Address review comments

x-pack/filebeat/input/o365audit/auth/cert.go

@@ -48,7 +48,7 @@ func loadConfigCerts(cfg tlscommon.CertificateConfig) (cert *x509.Certificate, k
 	if err != nil {
 		return nil, nil, errors.Wrapf(err, "error loading X509 certificate from '%s'", cfg.Certificate)
 	}
-	if tlsCert == nil || len(tlsCert.Certificate) < 1 {
+	if tlsCert == nil || len(tlsCert.Certificate) == 0 {
 		return nil, nil, fmt.Errorf("no certificates loaded from '%s'", cfg.Certificate)
 	}
 	cert, err = x509.ParseCertificate(tlsCert.Certificate[0])

x-pack/filebeat/input/o365audit/auth/secret.go

@@ -9,6 +9,8 @@ import (
 	"github.com/pkg/errors"
 )
 
+// NewProviderFromClientSecret returns a token provider that uses a secret
+// for authentication.
 func NewProviderFromClientSecret(endpoint, resource, applicationID, tenantID, secret string) (p TokenProvider, err error) {
 	oauth, err := adal.NewOAuthConfig(endpoint, tenantID)
 	if err != nil {

x-pack/filebeat/input/o365audit/config.go

@@ -27,17 +27,14 @@ type Config struct {
 
 	// TenantID (aka. Directory ID) is a list of tenants for which to fetch
 	// the audit logs. This can be a string or a list of strings.
-	TenantID interface{} `config:"tenant_id,replace" validate:"required"`
+	TenantID stringList `config:"tenant_id,replace" validate:"required"`
 
 	// Content-Type is a list of content-types to fetch.
 	// This can be a string or a list of strings.
-	ContentType interface{} `config:"content_type,replace"`
+	ContentType stringList `config:"content_type,replace"`
 
 	// API contains settings to adapt to changes on the API.
 	API APIConfig `config:"api"`
-
-	tenants      []string
-	contentTypes []string
 }
 
 // APIConfig contains advanced settings that are only supposed to be changed
@@ -149,36 +146,32 @@ func (c *Config) Validate() (err error) {
 			return errors.Wrap(err, "invalid certificate config")
 		}
 	}
-	if c.tenants, err = asStringList(c.TenantID); err != nil {
-		return errors.Wrap(err, "error validating tenant_id")
-	}
-	if c.contentTypes, err = asStringList(c.ContentType); err != nil {
-		return errors.Wrap(err, "error validating content_type")
-	}
 	return nil
 }
 
-// A helper to allow defining a field either as a string or a list of strings.
-func asStringList(value interface{}) (list []string, err error) {
+type stringList []string
+
+// Unpack populates the stringList with either a single string value or an array.
+func (s *stringList) Unpack(value interface{}) error {
 	switch v := value.(type) {
 	case string:
-		list = []string{v}
+		*s = []string{v}
 	case []string:
-		list = v
+		*s = v
 	case []interface{}:
-		list = make([]string, len(v))
+		*s = make([]string, len(v))
 		for idx, ival := range v {
 			str, ok := ival.(string)
 			if !ok {
-				return nil, fmt.Errorf("string value required. Found %v (type %T) at position %d",
+				return fmt.Errorf("string value required. Found %v (type %T) at position %d",
 					ival, ival, idx+1)
 			}
-			list[idx] = str
+			(*s)[idx] = str
 		}
 	default:
-		return nil, fmt.Errorf("array of strings required. Found %v (type %T)", value, value)
+		return fmt.Errorf("array of strings required. Found %v (type %T)", value, value)
 	}
-	return list, nil
+	return nil
 }
 
 // NewTokenProvider returns an auth.TokenProvider for the given tenantID.

x-pack/filebeat/input/o365audit/input.go

@@ -108,17 +108,17 @@ func newInput(
 	}()
 
 	pollers := make(map[stream]*poll.Poller)
-	for _, tenantID := range config.tenants {
+	for _, tenantID := range config.TenantID {
 		// MaxRequestsPerMinute limitation is per tenant.
-		delay := time.Duration(len(config.contentTypes)) * time.Minute / time.Duration(config.API.MaxRequestsPerMinute)
+		delay := time.Duration(len(config.ContentType)) * time.Minute / time.Duration(config.API.MaxRequestsPerMinute)
 		auth, err := config.NewTokenProvider(tenantID)
 		if err != nil {
 			return nil, err
 		}
 		if _, err = auth.Token(); err != nil {
 			return nil, errors.Wrapf(err, "unable to acquire authentication token for tenant:%s", tenantID)
 		}
-		for _, contentType := range config.contentTypes {
+		for _, contentType := range config.ContentType {
 			key := stream{
 				tenantID:    tenantID,
 				contentType: contentType,