Ruler: protect overrides map a mutex when creating a new appender

pkg/ruler/registry.go

@@ -128,8 +128,12 @@ func (r *walRegistry) get(tenant string) storage.Storage {
 }
 
 func (r *walRegistry) Appender(ctx context.Context) storage.Appender {
+	// concurrency-safe retrieval of remote-write config for this tenant, using the global remote-write for defaults
+	r.overridesMu.Lock()
 	tenant, _ := user.ExtractOrgID(ctx)
 	rwCfg, err := r.getTenantRemoteWriteConfig(tenant, r.config.RemoteWrite)
+	r.overridesMu.Unlock()
+
 	if err != nil {
 		level.Error(r.logger).Log("msg", "error retrieving remote-write config; discarding samples", "user", tenant, "err", err)
 		return discardingAppender{}

pkg/ruler/registry_test.go

@@ -405,6 +405,30 @@ func TestTenantRemoteWriteConfigWithOverrideConcurrentAccess(t *testing.T) {
 	})
 }
 
+func TestAppenderConcurrentAccess(t *testing.T) {
+	require.NotPanics(t, func() {
+		reg := setupRegistry(t, cfg, newFakeLimits())
+		var wg sync.WaitGroup
+		for i := 0; i < 1000; i++ {
+			wg.Add(1)
+			go func(reg *walRegistry) {
+				defer wg.Done()
+
+				_ = reg.Appender(user.InjectOrgID(context.Background(), enabledRWTenant))
+			}(reg)
+
+			wg.Add(1)
+			go func(reg *walRegistry) {
+				defer wg.Done()
+
+				_ = reg.Appender(user.InjectOrgID(context.Background(), additionalHeadersRWTenant))
+			}(reg)
+		}
+
+		wg.Wait()
+	})
+}
+
 func TestTenantRemoteWriteConfigWithoutOverride(t *testing.T) {
 	reg := setupRegistry(t, backCompatCfg, newFakeLimitsBackwardCompat())