Allow serving custom error page

html/html.go

@@ -4,6 +4,8 @@ import (
 	_ "embed"
 	"net/http"
 	"text/template" // do not use html/template here, we sanitize the message before passing it to the template
+	"os"
+	"path"
 
 	"codeberg.org/codeberg/pages/server/context"
 	"github.com/microcosm-cc/bluemonday"
@@ -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)
+}