Allow a configurable env var prefix [#19]

I'd like to keep Prefix's scope fairly limited, because it has potential
to spiral into something magnificently complex. You get one prefix
setting, it goes in `settings` not `environment`, end of feature.

internal/helm/config.go

@@ -42,9 +42,17 @@ func (cfg *Config) Populate() error {
 		return err
 	}
 
+	prefix := cfg.Prefix
+
 	if err := envconfig.Process("", cfg); err != nil {
 		return err
 	}
 
+	if prefix != "" {
+		if err := envconfig.Process(cfg.Prefix, cfg); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }

internal/helm/config_test.go

@@ -18,6 +18,7 @@ func TestConfigTestSuite(t *testing.T) {
 }
 
 func (suite *ConfigTestSuite) TestPopulateWithPluginPrefix() {
+	suite.unsetenv("PLUGIN_PREFIX")
 	suite.unsetenv("HELM_COMMAND")
 	suite.unsetenv("UPDATE_DEPENDENCIES")
 	suite.unsetenv("DEBUG")
@@ -35,6 +36,7 @@ func (suite *ConfigTestSuite) TestPopulateWithPluginPrefix() {
 }
 
 func (suite *ConfigTestSuite) TestPopulateWithNoPrefix() {
+	suite.unsetenv("PLUGIN_PREFIX")
 	suite.unsetenv("PLUGIN_HELM_COMMAND")
 	suite.unsetenv("PLUGIN_UPDATE_DEPENDENCIES")
 	suite.unsetenv("PLUGIN_DEBUG")
@@ -51,14 +53,56 @@ func (suite *ConfigTestSuite) TestPopulateWithNoPrefix() {
 	suite.True(cfg.Debug)
 }
 
+func (suite *ConfigTestSuite) TestPopulateWithConfigurablePrefix() {
+	suite.unsetenv("API_SERVER")
+	suite.unsetenv("PLUGIN_API_SERVER")
+
+	suite.setenv("PLUGIN_PREFIX", "prix_fixe")
+	suite.setenv("PRIX_FIXE_API_SERVER", "your waiter this evening")
+
+	cfg := Config{}
+	suite.Require().NoError(cfg.Populate())
+
+	suite.Equal("prix_fixe", cfg.Prefix)
+	suite.Equal("your waiter this evening", cfg.APIServer)
+}
+
+func (suite *ConfigTestSuite) TestPrefixSettingDoesNotAffectPluginPrefix() {
+	suite.setenv("PLUGIN_PREFIX", "IXFREP")
+	suite.setenv("PLUGIN_HELM_COMMAND", "wake me up")
+	suite.setenv("IXFREP_PLUGIN_HELM_COMMAND", "send me to sleep inside")
+
+	cfg := Config{}
+	suite.Require().NoError(cfg.Populate())
+
+	suite.Equal("wake me up", cfg.Command)
+}
+
+func (suite *ConfigTestSuite) TestPrefixSettingMustHavePluginPrefix() {
+	suite.unsetenv("PLUGIN_PREFIX")
+	suite.setenv("PREFIX", "refpix")
+	suite.setenv("HELM_COMMAND", "gimme more")
+	suite.setenv("REFPIX_HELM_COMMAND", "gimme less")
+
+	cfg := Config{}
+	suite.Require().NoError(cfg.Populate())
+
+	suite.Equal("gimme more", cfg.Command)
+}
+
 func (suite *ConfigTestSuite) TestPopulateWithConflictingVariables() {
 	suite.setenv("PLUGIN_HELM_COMMAND", "execute order 66")
-	suite.setenv("HELM_COMMAND", "defend the jedi")
+	suite.setenv("HELM_COMMAND", "defend the jedi") // values from the `environment` block override those from `settings`
+
+	suite.setenv("PLUGIN_PREFIX", "prod")
+	suite.setenv("TIMEOUT", "5m0s")
+	suite.setenv("PROD_TIMEOUT", "2m30s") // values from prefixed env vars override those from non-prefixed ones
 
 	cfg := Config{}
 	suite.Require().NoError(cfg.Populate())
 
 	suite.Equal("defend the jedi", cfg.Command)
+	suite.Equal("2m30s", cfg.Timeout)
 }
 
 func (suite *ConfigTestSuite) setenv(key, val string) {