storage_service: get_range_to_address_map prevent use after free

The implementation of get_range_to_address_map has a default behaviour,
when getting an empty keypsace, it uses the first non-system keyspace
(first here is basically, just a keyspace).

The current implementation has two issues, first, it uses a reference to
a string that is held on a stack of another function. In other word,
there's a use after free that is not clear why we never hit.

The second, it calls get_non_system_keyspaces twice. Though this is not
a bug, it's redundant (get_non_system_keyspaces uses a loop, so calling
that function does have a cost).

This patch solves both issues, by chaning the implementation to hold a
string instead of a reference to a string.

Second, it stores the results from get_non_system_keyspaces and reuse
them it's more efficient and holds the returned values on the local
stack.

Fixes scylladb#6465

Signed-off-by: Amnon Heiman <[email protected]>

service/storage_service.cc

@@ -1040,12 +1040,16 @@ storage_service::is_local_dc(const inet_address& targetHost) const {
 std::unordered_map<dht::token_range, std::vector<inet_address>>
 storage_service::get_range_to_address_map(const sstring& keyspace,
         const std::vector<token>& sorted_tokens) const {
+    sstring ks = keyspace;
     // some people just want to get a visual representation of things. Allow null and set it to the first
     // non-system keyspace.
-    if (keyspace == "" && _db.local().get_non_system_keyspaces().empty()) {
-        throw std::runtime_error("No keyspace provided and no non system kespace exist");
+    if (keyspace == "") {
+        auto keyspaces = _db.local().get_non_system_keyspaces();
+        if (keyspaces.empty()) {
+            throw std::runtime_error("No keyspace provided and no non system kespace exist");
+        }
+        ks = keyspaces[0];
     }
-    const sstring& ks = (keyspace == "") ? _db.local().get_non_system_keyspaces()[0] : keyspace;
     return construct_range_to_endpoint_map(ks, get_all_ranges(sorted_tokens));
 }