Feature 10: Recipe grounding and photos (#78)

Co-authored-by: dev_agent <dev_agent@cubecraftlabs.com>

internal/server/media.go

@@ -0,0 +1,75 @@
+package server
+
+import (
+	"context"
+	"errors"
+	"io/fs"
+	"log/slog"
+	"net/http"
+	"os"
+	"strconv"
+	"strings"
+
+	"github.com/go-chi/chi/v5"
+)
+
+// photoCacheControl lets the browser keep a photo for a year without
+// revalidating: a photo's files never change once stored, and a new image
+// gets a new id.
+const photoCacheControl = "private, max-age=31536000, immutable"
+
+// PhotoFiles opens a cached photo's file at one of its stored sizes. An
+// unknown photo or size is an error matching fs.ErrNotExist.
+type PhotoFiles interface {
+	OpenPhoto(ctx context.Context, id string, size int) (*os.File, error)
+}
+
+// getPhoto answers GET /media/photos/{id}-{size}.jpg with the stored JPEG.
+func getPhoto(logger *slog.Logger, photos PhotoFiles) http.HandlerFunc {
+	notFound := apiErrorHandler(logger, http.StatusNotFound, "not_found", "not found")
+	return func(w http.ResponseWriter, r *http.Request) {
+		id, size, ok := parsePhotoName(chi.URLParam(r, "name"))
+		if !ok {
+			notFound(w, r)
+			return
+		}
+		f, err := photos.OpenPhoto(r.Context(), id, size)
+		if errors.Is(err, fs.ErrNotExist) {
+			notFound(w, r)
+			return
+		}
+		if err != nil {
+			logger.ErrorContext(r.Context(), "open photo", "id", id, "size", size, "error", err)
+			writeError(w, r, logger, http.StatusInternalServerError, "internal", "internal error")
+			return
+		}
+		defer f.Close()
+		info, err := f.Stat()
+		if err != nil {
+			logger.ErrorContext(r.Context(), "stat photo", "id", id, "size", size, "error", err)
+			writeError(w, r, logger, http.StatusInternalServerError, "internal", "internal error")
+			return
+		}
+		w.Header().Set("Content-Type", "image/jpeg")
+		w.Header().Set("Cache-Control", photoCacheControl)
+		http.ServeContent(w, r, "", info.ModTime(), f)
+	}
+}
+
+// parsePhotoName splits "{id}-{size}.jpg" into its id and size, refusing
+// a size not written in its canonical decimal form.
+func parsePhotoName(name string) (id string, size int, ok bool) {
+	stem, ok := strings.CutSuffix(name, ".jpg")
+	if !ok {
+		return "", 0, false
+	}
+	i := strings.LastIndexByte(stem, '-')
+	if i < 0 {
+		return "", 0, false
+	}
+	size, err := strconv.Atoi(stem[i+1:])
+	if err != nil || strconv.Itoa(size) != stem[i+1:] {
+		return "", 0, false
+	}
+	return stem[:i], size, true
+}