add go templating engine for error page and make errors more clear (#260)

ping #199
closes #213

Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/260
Co-authored-by: crapStone <crapstone01@gmail.com>
Co-committed-by: crapStone <crapstone01@gmail.com>

html/html.go

@@ -1,9 +1,53 @@
 package html
 
-import _ "embed"
+import (
+	_ "embed"
+	"net/http"
+	"text/template" // do not use html/template here, we sanitize the message before passing it to the template
 
-//go:embed 404.html
-var NotFoundPage string
+	"codeberg.org/codeberg/pages/server/context"
+	"github.com/microcosm-cc/bluemonday"
+	"github.com/rs/zerolog/log"
+)
 
-//go:embed error.html
-var ErrorPage string
+//go:embed templates/error.html
+var errorPage string
+
+var (
+	errorTemplate = template.Must(template.New("error").Parse(errorPage))
+	sanitizer     = createBlueMondayPolicy()
+)
+
+type TemplateContext struct {
+	StatusCode int
+	StatusText string
+	Message    string
+}
+
+// ReturnErrorPage sets the response status code and writes the error page to the response body.
+// The error page contains a sanitized version of the message and the statusCode both in text and numeric form.
+//
+// Currently, only the following html tags are supported: <code>
+func ReturnErrorPage(ctx *context.Context, msg string, statusCode int) {
+	ctx.RespWriter.Header().Set("Content-Type", "text/html; charset=utf-8")
+	ctx.RespWriter.WriteHeader(statusCode)
+
+	templateContext := TemplateContext{
+		StatusCode: statusCode,
+		StatusText: http.StatusText(statusCode),
+		Message:    sanitizer.Sanitize(msg),
+	}
+
+	err := errorTemplate.Execute(ctx.RespWriter, templateContext)
+	if err != nil {
+		log.Err(err).Str("message", msg).Int("status", statusCode).Msg("could not write response")
+	}
+}
+
+func createBlueMondayPolicy() *bluemonday.Policy {
+	p := bluemonday.NewPolicy()
+
+	p.AllowElements("code")
+
+	return p
+}