Allow serving custom error page (#393)

It might be useful for those self-hosting a Codeberg Pages instance to be able to serve a different error page than the one embedded with go:embed

Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/393
Co-authored-by: Gnarwhal <git.aspect893@passmail.net>
Co-committed-by: Gnarwhal <git.aspect893@passmail.net>

README.md

@@ -84,6 +84,15 @@ This will trigger a build of the PR which will build a docker image to be used f
 - `PAGES_DOMAIN` (default: `codeberg.page`): main domain for pages.
 - `RAW_DOMAIN` (default: `raw.codeberg.page`): domain for raw resources (must be subdomain of `PAGES_DOMAIN`).
 
+### Custom Error Page
+
+A custom error page template can be served by creating `custom/error.html`.
+Data available to the template includes:
+
+- `{{ .StatusCode }}`: The HTTP status code (e.g. 404)
+- `{{ .StatusText }}`: The textual name associated with the status code (e.g. Not Found)
+- `{{ .Message }}`: The reason for the error
+
 ## Contributing to the development
 
 The Codeberg team is very open to your contribution.

flake.lock

@@ -5,11 +5,11 @@
         "systems": "systems"
       },
       "locked": {
-        "lastModified": 1726560853,
-        "narHash": "sha256-X6rJYSESBVr3hBoH0WbKE5KvhPU5bloyZ2L4K60/fPQ=",
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
         "owner": "numtide",
         "repo": "flake-utils",
-        "rev": "c1dfcf08411b08f6b8615f7d8971a2bfa81d5e8a",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
         "type": "github"
       },
       "original": {
@@ -20,8 +20,8 @@
     "nixpkgs": {
       "locked": {
         "lastModified": 0,
-        "narHash": "sha256-8fnFlXBgM/uSvBlLWjZ0Z0sOdRBesyNdH0+esxqizGc=",
-        "path": "/nix/store/rqicl40jqw5d93l43s1xq0bm6yy0gpx4-source",
+        "narHash": "sha256-x07g4NcqGP6mQn6AISXJaks9sQYDjZmTMBlKIvajvyc=",
+        "path": "/nix/store/2w8kz6zh3aq80f1dypiin222fry1rv51-source",
         "type": "path"
       },
       "original": {

html/html.go

@@ -3,6 +3,8 @@ package html
 import (
 	_ "embed"
 	"net/http"
+	"os"
+	"path"
 	"text/template" // do not use html/template here, we sanitize the message before passing it to the template
 
 	"codeberg.org/codeberg/pages/server/context"
@@ -14,7 +16,7 @@ import (
 var errorPage string
 
 var (
-	errorTemplate = template.Must(template.New("error").Parse(errorPage))
+	errorTemplate = template.Must(template.New("error").Parse(loadCustomTemplateOrDefault()))
 	sanitizer     = createBlueMondayPolicy()
 )
 
@@ -51,3 +53,19 @@ func createBlueMondayPolicy() *bluemonday.Policy {
 
 	return p
 }
+
+func loadCustomTemplateOrDefault() string {
+	contents, err := os.ReadFile("custom/error.html")
+	if err != nil {
+		if !os.IsNotExist(err) {
+			wd, wdErr := os.Getwd()
+			if wdErr != nil {
+				log.Err(err).Msg("could not load custom error page 'custom/error.html'")
+			} else {
+				log.Err(err).Msgf("could not load custom error page '%v'", path.Join(wd, "custom/error.html"))
+			}
+		}
+		return errorPage
+	}
+	return string(contents)
+}