From 3f4e85f1956827e4665b286455dd2171264947f2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Pawe=C5=82?= <pawel@bauer.net.pl>
Date: Wed, 9 Sep 2026 14:52:51 +0200
Subject: [PATCH] Serve a proper 404 error page for missing static files

http.FileServer previously fell through to the default net/http 404
response for unmatched static-asset requests. Intercepts that response
and redirects to Gokapi's own styled error page instead, undoing the
cache headers that were speculatively set for the (nonexistent) file
first.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0168YUwGEzEHTqd9CwzXE53d
---
 internal/webserver/Webserver.go      | 35 +++++++++++++++++++++++++---
 internal/webserver/Webserver_test.go |  8 +++++++
 2 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/internal/webserver/Webserver.go b/internal/webserver/Webserver.go
index 6b707e5b..2f832351 100644
--- a/internal/webserver/Webserver.go
+++ b/internal/webserver/Webserver.go
@@ -50,8 +50,6 @@ import (
 	"github.com/forceu/gokapi/internal/webserver/ssl"
 )
 
-// TODO add 404 handler
-
 // staticFolderEmbedded is the embedded version of the "static" folder
 // This contains JS files, CSS, images etc
 //
@@ -175,8 +173,39 @@ func filesystemHandler(webserverDir fs.FS) http.HandlerFunc {
 			return
 		}
 		addCacheHeader(w)
-		http.FileServer(http.FS(webserverDir)).ServeHTTP(w, r)
+		nfw := &notFoundInterceptor{ResponseWriter: w}
+		http.FileServer(http.FS(webserverDir)).ServeHTTP(nfw, r)
+		if nfw.intercepted {
+			// The response was cached as servable content above; a 404 means that
+			// assumption was wrong, so undo it before showing the error page.
+			w.Header().Del("cdn-cache-control")
+			w.Header().Del("Cloudflare-CDN-Cache-Control")
+			w.Header().Del("cache-control")
+			errorHandling.RedirectGenericErrorPage(w, r, errorHandling.TypeFileNotFound)
+		}
+	}
+}
+
+// notFoundInterceptor suppresses http.FileServer's built-in 404 response, so that a
+// styled Gokapi error page can be shown instead of the plain stdlib fallback page.
+type notFoundInterceptor struct {
+	http.ResponseWriter
+	intercepted bool
+}
+
+func (nfw *notFoundInterceptor) WriteHeader(statusCode int) {
+	if statusCode == http.StatusNotFound {
+		nfw.intercepted = true
+		return
+	}
+	nfw.ResponseWriter.WriteHeader(statusCode)
+}
+
+func (nfw *notFoundInterceptor) Write(b []byte) (int, error) {
+	if nfw.intercepted {
+		return len(b), nil
 	}
+	return nfw.ResponseWriter.Write(b)
 }
 
 func handleFavicon(w http.ResponseWriter, r *http.Request) {
diff --git a/internal/webserver/Webserver_test.go b/internal/webserver/Webserver_test.go
index f5e2bf8a..e6acd3f0 100644
--- a/internal/webserver/Webserver_test.go
+++ b/internal/webserver/Webserver_test.go
@@ -74,6 +74,14 @@ func TestStaticDirs(t *testing.T) {
 		RequiredContent: []string{".btn-secondary:hover"},
 	})
 }
+func TestStaticFileNotFound(t *testing.T) {
+	t.Parallel()
+	test.HttpPageResult(t, test.HttpTestConfig{
+		Url:                "http://localhost:53843/this/file/does/not/exist.xyz",
+		RedirectUrl:        "error",
+		IgnoreRedirectParm: true,
+	})
+}
 
 func postValues(username, password, csrf string) []test.PostBody {
 	return []test.PostBody{