SERVER-50367 Add IDL support for feature flags

6 files changed, 155 insertions, 0 deletions

diff --git a/buildscripts/idl/idl/binder.py b/buildscripts/idl/idl/binder.py
index 4d9bd3ff79c..8e01fe83b02 100644
--- a/buildscripts/idl/idl/binder.py
+++ b/buildscripts/idl/idl/binder.py
@@ -1005,6 +1005,22 @@ def _bind_server_parameter(ctxt, param):
         return None
 
 
+def _bind_feature_flags(param):
+    # type: (syntax.FeatureFlag) -> ast.ServerParameter
+    """Bind a FeatureFlag as a serverParameter setting."""
+    ast_param = ast.ServerParameter(param.file_name, param.line, param.column)
+    ast_param.name = param.name
+    ast_param.description = param.description
+
+    ast_param.set_at = "ServerParameterType::kStartupOnly"
+
+    ast_param.cpp_vartype = "bool"
+    ast_param.default = _bind_expression(param.default)
+    ast_param.cpp_varname = param.cpp_varname
+
+    return ast_param
+
+
 def _is_invalid_config_short_name(name):
     # type: (str) -> bool
     """Check if a given name is valid as a short name."""
@@ -1168,6 +1184,9 @@ def bind(parsed_spec):
         if not struct.imported:
             bound_spec.structs.append(_bind_struct(ctxt, parsed_spec, struct))
 
+    for feature_flag in parsed_spec.feature_flags:
+        bound_spec.server_parameters.append(_bind_feature_flags(feature_flag))
+
     for server_parameter in parsed_spec.server_parameters:
         bound_spec.server_parameters.append(_bind_server_parameter(ctxt, server_parameter))
 
diff --git a/buildscripts/idl/idl/parser.py b/buildscripts/idl/idl/parser.py
index 7a3a1e376d2..08f76bf0273 100644
--- a/buildscripts/idl/idl/parser.py
+++ b/buildscripts/idl/idl/parser.py
@@ -608,6 +608,25 @@ def _parse_server_parameter(ctxt, spec, name, node):
     spec.server_parameters.append(param)
 
 
+def _parse_feature_flag(ctxt, spec, name, node):
+    # type: (errors.ParserContext, syntax.IDLSpec, str, Union[yaml.nodes.MappingNode, yaml.nodes.ScalarNode, yaml.nodes.SequenceNode]) -> None
+    """Parse a feature_flags section in the IDL file."""
+    if not ctxt.is_mapping_node(node, "feature_flags"):
+        return
+
+    param = syntax.FeatureFlag(ctxt.file_name, node.start_mark.line, node.start_mark.column)
+    param.name = name
+
+    _generic_parser(
+        ctxt, node, "feature_flags", param, {
+            "description": _RuleDesc('scalar', _RuleDesc.REQUIRED),
+            "cpp_varname": _RuleDesc('scalar'),
+            "default": _RuleDesc('scalar_or_mapping', mapping_parser_func=_parse_expression),
+        })
+
+    spec.feature_flags.append(param)
+
+
 def _parse_config_option(ctxt, spec, name, node):
     # type: (errors.ParserContext, syntax.IDLSpec, str, Union[yaml.nodes.MappingNode, yaml.nodes.ScalarNode, yaml.nodes.SequenceNode]) -> None
     """Parse a configs section in the IDL file."""
@@ -727,6 +746,8 @@ def _parse(stream, error_file_name):
             _parse_mapping(ctxt, spec, second_node, "server_parameters", _parse_server_parameter)
         elif first_name == "configs":
             _parse_mapping(ctxt, spec, second_node, "configs", _parse_config_option)
+        elif first_name == "feature_flags":
+            _parse_mapping(ctxt, spec, second_node, "feature_flags", _parse_feature_flag)
         else:
             ctxt.add_unknown_root_node_error(first_node)
 
diff --git a/buildscripts/idl/idl/syntax.py b/buildscripts/idl/idl/syntax.py
index 3c9bb7bedf1..9b3530a7db7 100644
--- a/buildscripts/idl/idl/syntax.py
+++ b/buildscripts/idl/idl/syntax.py
@@ -67,6 +67,7 @@ class IDLSpec(object):
         self.imports = None  # type: Optional[Import]
         self.server_parameters = []  # type: List[ServerParameter]
         self.configs = []  # type: List[ConfigOption]
+        self.feature_flags = []  # type: List[FeatureFlag]
 
 
 def parse_array_type(name):
@@ -527,6 +528,22 @@ class ServerParameter(common.SourceLocation):
         super(ServerParameter, self).__init__(file_name, line, column)
 
