Serve behind route prefix (#283)

Add RoutePrefix to LandingConfig, update html to use the new RoutePrefix when provided

---------

Signed-off-by: kwilt <kwilt@pm.me>

Author: kwilt

web/landing_page.go

@@ -22,11 +22,13 @@ import (
 	"bytes"
 	_ "embed"
 	"net/http"
+	"strings"
 	"text/template"
 )
 
 // Config represents the configuration of the web listener.
 type LandingConfig struct {
+	RoutePrefix string         // The route prefix for the exporter.
 	HeaderColor string         // Used for the landing page header.
 	CSS         string         // CSS style tag for the landing page.
 	Name        string         // The name of the exporter, generally suffixed by _exporter.
@@ -62,6 +64,7 @@ type LandingLinks struct {
 
 type LandingPageHandler struct {
 	landingPage []byte
+	routePrefix string
 }
 
 var (
@@ -93,6 +96,15 @@ func NewLandingPage(c LandingConfig) (*LandingPageHandler, error) {
 		}
 		c.CSS = buf.String()
 	}
+	if c.RoutePrefix == "" {
+		c.RoutePrefix = "/"
+	} else if !strings.HasSuffix(c.RoutePrefix, "/") {
+		c.RoutePrefix += "/"
+	}
+	// Strip leading '/' from Links if present
+	for i, link := range c.Links {
+		c.Links[i].Address = strings.TrimPrefix(link.Address, "/")
+	}
 	t := template.Must(template.New("landing page").Parse(landingPagehtmlContent))
 
 	buf.Reset()
@@ -102,11 +114,12 @@ func NewLandingPage(c LandingConfig) (*LandingPageHandler, error) {
 
 	return &LandingPageHandler{
 		landingPage: buf.Bytes(),
+		routePrefix: c.RoutePrefix,
 	}, nil
 }
 
 func (h *LandingPageHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	if r.URL.Path != "/" {
+	if r.URL.Path != h.routePrefix {
 		http.NotFound(w, r)
 		return
 	}

web/landing_page.html

@@ -15,13 +15,13 @@ <h1>{{.Name}}</h1>
       <div>
         <ul>
           {{ range .Links }}
-          <li><a href="{{ .Address }}">{{.Text}}</a>{{if .Description}}: {{.Description}}{{end}}</li>
+          <li><a href="{{if $.RoutePrefix}}{{ $.RoutePrefix }}{{end}}{{ .Address }}">{{.Text}}</a>{{if .Description}}: {{.Description}}{{end}}</li>
           {{ end }}
         </ul>
       </div>
       {{ if .Form.Action }}
       <div>
-      <form action="{{ .Form.Action}}">
+      <form action="{{if $.RoutePrefix}}{{ $.RoutePrefix }}{{end}}{{ .Form.Action }}">
       {{ range .Form.Inputs }}
       <label>{{ .Label }}:</label>&nbsp;<input type="{{ .Type }}" name="{{ .Name }}" placeholder="{{ .Placeholder }}" value="{{ .Value }}"><br>
       {{ end }}
@@ -33,8 +33,8 @@ <h1>{{.Name}}</h1>
       <div id="pprof">
       Download a detailed report of resource usage (pprof format, from the Go runtime):
       <ul>
-        <li><a href="debug/pprof/heap">heap usage (memory)</a>
-        <li><a href="debug/pprof/profile?seconds=60">CPU usage (60 second profile)</a>
+        <li><a href="{{if $.RoutePrefix}}{{ $.RoutePrefix }}{{end}}debug/pprof/heap">heap usage (memory)</a>
+        <li><a href="{{if $.RoutePrefix}}{{ $.RoutePrefix }}{{end}}debug/pprof/profile?seconds=60">CPU usage (60 second profile)</a>
       </ul>
       To visualize and share profiles you can upload to <a href="https://pprof.me" target="_blank">pprof.me</a>
       </div>