feat: fail fast when no monitoring targets configured

internal/config/config.go

@@ -23,6 +23,11 @@ const (
 	defaultTLSExpiryWarning = 7
 )
 
+// ErrNoTargets is returned when no monitoring targets are configured.
+var ErrNoTargets = errors.New(
+	"no monitoring targets configured: set DNSWATCHER_TARGETS environment variable",
+)
+
 // Params contains dependencies for Config.
 type Params struct {
 	fx.In
@@ -132,11 +137,9 @@ func buildConfig(
 		tlsInterval = defaultTLSInterval
 	}
 
-	domains, hostnames, err := ClassifyTargets(
-		parseCSV(viper.GetString("TARGETS")),
-	)
+	domains, hostnames, err := parseAndValidateTargets()
 	if err != nil {
-		return nil, fmt.Errorf("invalid targets configuration: %w", err)
+		return nil, err
 	}
 
 	cfg := &Config{
@@ -162,6 +165,23 @@ func buildConfig(
 	return cfg, nil
 }
 
+func parseAndValidateTargets() ([]string, []string, error) {
+	domains, hostnames, err := ClassifyTargets(
+		parseCSV(viper.GetString("TARGETS")),
+	)
+	if err != nil {
+		return nil, nil, fmt.Errorf(
+			"invalid targets configuration: %w", err,
+		)
+	}
+
+	if len(domains) == 0 && len(hostnames) == 0 {
+		return nil, nil, ErrNoTargets
+	}
+
+	return domains, hostnames, nil
+}
+
 func parseCSV(input string) []string {
 	if input == "" {
 		return nil