fix dynamic_config: don't store defaults to cache

Changelog entry (fixes):

* Dynamic config variables, for which the @ref dynamic_config_defaults "default value" is currently used, are no longer stored into the cache file. This fixes an interaction where the service would ignore a changed default value for a config variable.

---

Let's look at a setup where the config service only sends configs which were explicitly overridden compared to the defaults.

If

1. defaults change (e.g. by modifying `dynamic_config_fallback.json`),
2. the service restarts, and
3. there is an old config cache file

then the cache will contain all configs, including the configs that the config service did not send, for which the defaults were used.

The behavior will be as if the old defaults were somehow stuck, which is erroneous.
commit_hash:68b4c2aa0ebca6d663cf636b7b4249b64f764cda

Author: Anton3

core/functional_tests/dynamic_configs/tests/conftest.py

@@ -5,6 +5,15 @@
 pytest_plugins = ['pytest_userver.plugins.core']
 
 
+@pytest.fixture(name='_userver_config_dumps', scope='session')
+def _userver_config_dumps_fixture(_userver_config_dumps):
+    def patch_config(config_yaml, config_vars) -> None:
+        _userver_config_dumps(config_yaml, config_vars)
+        config_vars['userver-dumps-periodic'] = True
+
+    return patch_config
+
+
 # For test_fixtures.py
 @pytest.fixture(scope='session')
 def dynamic_config_fallback_patch() -> dict[str, Any]:

core/functional_tests/dynamic_configs/tests/test_cache_updates.py

@@ -1,4 +1,13 @@
-async def test_inc_update(service_client, testpoint):
+import json
+import pathlib
+
+
+def _load_dynamic_config_cache(service_config):
+    cache_path = pathlib.Path(service_config['components_manager']['components']['dynamic-config']['fs-cache-path'])
+    return json.loads(cache_path.read_text())
+
+
+async def test_inc_update(service_client, service_config, testpoint):
     @testpoint('reset-cache-dynamic-config-client-updater')
     def tp_reset(data):
         pass
@@ -15,3 +24,6 @@ def tp_reset(data):
 
     assert tp_reset.times_called == 1
     assert tp_reset.next_call() == {'data': {'update_type': 'full'}}
+
+    cache = _load_dynamic_config_cache(service_config)
+    assert cache == {'HTTP_CLIENT_CONNECTION_POOL_SIZE': 777}

core/src/dynamic_config/storage/component.cpp

@@ -221,9 +221,9 @@ void DynamicConfig::Impl::DoSetConfig(const dynamic_config::DocsMap& value) {
 
 void DynamicConfig::Impl::SetConfig(std::string_view updater, dynamic_config::DocsMap&& value) {
     LOG_DEBUG() << "Setting new dynamic config value from '" << updater << "'";
+    WriteFsCache(value);
     value.MergeMissing(fallback_config_);
     DoSetConfig(value);
-    WriteFsCache(value);
 }
 
 ComponentHealth DynamicConfig::Impl::GetComponentHealth() const {
@@ -327,7 +327,7 @@ void DynamicConfig::Impl::WriteFsCache(const dynamic_config::DocsMap& docs_map)
         fs::CreateDirectories(fs_task_processor_, boost::filesystem::path(fs_cache_path_).parent_path().string());
         fs::RewriteFileContentsAtomically(fs_task_processor_, fs_cache_path_, contents, mode);
 
-        LOG_INFO() << "Successfully wrote dynamic_config from FS cache";
+        LOG_INFO() << "Successfully wrote dynamic_config to FS cache";
     } catch (const std::exception& e) {
         LOG_ERROR() << "Failed to save config to FS cache '" << fs_cache_path_ << "': " << e;
     }