Fix sending invalid Alertmanager configuration

internal/controller/alertmanager_controller.go

@@ -46,6 +46,7 @@ func SetupAlertmanagerReconciler(mgr ctrl.Manager, conf config.Config) error {
 
 	// Setup the controller
 	return ctrl.NewControllerManagedBy(mgr).
+		Named("alertmanager").
 		For(&v1.Secret{}, builder.WithPredicates(secretPredicate)).
 		Watches(&v1.Pod{}, p, builder.WithPredicates(podPredicate)).
 		Complete(r)

pkg/alertmanager/alertmanager.go

@@ -5,10 +5,8 @@ import (
 	"context"
 	"fmt"
 	"io"
-	"maps"
 	"net/http"
 	"path"
-	"slices"
 	"strings"
 
 	"github.com/pkg/errors"
@@ -89,40 +87,39 @@ func (s Service) Configure(ctx context.Context, secret *v1.Secret) error {
 }
 
 // configure sends the configuration and templates to Mimir Alertmanager's API
+// It is the caller responsibility to make sure templates names are valid (do not contain any path), and that templates are referenced in the configuration.
 // https://grafana.com/docs/mimir/latest/references/http-api/#set-alertmanager-configuration
 func (s Service) configure(ctx context.Context, alertmanagerConfigContent []byte, templates map[string]string, tenantID string) error {
 	logger := log.FromContext(ctx)
 
-	// Load alertmanager configuration
-	alertmanagerConfig, err := config.Load(string(alertmanagerConfigContent))
+	// Validate Alertmanager configuration
+	// The returned config is not used, as transforming it via String() would produce an invalid configuration with all secrets replaced with <redacted>.
+	_, err := config.Load(string(alertmanagerConfigContent))
 	if err != nil {
 		return errors.WithStack(fmt.Errorf("alertmanager: failed to load configuration: %w", err))
 	}
 
-	// Set template names
-	// Values set here must match the keys set in requestData.TemplateFiles
-	alertmanagerConfig.Templates = slices.Collect(maps.Keys(templates))
-	alertmanagerConfigString := alertmanagerConfig.String()
-
 	// Prepare request for Alertmanager API
 	requestData := configRequest{
-		AlertmanagerConfig: alertmanagerConfigString,
+		AlertmanagerConfig: string(alertmanagerConfigContent),
 		TemplateFiles:      templates,
 	}
 	data, err := yaml.Marshal(requestData)
 	if err != nil {
 		return errors.WithStack(fmt.Errorf("alertmanager: failed to marshal yaml: %w", err))
 	}
+	dataLen := len(data)
 
 	url := s.alertmanagerURL + alertmanagerAPIPath
-	logger.WithValues("url", url, "data_size", len(data), "config_size", len(alertmanagerConfigString), "templates_count", len(templates)).Info("Alertmanager: sending configuration")
+	logger.WithValues("url", url, "data_size", dataLen, "config_size", len(alertmanagerConfigContent), "templates_count", len(templates)).Info("Alertmanager: sending configuration")
 
 	// Send request to Alertmanager's API
 	req, err := http.NewRequest(http.MethodPost, url, bytes.NewBuffer(data))
 	if err != nil {
 		return errors.WithStack(fmt.Errorf("alertmanager: failed to create request: %w", err))
 	}
 	req.Header.Set(common.OrgIDHeader, tenantID)
+	req.ContentLength = int64(dataLen)
 
 	resp, err := http.DefaultClient.Do(req)
 	if err != nil {