gossiper: add num_endpoints

Return the number of endpoints tracked by gossiper.
This is useful when the caller doesn't need
access to the endpoint states map.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

alternator/executor.cc

@@ -4469,7 +4469,7 @@ future<executor::request_return_type> executor::describe_continuous_backups(clie
 // manually create the keyspace to override this predefined behavior.
 static future<std::vector<mutation>> create_keyspace(std::string_view keyspace_name, service::storage_proxy& sp, gms::gossiper& gossiper, api::timestamp_type ts) {
     sstring keyspace_name_str(keyspace_name);
-    int endpoint_count = gossiper.get_endpoint_states().size();
+    int endpoint_count = gossiper.num_endpoints();
     int rf = 3;
     if (endpoint_count < rf) {
         rf = 1;

gms/gossiper.hh

@@ -428,6 +428,10 @@ public:
 
     std::vector<inet_address> get_endpoints() const;
 
+    size_t num_endpoints() const noexcept {
+        return _endpoint_state_map.size();
+    }
+
     bool uses_host_id(inet_address endpoint) const;
 
     locator::host_id get_host_id(inet_address endpoint) const;

service/migration_manager.cc

@@ -222,11 +222,11 @@ void migration_manager::schedule_schema_pull(const gms::inet_address& endpoint,
 }
 
 bool migration_manager::have_schema_agreement() {
-    const auto known_endpoints = _gossiper.get_endpoint_states();
-    if (known_endpoints.size() == 1) {
+    if (_gossiper.num_endpoints() == 1) {
         // Us.
         return true;
     }
+    const auto known_endpoints = _gossiper.get_endpoint_states();
     auto our_version = _storage_proxy.get_db().local().get_version();
     bool match = false;
     for (auto& x : known_endpoints) {