Add max_databases setting

`max_databases` allows to limit max number of databases a user
can create. In case the user tries to create a new database and
the current number of databases is equal or larger then `max_databases`
an Error is thrown.

src/Core/Settings.h

@@ -279,6 +279,8 @@ class IColumn;
     M(UInt64, max_concurrent_queries_for_all_users, 0, "The maximum number of concurrent requests for all users.", 0) \
     M(UInt64, max_concurrent_queries_for_user, 0, "The maximum number of concurrent requests per user.", 0) \
     \
+    M(UInt64, max_databases, 0, "The maximum number of created databases.", 0) \
+    \
     M(Bool, insert_deduplicate, true, "For INSERT queries in the replicated table, specifies that deduplication of inserting blocks should be performed", 0) \
     M(Bool, async_insert_deduplicate, false, "For async INSERT queries in the replicated table, specifies that deduplication of inserting blocks should be performed", 0) \
     \

src/Interpreters/InterpreterCreateQuery.cpp

@@ -97,6 +97,7 @@ namespace DB
 namespace ErrorCodes
 {
     extern const int ACCESS_DENIED;
+    extern const int CANNOT_CREATE_DATABASE;
     extern const int TABLE_ALREADY_EXISTS;
     extern const int DICTIONARY_ALREADY_EXISTS;
     extern const int EMPTY_LIST_OF_COLUMNS_PASSED;
@@ -1806,6 +1807,19 @@ BlockIO InterpreterCreateQuery::createReplicatedDatabaseByClient() {
     return {};
 }
 
+void InterpreterCreateQuery::checkMaxDatabases() {
+    if (internal)
+        return;
+    auto max_databases = getContext()->getSettingsRef().max_databases;
+    if (!max_databases)
+        return;
+    auto databases = DatabaseCatalog::instance().getDatabases();
+    auto databases_cnt = databases.size();
+    if (databases_cnt >= max_databases) {
+        throw Exception(ErrorCodes::CANNOT_CREATE_DATABASE, "Cannot create database: the number of databases {} + 1 exceeds the limit of {}", databases_cnt, max_databases);
+    }
+}
+
 void InterpreterCreateQuery::checkDatabaseNameAllowed() {
     auto & create = query_ptr->as<ASTCreateQuery &>();
     if (internal)
@@ -1841,8 +1855,10 @@ BlockIO InterpreterCreateQuery::execute()
     FunctionNameNormalizer().visit(query_ptr.get());
     auto & create = query_ptr->as<ASTCreateQuery &>();
     bool is_create_database = create.database && !create.table;
-    if (is_create_database)
+    if (is_create_database) {
         checkDatabaseNameAllowed();
+        checkMaxDatabases();
+    }
     auto context = getContext();
     auto username = context->getUserName();
     auto user_with_interect_db_creation = context->getServerSettings().getString("user_with_indirect_database_creation");

src/Interpreters/InterpreterCreateQuery.h

@@ -93,6 +93,7 @@ class InterpreterCreateQuery : public IInterpreter, WithMutableContext
     };
 
     BlockIO createReplicatedDatabaseByClient();
+    void checkMaxDatabases();
     void checkDatabaseNameAllowed();
     BlockIO createDatabase(ASTCreateQuery & create);
     BlockIO createTable(ASTCreateQuery & create);