Ability to serve exporters on route prefix with built in support for …

…serving the pprof endpoints accessible on the landing page

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

web/landing_page.go

@@ -21,23 +21,40 @@ package web
 import (
 	"bytes"
 	_ "embed"
+	"errors"
+	"fmt"
+	"net"
 	"net/http"
+	"net/http/pprof"
+	"net/url"
+	"os"
+	"path"
 	"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.
-	Description string         // A short description about the exporter.
-	Form        LandingForm    // A POST form.
-	Links       []LandingLinks // Links displayed on the landing page.
-	ExtraHTML   string         // Additional HTML to be embedded.
-	ExtraCSS    string         // Additional CSS to be embedded.
-	Version     string         // The version displayed.
+	RoutePrefix      string         // The route prefix for the exporter.
+	ExternalURL      string         // The external URL for the exporter.
+	ListenAddresses  []string       // The listen address for the exporter.
+	UseSystemdSocket bool           // Use systemd socket.
+	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.
+	Description      string         // A short description about the exporter.
+	Form             LandingForm    // A POST form.
+	Links            []LandingLinks // Links displayed on the landing page.
+	ExtraHTML        string         // Additional HTML to be embedded.
+	ExtraCSS         string         // Additional CSS to be embedded.
+	Version          string         // The version displayed.
+	Logger           Logger         // Logging interface
+}
+
+type Logger interface {
+	Error(msg string, keysAndValues ...interface{})
+	Info(msg string, keysAndValues ...interface{})
+	Debug(msg string, keysAndValues ...interface{})
 }
 
 // LandingForm provides a configuration struct for creating a POST form on the landing page.
@@ -65,6 +82,7 @@ type LandingLinks struct {
 type LandingPageHandler struct {
 	landingPage []byte
 	routePrefix string
+	pprofMux    *http.ServeMux
 }
 
 var (
@@ -74,16 +92,97 @@ var (
 	landingPagecssContent string
 )
 
-func NewLandingPage(c LandingConfig) (*LandingPageHandler, error) {
+func NewLandingPage(c LandingConfig) (*LandingPageHandler, string, error) {
 	var buf bytes.Buffer
 
+	// Setup URL and Prefix logic
+	if c.ExternalURL == "" && c.UseSystemdSocket {
+		return nil, "", fmt.Errorf("cannot automatically infer external URL with systemd socket listener")
+	}
+
+	if c.ExternalURL == "" && len(c.ListenAddresses) > 1 {
+		c.Logger.Info("Inferring external URL from first provided listen address")
+	}
+
+	if len(c.ListenAddresses) == 0 {
+		return nil, "", fmt.Errorf("no listen addresses provided")
+	}
+
+	// Compute external URL
+	var parsedExternalURL *url.URL
+	var err error
+	if c.ExternalURL == "" {
+		hostname, err := os.Hostname()
+		if err != nil {
+			return nil, "", err
+		}
+		_, port, err := net.SplitHostPort(c.ListenAddresses[0])
+		if err != nil {
+			return nil, "", err
+		}
+		c.ExternalURL = fmt.Sprintf("http://%s:%s/", hostname, port)
+	}
+
+	if strings.HasPrefix(c.ExternalURL, "\"") || strings.HasPrefix(c.ExternalURL, "'") ||
+		strings.HasSuffix(c.ExternalURL, "\"") || strings.HasSuffix(c.ExternalURL, "'") {
+		return nil, "", errors.New("URL must not begin or end with quotes")
+	}
+
+	parsedExternalURL, err = url.Parse(c.ExternalURL)
+	if err != nil {
+		return nil, "", fmt.Errorf("failed to determine external URL: %w", err)
+	}
+
+	// Ensure Path component of ExternalURL is formatted without trailing slashes,
+	// contains a leading slash, and is not empty
+	pathPrefix := strings.TrimRight(parsedExternalURL.Path, "/")
+	if pathPrefix != "" && !strings.HasPrefix(pathPrefix, "/") {
+		pathPrefix = "/" + pathPrefix
+	}
+	parsedExternalURL.Path = pathPrefix
+
+	if c.RoutePrefix == "" {
+		c.RoutePrefix = parsedExternalURL.Path
+		c.Logger.Info("RoutePrefix is empty, defaulting to ExternalURL path", "url", parsedExternalURL.Path)
+	} else {
+		c.Logger.Info("RoutePrefix is set", "RoutePrefix", c.RoutePrefix)
+	}
+
+	c.RoutePrefix = "/" + strings.Trim(c.RoutePrefix, "/")
+	if c.RoutePrefix != "/" {
+		c.RoutePrefix += "/"
+	}
+
+	if c.RoutePrefix == "" {
+		c.RoutePrefix = "/"
+	} else if !strings.HasSuffix(c.RoutePrefix, "/") {
+		c.RoutePrefix += "/"
+	}
+
+	// Validate RoutePrefix
+	if !strings.HasPrefix(c.RoutePrefix, "/") {
+		return nil, "", fmt.Errorf("route prefix must start with '/'")
+	}
+
+	// Redirect over externalURL for root path only if routePrefix is different from "/"
+	if c.RoutePrefix != "/" {
+		http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+			if r.URL.Path != "/" {
+				http.NotFound(w, r)
+				return
+			}
+			http.Redirect(w, r, parsedExternalURL.String(), http.StatusFound)
+		})
+	}
+
 	length := 0
 	for _, input := range c.Form.Inputs {
 		inputLength := len(input.Label)
 		if inputLength > length {
 			length = inputLength
 		}
 	}
+
 	c.Form.Width = (float64(length) + 1) / 2
 	if c.CSS == "" {
 		if c.HeaderColor == "" {
@@ -92,15 +191,11 @@ func NewLandingPage(c LandingConfig) (*LandingPageHandler, error) {
 		}
 		cssTemplate := template.Must(template.New("landing css").Parse(landingPagecssContent))
 		if err := cssTemplate.Execute(&buf, c); err != nil {
-			return nil, err
+			return nil, "", err
 		}
 		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, "/")
@@ -109,16 +204,26 @@ func NewLandingPage(c LandingConfig) (*LandingPageHandler, error) {
 
 	buf.Reset()
 	if err := t.Execute(&buf, c); err != nil {
-		return nil, err
+		return nil, "", err
 	}
 
+	// Create a new ServeMux for pprof endpoints in the LandingPage
+	pprofMux := http.NewServeMux()
+	pprofMux.HandleFunc(path.Join(c.RoutePrefix, "debug/pprof/profile"), pprof.Profile)
+	pprofMux.HandleFunc(path.Join(c.RoutePrefix, "debug/pprof/heap"), pprof.Handler("heap").ServeHTTP)
+
 	return &LandingPageHandler{
 		landingPage: buf.Bytes(),
 		routePrefix: c.RoutePrefix,
-	}, nil
+		pprofMux:    pprofMux,
+	}, c.RoutePrefix, nil
 }
 
 func (h *LandingPageHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if strings.HasPrefix(r.URL.Path, path.Join(h.routePrefix, "debug/pprof/")) {
+		h.pprofMux.ServeHTTP(w, r)
+		return
+	}
 	if r.URL.Path != h.routePrefix {
 		http.NotFound(w, r)
 		return