Add error reponses and render helpers

backend/web/error.go

@@ -1,25 +1,44 @@
 package web
 
 import (
+	"io"
 	"net/http"
 
 	"github.com/theandrew168/bloggulus/backend/web/page"
 )
 
-func HandleNotFoundPage() http.Handler {
+func NotFoundResponse(w http.ResponseWriter, r *http.Request) {
+	tmpl := page.NewError()
+
+	code := 404
+	data := page.ErrorData{
+		StatusCode: code,
+		StatusText: http.StatusText(code),
+		Message:    "Sorry, this page could not be found.",
+	}
+
+	RenderError(w, r, code, func(w io.Writer) error {
+		return tmpl.Render(w, data)
+	})
+}
+
+func InternalServerErrorResponse(w http.ResponseWriter, r *http.Request, err error) {
 	tmpl := page.NewError()
+
+	code := 500
+	data := page.ErrorData{
+		StatusCode: code,
+		StatusText: http.StatusText(code),
+		Message:    "Sorry, something went wrong.",
+	}
+
+	RenderError(w, r, code, func(w io.Writer) error {
+		return tmpl.Render(w, data)
+	})
+}
+
+func HandleNotFoundPage() http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		w.WriteHeader(404)
-
-		data := page.ErrorData{
-			StatusCode: 404,
-			StatusText: http.StatusText(404),
-			Message:    "This page could not be found.",
-		}
-		err := tmpl.Render(w, data)
-		if err != nil {
-			http.Error(w, err.Error(), 500)
-			return
-		}
+		NotFoundResponse(w, r)
 	})
 }

backend/web/render.go

@@ -0,0 +1,50 @@
+package web
+
+import (
+	"bytes"
+	"io"
+	"log/slog"
+	"net/http"
+)
+
+// Generic function type for rendering data to an io.Writer.
+type RenderFunc func(w io.Writer) error
+
+// Render an HTML template based on a given RenderFunc.
+func Render(w http.ResponseWriter, r *http.Request, code int, render RenderFunc) {
+	// Write the template to the buffer, instead of straight to the http.ResponseWriter.
+	// If there's an error, call our serverError() helper and then return.
+	var buf bytes.Buffer
+	err := render(&buf)
+	if err != nil {
+		InternalServerErrorResponse(w, r, err)
+		return
+	}
+
+	// If the template is written to the buffer without any errors, we are safe
+	// to go ahead and write the HTTP status code to http.ResponseWriter.
+	w.WriteHeader(code)
+
+	// Write the contents of the buffer to the http.ResponseWriter. Note: this
+	// is another time where we pass our http.ResponseWriter to a function that
+	// takes an io.Writer.
+	buf.WriteTo(w)
+}
+
+// Render an HTML error template based on a given RenderFunc. This function
+// differs from Render in that it doesn't call InternalServerErrorResponse
+// if something goes wrong since that could cause an infinite recursion.
+func RenderError(w http.ResponseWriter, r *http.Request, code int, render RenderFunc) {
+	var buf bytes.Buffer
+	err := render(&buf)
+	if err != nil {
+		slog.Error(err.Error())
+
+		code = http.StatusInternalServerError
+		http.Error(w, http.StatusText(code), code)
+		return
+	}
+
+	w.WriteHeader(code)
+	buf.WriteTo(w)
+}