Apply defaults to undefined sections (grafana#546)

CHANGELOG.md

@@ -3,14 +3,18 @@ cross-compilation issue, but will return in v0.13.0.
 
 # Main (unreleased)
 
+- [BUGFIX] Ensure defaults are applied to undefined sections in config file.
+  This fixes a problem where integrations didn't work if `prometheus:` wasn't
+  configured. (@rfratto)
+
 # 0.14.0-rc.3 (2021-04-15)
 
 - [ENHANCEMENT] Add  `headers` field in `remote_write` config for Tempo. `headers`
   specifies HTTP headers to forward to the remote endpoint. (@alexbiehl)
 - [CHANGE] Add `tempo_spanmetrics` namespace in spanmetrics (@mapno)
 
 - [BUGFIX] Grafana Agent running as a Windows service should start automatically on startup
-  (@mattdurham) 
+  (@mattdurham)
 
 - [BUGFIX] Validate that incoming scraped metrics do not have an empty label
   set or a label set with duplicate labels, mirroring the behavior of

pkg/config/config.go

@@ -18,6 +18,13 @@ import (
 	"gopkg.in/yaml.v2"
 )
 
+// DefaultConfig holds default settings for all the subsystems.
+var DefaultConfig = Config{
+	// All subsystems with a DefaultConfig should be listed here.
+	Prometheus:   prom.DefaultConfig,
+	Integrations: integrations.DefaultManagerConfig,
+}
+
 // Config contains underlying configurations for the agent
 type Config struct {
 	Server       server.Config              `yaml:"server,omitempty"`
@@ -33,6 +40,13 @@ type Config struct {
 	ReloadPort    int    `yaml:"-"`
 }
 
+// UnmarshalYAML implements yaml.Unmarshaler.
+func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	*c = DefaultConfig
+	type config Config
+	return unmarshal((*config)(c))
+}
+
 // ApplyDefaults sets default values in the config
 func (c *Config) ApplyDefaults() error {
 	if err := c.Prometheus.ApplyDefaults(); err != nil {
@@ -103,8 +117,9 @@ func Load(fs *flag.FlagSet, args []string) (*Config, error) {
 // doesn't require having a literal file on disk.
 func load(fs *flag.FlagSet, args []string, loader func(string, bool, *Config) error) (*Config, error) {
 	var (
+		cfg = DefaultConfig
+
 		printVersion    bool
-		cfg             Config
 		file            string
 		configExpandEnv bool
 	)

pkg/config/config_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 	"time"
 
+	"github.com/grafana/agent/pkg/integrations"
+	"github.com/grafana/agent/pkg/prom"
 	"github.com/grafana/agent/pkg/prom/instance"
 	"github.com/prometheus/common/model"
 	promCfg "github.com/prometheus/prometheus/config"
@@ -120,3 +122,12 @@ prometheus:
 		require.Error(t, err)
 	})
 }
+
+func TestConfig_Defaults(t *testing.T) {
+	var c Config
+	err := LoadBytes([]byte(`{}`), false, &c)
+	require.NoError(t, err)
+
+	require.Equal(t, prom.DefaultConfig, c.Prometheus)
+	require.Equal(t, integrations.DefaultManagerConfig, c.Integrations)
+}