go: add a 404 handler

main.go

@@ -2,9 +2,12 @@ package main
 
 import (
 	"embed"
+	"fmt"
 	"io/fs"
 	"log"
 	"net/http"
+	"os"
+	"path"
 	"time"
 )
 
@@ -13,18 +16,64 @@ var version = "development"
 //go:embed public/*
 var embeddedPublic embed.FS
 
+// bytes of the 404.html page.
+var b404 []byte
+
+// usrIP is a good way to read the src IP this way because we trust our proxy.
+// largely from: https://stackoverflow.com/a/55738279
+func usrIP(r *http.Request) string {
+	ip := r.Header.Get("x-forwarded-for")
+	if ip == "" {
+		ip = r.Header.Get("x-real-ip")
+	}
+
+	if ip == "" {
+		ip = r.RemoteAddr
+	}
+
+	return ip
+}
+
+// notFound writes back 404 and the 404 page.
+func notFound(w http.ResponseWriter, r *http.Request) {
+	w.WriteHeader(http.StatusNotFound)
+	w.Write(b404)
+}
+
+// handleNotFound allows us to override the response on e.g. 404.
+// inspired by https://stackoverflow.com/a/62747667
+func handleNotFound(fs http.FileSystem) http.Handler {
+	fileServer := http.FileServer(fs)
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		p := r.URL.Path
+		// so as not to allow path traversals.
+		cleanedPath := path.Clean(p)
+		_, err := fs.Open(cleanedPath)
+		if os.IsNotExist(err) {
+			log.Printf("Error 404 Not Found when serving path: %s, cleaned path: %s, IP+port: %s",
+				p, cleanedPath, usrIP(r))
+			notFound(w, r)
+			return
+		}
+		fileServer.ServeHTTP(w, r)
+	})
+}
+
 func main() {
 	// TODO: ENV WHATPORT
-	// TODO: handler for / + 404
+	// TODO: add /ip endpoint that returns the src IP.
+	f404, err := embeddedPublic.ReadFile("public/404.html")
+	if err != nil {
+		log.Fatalf("no 404.html in the folder, weird: %s", fmt.Errorf("err: %w", err))
+	}
+
+	b404 = f404
+
 	root, err := fs.Sub(embeddedPublic, "public")
 	if err != nil {
 		log.Fatal(err)
 	}
 
-	fs := http.FileServer(http.FS(root))
-
-	http.Handle("/", fs)
-
 	log.Printf("app built from revision '%s'\n", version)
 	log.Print("Listening on :1314...")
 
@@ -33,7 +82,7 @@ func main() {
 		ReadTimeout:  15 * time.Second,
 		WriteTimeout: 15 * time.Second,
 		Addr:         ":1314",
-		Handler:      nil,
+		Handler:      handleNotFound(http.FS(root)),
 	}
 
 	err = srv.ListenAndServe()