raft: Fix use-after-free error in raft_store_snapshot().

raft_store_snapshot() constructs a new snapshot in a local variable then
destroys the current snapshot and replaces it by the new one.  Until now,
it has not cloned the data in the new snapshot until it did the
replacement.  This led to the unexpected consequence that, if 'servers' in
the old and new snapshots was the same, then it would first be freed and
later cloned, which could cause a segfault.

Multiple people reported the crash.  Gurucharan Shetty provided a
reproduction case.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Mark Michelson <mmichels@redhat.com>

ovsdb/raft.c

@@ -3838,22 +3838,22 @@ raft_store_snapshot(struct raft *raft, const struct json *new_snapshot_data)
     }
 
     uint64_t new_log_start = raft->last_applied + 1;
-    const struct raft_entry new_snapshot = {
+    struct raft_entry new_snapshot = {
         .term = raft_get_term(raft, new_log_start - 1),
-        .data = CONST_CAST(struct json *, new_snapshot_data),
+        .data = json_clone(new_snapshot_data),
         .eid = *raft_get_eid(raft, new_log_start - 1),
-        .servers = CONST_CAST(struct json *,
-                              raft_servers_for_index(raft, new_log_start - 1)),
+        .servers = json_clone(raft_servers_for_index(raft, new_log_start - 1)),
     };
     struct ovsdb_error *error = raft_save_snapshot(raft, new_log_start,
                                                    &new_snapshot);
     if (error) {
+        raft_entry_uninit(&new_snapshot);
         return error;
     }
 
     raft->log_synced = raft->log_end - 1;
     raft_entry_uninit(&raft->snap);
-    raft_entry_clone(&raft->snap, &new_snapshot);
+    raft->snap = new_snapshot;
     for (size_t i = 0; i < new_log_start - raft->log_start; i++) {
         raft_entry_uninit(&raft->entries[i]);
     }