prober: add a status page handler

This change adds an HTTP handler with a table showing a list of all
probes, their status, and a button that allows triggering a specific
probe.

Updates tailscale/corp#20583

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

prober/status.go

@@ -0,0 +1,124 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package prober
+
+import (
+	"embed"
+	"fmt"
+	"html/template"
+	"net/http"
+	"strings"
+	"time"
+
+	"tailscale.com/tsweb"
+	"tailscale.com/util/mak"
+)
+
+//go:embed status.html
+var statusFiles embed.FS
+var statusTpl = template.Must(template.ParseFS(statusFiles, "status.html"))
+
+type statusHandlerOpt func(*statusHandlerParams)
+type statusHandlerParams struct {
+	title string
+
+	pageLinks  map[string]string
+	probeLinks map[string]string
+}
+
+// WithTitle sets the title of the status page.
+func WithTitle(title string) statusHandlerOpt {
+	return func(opts *statusHandlerParams) {
+		opts.title = title
+	}
+}
+
+// WithPageLink adds a top-level link to the status page.
+func WithPageLink(text, url string) statusHandlerOpt {
+	return func(opts *statusHandlerParams) {
+		mak.Set(&opts.pageLinks, text, url)
+	}
+}
+
+// WithProbeLink adds a link to each probe on the status page.
+// The textTpl and urlTpl are Go templates that will be rendered
+// with the respective ProbeInfo struct as the data.
+func WithProbeLink(textTpl, urlTpl string) statusHandlerOpt {
+	return func(opts *statusHandlerParams) {
+		mak.Set(&opts.probeLinks, textTpl, urlTpl)
+	}
+}
+
+// StatusHandler is a handler for the probe overview HTTP endpoint.
+// It shows a list of probes and their current status.
+func (p *Prober) StatusHandler(opts ...statusHandlerOpt) tsweb.ReturnHandlerFunc {
+	params := &statusHandlerParams{
+		title: "Prober Status",
+	}
+	for _, opt := range opts {
+		opt(params)
+	}
+	return func(w http.ResponseWriter, r *http.Request) error {
+		type probeStatus struct {
+			ProbeInfo
+			TimeSinceLast time.Duration
+			Links         map[string]template.URL
+		}
+		vars := struct {
+			Title           string
+			Links           map[string]template.URL
+			TotalProbes     int64
+			UnhealthyProbes int64
+			Probes          map[string]probeStatus
+		}{
+			Title: params.title,
+		}
+
+		for text, url := range params.pageLinks {
+			mak.Set(&vars.Links, text, template.URL(url))
+		}
+
+		for name, info := range p.ProbeInfo() {
+			vars.TotalProbes++
+			if !info.Result {
+				vars.UnhealthyProbes++
+			}
+			s := probeStatus{ProbeInfo: info}
+			if !info.End.IsZero() {
+				s.TimeSinceLast = time.Since(info.End)
+			}
+			for textTpl, urlTpl := range params.probeLinks {
+				text, err := renderTemplate(textTpl, info)
+				if err != nil {
+					return tsweb.Error(500, err.Error(), err)
+				}
+				url, err := renderTemplate(urlTpl, info)
+				if err != nil {
+					return tsweb.Error(500, err.Error(), err)
+				}
+				mak.Set(&s.Links, text, template.URL(url))
+			}
+			mak.Set(&vars.Probes, name, s)
+		}
+
+		if err := statusTpl.ExecuteTemplate(w, "status", vars); err != nil {
+			return tsweb.HTTPError{Code: 500, Err: err, Msg: "error rendering status page"}
+		}
+		return nil
+	}
+}
+
+// renderTemplate renders the given Go template with the provided data
+// and returns the result as a string.
+func renderTemplate(tpl string, data any) (string, error) {
+	t, err := template.New("").Parse(tpl)
+	if err != nil {
+		return "", fmt.Errorf("error parsing template %q: %w", tpl, err)
+	}
+	var buf strings.Builder
+	if err := t.ExecuteTemplate(&buf, "", data); err != nil {
+		return "", fmt.Errorf("error rendering template %q with data %v: %w", tpl, data, err)
+	}
+	return buf.String(), nil
+}