exporttree remotes..

Command/Export.hs

@@ -294,9 +294,8 @@ startExport r db cvar allfilledvar ti = do
 
 performExport :: Remote -> ExportHandle -> Key -> AssociatedFile -> Sha -> ExportLocation -> MVar AllFilled -> CommandPerform
 performExport r db ek af contentsha loc allfilledvar = do
-	let storer = storeExport (exportActions r)
 	sent <- tryNonAsync $ if not (isGitShaKey ek)
-		then ifM (inAnnex ek)
+		then tryrenameannexobject $ ifM (inAnnex ek)
 			( notifyTransfer Upload af $
 				-- alwaysUpload because the same key
 				-- could be used for more than one export
@@ -328,6 +327,24 @@ performExport r db ek af contentsha loc allfilledvar = do
 		Left err -> do
 			failedsend
 			throwM err
+  where
+	storer = storeExport (exportActions r)
+
+	tryrenameannexobject fallback
+		| annexObjects (Remote.config r) = do
+			case renameExport (exportActions r) of
+				Just renameaction -> do
+					locs <- loggedLocations ek
+					gc <- Annex.getGitConfig
+	  				let objloc = exportAnnexObjectLocation gc ek
+					if Remote.uuid r `elem` locs
+						then tryNonAsync (renameaction ek objloc loc) >>= \case
+							Right (Just ()) -> return True
+							Left _err -> fallback
+							Right Nothing -> fallback
+						else fallback
+				Nothing -> fallback
+		| otherwise = fallback
 
 cleanupExport :: Remote -> ExportHandle -> Key -> ExportLocation -> Bool -> CommandCleanup
 cleanupExport r db ek loc sent = do

doc/todo/git-annex_proxies.mdwn

@@ -33,21 +33,6 @@ Planned schedule of work:
 * Working on `exportreeplus` branch which is groundwork for proxying to
   exporttree=yes special remotes.
 
-* `git-annex export` when exporting a file should rename it from the
-  annexobjects location when it's present there. This will avoid redundant
-  uploads.
-
-  An example case where this is important: Proxying to a exporttree remote,
-  a file is uploaded to it but is not yet in an exported tree. When the
-  exported tree is pushed, the remote needs to be updated by exporting to
-  it. In this case, the proxy doesn't have a copy of the file, so it would
-  need to download it from annexobjects before uploading it to the final
-  location. With this optimisation, it can just rename it.
-
-  However: If a key is used twice in an exported tree, it seems a proxy
-  will need to download and reupload anyway. Unless a copy operation is
-  added to exporttree remotes..
-
 ## items deferred until later for p2p protocol over http
 
 * `git-annex p2phttp` should support serving several repositories at the same