app/vmauth: adds dryRun flag for config validation

This flag only validates configuration file pointed by `auth.config`
flag.

 Related issue:
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7505

Signed-off-by: f41gh7 <nik@victoriametrics.com>

app/vmauth/main.go

@@ -61,6 +61,7 @@ var (
 		"See https://docs.victoriametrics.com/vmauth/#backend-tls-setup")
 	backendTLSServerName = flag.String("backend.TLSServerName", "", "Optional TLS ServerName, which must be sent to HTTPS backend. "+
 		"See https://docs.victoriametrics.com/vmauth/#backend-tls-setup")
+	dryRun = flag.Bool("dryRun", false, "Whether to check only config files without running vmauth. The auth configuration file is validated. The -auth.config flag must be specified.")
 )
 
 func main() {
@@ -71,6 +72,16 @@ func main() {
 	buildinfo.Init()
 	logger.Init()
 
+	if *dryRun {
+		if len(*authConfigPath) == 0 {
+			logger.Fatalf("missing required `-auth.config` command-line flag")
+		}
+		if _, err := reloadAuthConfig(); err != nil {
+			logger.Fatalf("failed to parse %q: %s", *authConfigPath, err)
+		}
+		return
+	}
+
 	listenAddrs := *httpListenAddrs
 	if len(listenAddrs) == 0 {
 		listenAddrs = []string{":8427"}

docs/changelog/CHANGELOG.md

@@ -24,6 +24,8 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * FEATURE: [vmui](https://docs.victoriametrics.com/#vmui): add `Raw Query` tab for displaying raw data. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7024).
 * FEATURE: [stream aggregation](https://docs.victoriametrics.com/stream-aggregation/): add `ignore_first_sample_interval` param to [aggregation config](https://docs.victoriametrics.com/stream-aggregation/#stream-aggregation-config). It allows users to control the time interval when aggregation skips sending aggregated samples to avoid unexpected spikes in values. By default, this interval is set to x2 of `staleness_interval`. The new setting is applicable only to `total`, `total_prometheus`, `increase`, `increase_prometheus` and `histogram_bucket` outputs. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7116) for details. Thanks to @iyuroch for the [pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/7313).
 * FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth/): add `dump_request_on_errors` bool setting to [auth config](https://docs.victoriametrics.com/vmauth/#auth-config) for debugging HTTP requests that missed routing rules. This should improve debugability of vmauth settings.
+* FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth/): add `dryRun` flag to validate configuration. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7505) for details.
+
 
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent): Properly return `200 OK` HTTP status code when importing data via [Pushgateway protocol](https://docs.victoriametrics.com/#how-to-import-data-in-prometheus-exposition-format) using [multitenant URL format](https://docs.victoriametrics.com/cluster-victoriametrics/#url-format). See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3636) and [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/7571).
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent): Properly set `TCP` connection timeout for `Kubernetes API server` connection for metric scrapping with `kubernetes_sd_configs`. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7127).

docs/vmauth.md

@@ -1199,6 +1199,8 @@ See the docs at https://docs.victoriametrics.com/vmauth/ .
      Whether to discover backend IPs via periodic DNS queries to hostnames specified in url_prefix. This may be useful when url_prefix points to a hostname with dynamically scaled instances behind it. See https://docs.victoriametrics.com/vmauth/#discovering-backend-ips
   -discoverBackendIPsInterval duration
      The interval for re-discovering backend IPs if -discoverBackendIPs command-line flag is set. Too low value may lead to DNS errors (default 10s)
+  -dryRun
+        Whether to check only config files without running vmauth. The auth configuration file is validated. The -auth.config flag must be specified.
   -enableTCP6
      Whether to enable IPv6 for listening and dialing. By default, only IPv4 TCP and UDP are used
   -envflag.enable