lib/httpserver: expose command-line flags at /flags page

This should simplify debugging.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1695

app/victoria-metrics/main.go

@@ -95,6 +95,7 @@ func requestHandler(w http.ResponseWriter, r *http.Request) bool {
 			{"/api/v1/targets", "advanced information about discovered targets in JSON format"},
 			{"/config", "-promscrape.config contents"},
 			{"/metrics", "available service metrics"},
+			{"/flags", "command-line flags"},
 			{"/api/v1/status/tsdb", "tsdb status page"},
 			{"/api/v1/status/top_queries", "top queries"},
 			{"/api/v1/status/active_queries", "active queries"},

app/vmagent/main.go

@@ -161,6 +161,7 @@ func requestHandler(w http.ResponseWriter, r *http.Request) bool {
 			{"/api/v1/targets", "advanced information about discovered targets in JSON format"},
 			{"/config", "-promscrape.config contents"},
 			{"/metrics", "available service metrics"},
+			{"/flags", "command-line flags"},
 			{"/-/reload", "reload configuration"},
 		})
 		return true

app/vmalert/web.go

@@ -28,6 +28,7 @@ func initLinks() {
 		{path.Join(pathPrefix, "api/v1/groups"), "list all loaded groups and rules"},
 		{path.Join(pathPrefix, "api/v1/alerts"), "list all active alerts"},
 		{path.Join(pathPrefix, "api/v1/groupID/alertID/status"), "get alert status by ID"},
+		{path.Join(pathPrefix, "flags"), "command-line flags"},
 		{path.Join(pathPrefix, "metrics"), "list of application metrics"},
 		{path.Join(pathPrefix, "-/reload"), "reload configuration"},
 	}

docs/CHANGELOG.md

@@ -18,6 +18,7 @@ sort: 15
 * FEATURE: add trigonometric functions, which are going to be added in [Prometheus 2.31](https://github.com/prometheus/prometheus/pull/9239): [acosh](https://docs.victoriametrics.com/MetricsQL.html#acosh), [asinh](https://docs.victoriametrics.com/MetricsQL.html#asinh), [atan](https://docs.victoriametrics.com/MetricsQL.html#atan), [atanh](https://docs.victoriametrics.com/MetricsQL.html#atanh), [cosh](https://docs.victoriametrics.com/MetricsQL.html#cosh), [deg](https://docs.victoriametrics.com/MetricsQL.html#deg), [rad](https://docs.victoriametrics.com/MetricsQL.html#rad), [sinh](https://docs.victoriametrics.com/MetricsQL.html#sinh), [tan](https://docs.victoriametrics.com/MetricsQL.html#tan), [tanh](https://docs.victoriametrics.com/MetricsQL.html#tanh). Also add `atan2` binary operator. See [this pull request](https://github.com/prometheus/prometheus/pull/9248).
 * FEATURE: consistently return the same set of time series from [limitk](https://docs.victoriametrics.com/MetricsQL.html#limitk) function. This improves the usability of periodically refreshed graphs.
 * FEATURE: [vmui](https://docs.victoriametrics.com/#vmui): varios UX improvements. See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/1711).
+* FEATURE: add `/flags` page to all the VictoriaMetrics components. This page contains command-line flags passed to the component.
 
 * BUGFIX: vmstorage: fix `unaligned 64-bit atomic operation` panic on 32-bit architectures (arm and 386). The panic has been introduced in v1.67.0.
 * BUGFIX: vmalert, vmauth: prevent from frequent closing of TCP connections established to backends under high load. This should reduce the number of TCP sockets in `TIME_WAIT` state at `vmalert` and `vmauth` under high load. See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/1704).

lib/flagutil/flag.go

@@ -0,0 +1,15 @@
+package flagutil
+
+import (
+	"flag"
+	"fmt"
+	"io"
+)
+
+// WriteFlags writes all the explicitly set flags to w.
+func WriteFlags(w io.Writer) {
+	flag.Visit(func(f *flag.Flag) {
+		value := f.Value.String()
+		fmt.Fprintf(w, "-%s=%q\n", f.Name, value)
+	})
+}

lib/httpserver/httpserver.go

@@ -22,6 +22,7 @@ import (
 	"time"
 
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fasttime"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/flagutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/netutil"
 	"github.com/VictoriaMetrics/metrics"
@@ -279,6 +280,10 @@ func handlerWrapper(s *server, w http.ResponseWriter, r *http.Request, rh Reques
 		WritePrometheusMetrics(w)
 		metricsHandlerDuration.UpdateDuration(startTime)
 		return
+	case "/flags":
+		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		flagutil.WriteFlags(w)
+		return
 	default:
 		if strings.HasPrefix(r.URL.Path, "/debug/pprof/") {
 			pprofRequests.Inc()