oxenstored: Quota.merge: don't assume domain already exists

In Quota.merge, we merge two quota hashtables, orig_quota and mod_quota, putting
the results into dest_quota. These hashtables map domids to the number of
entries currently owned by that domain.

When mod_quota contains an entry for a domid that was not present in orig_quota
(or dest_quota), the call to get_entry caused Quota.merge to raise a Not_found
exception. This propagates back to the client as an ENOENT error, which is not
an appropriate return value from some operations, such as transaction_end.

This situation can arise when a transaction that introduces a domain (hence
calling Quota.add_entry) needs to be coalesced due to concurrent xenstore
activity.

This patch handles the merge in the case where mod_quota contains an entry not
present in orig_quota (or in dest_quota) by treating that hashtable as having
existing value 0.

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
(cherry picked from commit 82ff7cbed6)
(cherry picked from commit ba391da2e7)

tools/ocaml/xenstored/quota.ml

@@ -83,6 +83,6 @@ let add quota diff =
 	Hashtbl.iter (fun id nb -> set_entry quota id (get_entry quota id + nb)) diff.cur
 
 let merge orig_quota mod_quota dest_quota =
-	  Hashtbl.iter (fun id nb -> let diff = nb - (get_entry orig_quota id) in
+	  Hashtbl.iter (fun id nb -> let diff = nb - (try get_entry orig_quota id with Not_found -> 0) in
 				if diff <> 0 then
-					set_entry dest_quota id ((get_entry dest_quota id) + diff)) mod_quota.cur
+					set_entry dest_quota id ((try get_entry dest_quota id with Not_found -> 0) + diff)) mod_quota.cur