db::extensions: Allow extensions to modify (system) schemas

Allows extensions/config listeners to potentially augument
(system) schemas at boot time. This is only useful for schemas
who do not pass through system_schema tables.

db/extensions.cc

@@ -23,6 +23,7 @@
 #include "extensions.hh"
 #include "sstables/sstables.hh"
 #include "commitlog/commitlog_extensions.hh"
+#include "schema.hh"
 
 db::extensions::extensions()
 {}
@@ -36,3 +37,7 @@ void db::extensions::add_sstable_file_io_extension(sstring n, sstable_file_io_ex
 void db::extensions::add_commitlog_file_extension(sstring n, commitlog_file_extension_ptr f) {
     _commitlog_file_extensions[n] = std::move(f);
 }
+
+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);
+}

db/extensions.hh

@@ -53,6 +53,7 @@
 #include "stdx.hh"
 #include "bytes.hh"
 #include "cql3/statements/property_definitions.hh"
+#include "schema.hh"
 
 class schema_extension;
 
@@ -118,6 +119,14 @@ public:
      * Init time method to add sstable extension
      */
     void add_commitlog_file_extension(sstring n, commitlog_file_extension_ptr);
+
+    /**
+     * Allows forcible modification of schema extensions of a schema. This should
+     * not be done lightly however. In fact, it should only be done on startup
+     * at most, and thus this method is non-const, i.e. you can only use it on
+     * config apply.
+     */
+    void add_extension_to_schema(schema_ptr, const sstring&, shared_ptr<schema_extension>);
 private:
     std::map<sstring, schema_ext_create_func> _schema_extensions;
     std::map<sstring, sstable_file_io_extension> _sstable_file_io_extensions;

db/system_keyspace.hh

@@ -124,6 +124,7 @@ struct range_estimates {
 
 extern schema_ptr hints();
 extern schema_ptr batchlog();
+extern schema_ptr paxos();
 extern schema_ptr built_indexes(); // TODO (from Cassandra): make private
 
 namespace legacy {

schema.hh

@@ -69,6 +69,9 @@ bool check_compound(sstring comparator);
 void read_collections(schema_builder& builder, sstring comparator);
 }
 
+namespace db {
+class extensions;
+}
 // make sure these match the order we like columns back from schema
 enum class column_kind { partition_key, clustering_key, static_column, regular_column };
 
@@ -515,6 +518,11 @@ private:
     column_mapping _column_mapping;
     shared_ptr<query::partition_slice> _full_slice;
 
+    extensions_map& extensions() {
+        return _raw._extensions;
+    }
+
+    friend class db::extensions;
     friend class schema_builder;
 public:
     using row_column_ids_are_ordered_by_name = std::true_type;