 
+class FeatureFlag(common.SourceLocation):
+    """IDL FeatureFlag information."""
+
+    # pylint: disable=too-many-instance-attributes
+
+    def __init__(self, file_name, line, column):
+        # type: (str, int, int) -> None
+        """Construct a FeatureFlag."""
+        self.name = None  # type: str
+        self.description = None  # type: str
+        self.cpp_varname = None  # type: str
+        self.default = None  # type: Expression
+
+        super(FeatureFlag, self).__init__(file_name, line, column)
+
+
 class GlobalInitializer(common.SourceLocation):
     """Initializer details for custom registration/storage."""
 
diff --git a/src/mongo/idl/SConscript b/src/mongo/idl/SConscript
index 28bcb2724b5..dd13816c007 100644
--- a/src/mongo/idl/SConscript
+++ b/src/mongo/idl/SConscript
@@ -32,11 +32,13 @@ env.CppUnitTest(
     target='idl_test',
     source=[
         'config_option_test.cpp',
+        'feature_flag_test.cpp',
         'idl_test.cpp',
         'server_parameter_specialized_test.cpp',
         'server_parameter_with_storage_test.cpp',
         env.Idlc('config_option_no_init_test.idl')[0],
         env.Idlc('config_option_test.idl')[0],
+        env.Idlc('feature_flag_test.idl')[0],
         env.Idlc('server_parameter_specialized_test.idl')[0],
         env.Idlc('server_parameter_with_storage_test.idl')[0],
         env.Idlc('unittest.idl')[0],
diff --git a/src/mongo/idl/feature_flag_test.cpp b/src/mongo/idl/feature_flag_test.cpp
new file mode 100644
index 00000000000..cb111756272
--- /dev/null
+++ b/src/mongo/idl/feature_flag_test.cpp
@@ -0,0 +1,60 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#include "mongo/platform/basic.h"
+
+#include "mongo/idl/feature_flag_test_gen.h"
+#include "mongo/unittest/unittest.h"
+
+namespace mongo {
+
+namespace {
+
+ServerParameter* getServerParameter(const std::string& name) {
+    const auto& spMap = ServerParameterSet::getGlobal()->getMap();
+    const auto& spIt = spMap.find(name);
+    ASSERT(spIt != spMap.end());
+
+    auto* sp = spIt->second;
+    ASSERT(sp);
+    return sp;
+}
+
+TEST(IDLFeatureFlag, Basic) {
+    // true is set by "default" attribute in the IDL file.
+    ASSERT_EQ(feature_flags::gFeatureFlagToaster, true);
+
+    auto* featureFlagToaster = getServerParameter("featureFlagToaster");
+    ASSERT_OK(featureFlagToaster->setFromString("false"));
+    ASSERT_EQ(feature_flags::gFeatureFlagToaster, false);
+    ASSERT_NOT_OK(featureFlagToaster->setFromString("alpha"));
+}
+
+}  // namespace
+}  // namespace mongo
diff --git a/src/mongo/idl/feature_flag_test.idl b/src/mongo/idl/feature_flag_test.idl
new file mode 100644
index 00000000000..c756ca5a90f
--- /dev/null
+++ b/src/mongo/idl/feature_flag_test.idl
@@ -0,0 +1,36 @@
+#    Copyright (C) 2020-present MongoDB, Inc.
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the Server Side Public License, version 1,
+#    as published by MongoDB, Inc.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the 
+#    Server Side Public License for more details.
+#
+#    You should have received a copy of the Server Side Public License
+#    along with this program. If not, see 
+#    <http://www.mongodb.com/licensing/server-side-public-license>.
+#
+#    As a special exception, the copyright holders give permission to link the 
+#    code of portions of this program with the OpenSSL library under certain
+#    conditions as described in each individual source file and distribute
+#    linked combinations including the program with the OpenSSL library. You 
+#    must comply with the Server Side Public License in all respects for 
+#    all of the code used other than as permitted herein. If you modify file(s)
+#    with this exception, you may extend this exception to your version of the 
+#    file(s), but you are not obligated to do so. If you do not wish to do so, 
+#    delete this exception statement from your version. If you delete this
+#    exception statement from all source files in the program, then also delete
+#    it in the license file.
+#
+
+global:
+  cpp_namespace: "mongo::feature_flags"
+
+feature_flags:
+    featureFlagToaster:
+      description: "Create a feature flag"
+      cpp_varname: gFeatureFlagToaster
+      default: true