MM28858 Basic framework for use of feature flags. Enviroment variable overrides. (#15544)

* Basic framework for use of feature flags. Enviroment variable overrides.

* Use Apperr instead of error number increments.

* Undo random viper change.

* Update model/config_test.go

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

model/config.go

@@ -2882,6 +2882,7 @@ type Config struct {
 	GuestAccountsSettings     GuestAccountsSettings
 	ImageProxySettings        ImageProxySettings
 	CloudSettings             CloudSettings
+	FeatureFlags              *FeatureFlags `json:",omitempty"`
 }
 
 func (o *Config) Clone() *Config {
@@ -2969,6 +2970,10 @@ func (o *Config) SetDefaults() {
 	o.GuestAccountsSettings.SetDefaults()
 	o.ImageProxySettings.SetDefaults(o.ServiceSettings)
 	o.CloudSettings.SetDefaults()
+	if o.FeatureFlags == nil {
+		o.FeatureFlags = &FeatureFlags{}
+		o.FeatureFlags.SetDefaults()
+	}
 }
 
 func (o *Config) IsValid() *AppError {

model/config_test.go

@@ -1326,3 +1326,21 @@ func TestConfigMarketplaceDefaults(t *testing.T) {
 		require.Equal(t, "https://marketplace.example.com", *c.PluginSettings.MarketplaceUrl)
 	})
 }
+
+func TestSetDefaultFeatureFlagBehaviour(t *testing.T) {
+	cfg := Config{}
+	cfg.SetDefaults()
+
+	require.NotNil(t, cfg.FeatureFlags)
+	require.Equal(t, "off", cfg.FeatureFlags.TestFeature)
+
+	cfg = Config{
+		FeatureFlags: &FeatureFlags{
+			TestFeature: "somevalue",
+		},
+	}
+	cfg.SetDefaults()
+	require.NotNil(t, cfg.FeatureFlags)
+	require.Equal(t, "somevalue", cfg.FeatureFlags.TestFeature)
+
+}

model/feature_flags.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+type FeatureFlags struct {
+	// Exists only for unit and manual testing.
+	// When set to a value, will be returned by the ping endpoint.
+	TestFeature string
+}
+
+func (f *FeatureFlags) SetDefaults() {
+	f.TestFeature = "off"
+}