app/vmagent: add -dryRun option for checking all the configs mentioned in command-line flags without running vmagent

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/362

app/vmagent/main.go

@@ -40,6 +40,8 @@ var (
 		"Telnet put messages and HTTP /api/put messages are simultaneously served on TCP port. "+
 		"Usually :4242 must be set. Doesn't work if empty")
 	opentsdbHTTPListenAddr = flag.String("opentsdbHTTPListenAddr", "", "TCP address to listen for OpentTSDB HTTP put requests. Usually :4242 must be set. Doesn't work if empty")
+	dryRun                 = flag.Bool("dryRun", false, "Whether to check only config files without running vmagent. The following files are checked: "+
+		"-promscrape.config, -remoteWrite.relabelConfig, -remoteWrite.urlRelabelConfig . See also -promscrape.config.dryRun")
 )
 
 var (
@@ -55,6 +57,19 @@ func main() {
 	envflag.Parse()
 	buildinfo.Init()
 	logger.Init()
+
+	if *dryRun {
+		flag.Set("promscrape.config.strictParse", "true")
+		if err := remotewrite.CheckRelabelConfigs(); err != nil {
+			logger.Fatalf("error when checking relabel configs: %s", err)
+		}
+		if err := promscrape.CheckConfig(); err != nil {
+			logger.Fatalf("error when checking Prometheus config: %s", err)
+		}
+		logger.Infof("all the configs are ok; exitting with 0 status code")
+		return
+	}
+
 	logger.Infof("starting vmagent at %q...", *httpListenAddr)
 	startTime := time.Now()
 	remotewrite.Init()

app/vmagent/remotewrite/remotewrite.go

@@ -32,6 +32,21 @@ var (
 		"Disk usage is unlimited if the value is set to 0")
 )
 
+// CheckRelabelConfigs checks -remoteWrite.relabelConfig and -remoteWrite.urlRelabelConfig.
+func CheckRelabelConfigs() error {
+	if *relabelConfigPathGlobal != "" {
+		if _, err := promrelabel.LoadRelabelConfigs(*relabelConfigPathGlobal); err != nil {
+			return fmt.Errorf("cannot load -remoteWrite.relabelConfig=%q: %s", *relabelConfigPathGlobal, err)
+		}
+	}
+	for _, path := range *relabelConfigPaths {
+		if _, err := promrelabel.LoadRelabelConfigs(path); err != nil {
+			return fmt.Errorf("cannot load relabel configs from -remoteWrite.urlRelabelConfig=%q: %s", path, err)
+		}
+	}
+	return nil
+}
+
 var rwctxs []*remoteWriteCtx
 
 // Init initializes remotewrite.

lib/promscrape/scraper.go

@@ -37,6 +37,15 @@ var (
 		"See https://prometheus.io/docs/prometheus/latest/configuration/configuration/#scrape_config for details")
 )
 
+// CheckConfig checks -promscrape.config for errors and unsupported options.
+func CheckConfig() error {
+	if *promscrapeConfigFile == "" {
+		return fmt.Errorf("missing -promscrape.config option")
+	}
+	_, _, err := loadConfig(*promscrapeConfigFile)
+	return err
+}
+
 // Init initializes Prometheus scraper with config from the `-promscrape.config`.
 //
 // Scraped data is passed to pushData.