Make configuration serializable

config/config.go

@@ -42,6 +42,16 @@ func FromYAML(r io.Reader) (*Config, error) {
 	return c, nil
 }
 
+// ToYAML encodes the given configuration to the writer as YAML
+func ToYAML(w io.Writer, cfg *Config) error {
+	err := yaml.NewEncoder(w).Encode(cfg)
+	if err != nil {
+		return fmt.Errorf("failed to encode YAML: %w", err)
+	}
+
+	return nil
+}
+
 func (cfg *Config) TargetConfigByAddr(addr string) TargetConfig {
 	for _, t := range cfg.Targets {
 		if t.Addr == addr {

config/config_test.go

@@ -3,6 +3,7 @@
 package config
 
 import (
+	"bytes"
 	"os"
 	"reflect"
 	"testing"
@@ -65,3 +66,35 @@ func TestParseConfig(t *testing.T) {
 		t.Errorf("expected options.disable-ipv6 to be %v, got %v", expected, c.Options.DisableIPv6)
 	}
 }
+
+func TestRoundtrip(t *testing.T) {
+	f, err := os.Open("testdata/config_test.yml")
+	if err != nil {
+		t.Error("failed to open file", err)
+		t.FailNow()
+	}
+
+	c, err := FromYAML(f)
+	if err != nil {
+		t.Error("failed to read file", err)
+		t.FailNow()
+	}
+
+	buf := bytes.NewBuffer(nil)
+	err = ToYAML(buf, c)
+	if err != nil {
+		t.Error("failed to encode config", err)
+		t.FailNow()
+	}
+
+	after, err := FromYAML(buf)
+	if err != nil {
+		t.Error("failed to read config again", err)
+		t.FailNow()
+	}
+
+	if !reflect.DeepEqual(c, after) {
+		t.Error("config after Decode(Encode(cfg)) didn't match")
+		t.FailNow()
+	}
+}

config/duration.go

@@ -22,6 +22,10 @@ func (d *duration) UnmarshalYAML(unmashal func(interface{}) error) error {
 	return nil
 }
 
+func (d duration) MarshalYAML() (interface{}, error) {
+	return d.Duration().String(), nil
+}
+
 // Duration is a convenience getter.
 func (d duration) Duration() time.Duration {
 	return time.Duration(d)

config/target.go

@@ -25,3 +25,12 @@ func (d *TargetConfig) UnmarshalYAML(unmashal func(interface{}) error) error {
 
 	return nil
 }
+
+func (d TargetConfig) MarshalYAML() (interface{}, error) {
+	if d.Labels == nil {
+		return d.Addr, nil
+	}
+	ret := make(map[string]map[string]string)
+	ret[d.Addr] = d.Labels
+	return ret, nil
+}

target.go

@@ -76,7 +76,7 @@ func (t *target) addOrUpdateMonitor(monitor *mon.Monitor, opts targetOpts) error
 
 	addrs, err := t.resolver.LookupIPAddr(context.Background(), t.host)
 	if err != nil {
-		return fmt.Errorf("error resolving target: %w", err)
+		return fmt.Errorf("error resolving target '%s': %w", t.host, err)
 	}
 
 	var sanitizedAddrs []net.IPAddr