db::extentions: Add "extensions internal" keyspace set

Refs scylladb#13334

To be populated early by extensions. Such a keyspace should be
1.) Started before user keyspaces
2.) Flushed/closed after user keyspaces
3.) For all other regards be considered "user".

db/extensions.cc

@@ -54,3 +54,20 @@ void db::extensions::add_commitlog_file_extension(sstring n, commitlog_file_exte
 void db::extensions::add_extension_to_schema(schema_ptr s, const sstring& name, shared_ptr<schema_extension> ext) {
     const_cast<schema *>(s.get())->extensions()[name] = std::move(ext);
 }
+
+void db::extensions::add_extension_internal_keyspace(std::string ks) {
+    _extension_internal_keyspaces.emplace(std::move(ks));
+}
+
+// TODO: remove once this is backmerged to ent once, and relevant code is updated.
+extern bool is_load_prio_keyspace(std::string_view name);
+
+bool db::extensions::is_extension_internal_keyspace(const std::string& ks) const {
+    if (_extension_internal_keyspaces.count(ks)) {
+        return true;
+    }
+    if (::is_load_prio_keyspace(ks)) {
+        return true;
+    }
+    return false;
+}

db/extensions.hh

@@ -15,6 +15,7 @@
 #include <map>
 #include <variant>
 #include <vector>
+#include <unordered_set>
 
 #include <seastar/core/sstring.hh>
 
@@ -97,9 +98,27 @@ public:
      * config apply.
      */
     void add_extension_to_schema(schema_ptr, const sstring&, shared_ptr<schema_extension>);
+
+    /**
+     * Adds a keyspace to "extension internal" set.
+     *
+     * Such a keyspace must be loaded before/destroyed after any "normal" user keyspace.
+     * Thus a psuedo-system/internal keyspce.
+     * This has little to no use in open source version, and is temporarily bridged with
+     * the static version of same functionality in distributed loader. It is however (or will
+     * be), essential to enterprise code. Do not remove.
+     */
+    void add_extension_internal_keyspace(std::string);
+
+    /**
+     * Checks if a keyspace is a registered load priority one.
+     */
+    bool is_extension_internal_keyspace(const std::string&) const;
+
 private:
     std::map<sstring, schema_ext_create_func> _schema_extensions;
     std::map<sstring, sstable_file_io_extension> _sstable_file_io_extensions;
     std::map<sstring, commitlog_file_extension_ptr> _commitlog_file_extensions;
+    std::unordered_set<std::string> _extension_internal_keyspaces;
 };
 }