Keep max-age within an expiring image URL's lifetime (closes #63)

Both image routes sent Cache-Control: public, max-age=31536000,
immutable unconditionally, so a browser or proxy could keep serving an
image for a year after its signed or encrypted URL had expired. max-age
is now the whole seconds left until the URL expires, never negative and
at most one year; a URL with no expiry keeps one year. The 304 answer
uses the same value. An encrypted URL's expiry now reaches
ImageRequest.Expires through ToImageRequest. immutable stays: freshness
now ends no later than the URL's expiry. README.md documents the header.

Model: opus-5-5

internal/handlers/image_cache_control_internal_test.go

@@ -0,0 +1,203 @@
+package handlers
+
+import (
+	"image/color"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strconv"
+	"strings"
+	"testing"
+	"testing/fstest"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// photoPath is the path of the JPEG that newSignedHostServer serves.
+const photoPath = "/images/photo.jpg"
+
+// newSignedHostServer returns a router for both image routes, and the Handlers
+// behind it, whose fetcher serves a JPEG at photoPath on signedHost. signedHost
+// is not on the allowlist, so a /v1/image/ URL for it is served only with a
+// valid signature.
+func newSignedHostServer(t *testing.T) (*Handlers, http.Handler) {
+	t.Helper()
+
+	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.CacheConfig{
+		StateDir:    t.TempDir(),
+		CacheTTL:    time.Hour,
+		NegativeTTL: 5 * time.Minute,
+	})
+	if err != nil {
+		t.Fatalf("imgcache.NewCache() error = %v", err)
+	}
+
+	jpegData := generateTestJPEG(t, 100, 100, color.RGBA{255, 0, 0, 255})
+
+	svc, err := imgcache.NewService(&imgcache.ServiceConfig{
+		Cache: cache,
+		Fetcher: newMockFetcher(fstest.MapFS{
+			signedHost + photoPath: &fstest.MapFile{Data: jpegData},
+		}),
+		SigningKey: testSigningKey,
+	})
+	if err != nil {
+		t.Fatalf("imgcache.NewService() error = %v", err)
+	}
+
+	encGen, err := encurl.NewGenerator(testSigningKey)
+	if err != nil {
+		t.Fatalf("encurl.NewGenerator() error = %v", err)
+	}
+
+	h := &Handlers{
+		log:    slog.New(slog.DiscardHandler),
+		imgSvc: svc,
+		encGen: encGen,
+	}
+
+	r := chi.NewRouter()
+	r.Get("/v1/image/*", h.HandleImage())
+	r.Get("/v1/e/{token}/*", h.HandleImageEnc())
+
+	return h, r
+}
+
+// getMaxAge sends a GET for target to srv, requires a 200, and returns the
+// max-age of the response's Cache-Control header, which must read
+// "public, max-age=<seconds>, immutable".
+func getMaxAge(t *testing.T, srv http.Handler, target string) int {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, target, nil)
+	rec := httptest.NewRecorder()
+
+	srv.ServeHTTP(rec, req)
+
+	header := rec.Header().Get("Cache-Control")
+	t.Logf("GET %s: %d, Cache-Control: %s", target, rec.Code, header)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	value, hasPrefix := strings.CutPrefix(header, "public, max-age=")
+	value, hasSuffix := strings.CutSuffix(value, ", immutable")
+
+	maxAge, err := strconv.Atoi(value)
+	if !hasPrefix || !hasSuffix || err != nil {
+		t.Fatalf("Cache-Control = %q, want public, max-age=<seconds>, immutable",
+			header)
+	}
+
+	return maxAge
+}
+
+// TestHandleImage_SignedURL_MaxAgeEndsAtExp verifies that an image served
+// through a signed URL expiring in 60 seconds may be cached for at most those
+// 60 seconds. The lower bound of 50 shows the max-age is the time left, not 0.
+func TestHandleImage_SignedURL_MaxAgeEndsAtExp(t *testing.T) {
+	t.Parallel()
+
+	h, srv := newSignedHostServer(t)
+
+	signedURL, err := h.imgSvc.GenerateSignedURL("", &imgcache.ImageRequest{
+		SourceHost: signedHost,
+		SourcePath: photoPath,
+		Size:       imgcache.Size{Width: 50, Height: 50},
+		Format:     imgcache.FormatJPEG,
+	}, time.Minute)
+	if err != nil {
+		t.Fatalf("GenerateSignedURL() error = %v", err)
+	}
+
+	maxAge := getMaxAge(t, srv, signedURL)
+	if maxAge < 50 || maxAge > 60 {
+		t.Errorf("max-age = %d, want 50 to 60", maxAge)
+	}
+}
+
+// TestHandleImage_AllowlistedHost_MaxAge verifies the max-age of an image from
+// an allowlisted host, which is served without checking sig or exp. A URL with
+// no exp may be cached for a year. A URL whose exp has passed is the one request
+// that reaches the header after its expiry, and must get 0, never less.
+func TestHandleImage_AllowlistedHost_MaxAge(t *testing.T) {
+	t.Parallel()
+
+	pastExp := strconv.FormatInt(time.Now().Add(-time.Hour).Unix(), 10)
+
+	tests := []struct {
+		name       string
+		query      string
+		wantMaxAge int
+	}{
+		{"no exp", "", 31536000},
+		{"exp already past", "?exp=" + pastExp, 0},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			fix := setupTestHandler(t)
+
+			r := chi.NewRouter()
+			r.Get("/v1/image/*", fix.handler.HandleImage())
+
+			maxAge := getMaxAge(t, r,
+				"/v1/image/"+fix.goodHost+"/images/photo.jpg/50x50.jpeg"+tt.query)
+			if maxAge != tt.wantMaxAge {
+				t.Errorf("max-age = %d, want %d", maxAge, tt.wantMaxAge)
+			}
+		})
+	}
+}
+
+// TestHandleImageEnc_MaxAge verifies that an image served through an encrypted
+// URL with a 60 second TTL may be cached for at most those 60 seconds, that one
+// with a two-year TTL may be cached for a year, and that one made without a
+// TTL, which never expires, may be cached for a year.
+func TestHandleImageEnc_MaxAge(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		expiresAt   int64
+		wantAtLeast int
+		wantAtMost  int
+	}{
+		{"60 second TTL", time.Now().Add(time.Minute).Unix(), 50, 60},
+		{"two-year TTL", time.Now().Add(2 * 365 * 24 * time.Hour).Unix(), 31536000, 31536000},
+		{"no TTL", 0, 31536000, 31536000},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			h, srv := newSignedHostServer(t)
+
+			token, err := h.encGen.Generate(&encurl.Payload{
+				SourceHost: signedHost,
+				SourcePath: photoPath,
+				Width:      50,
+				Height:     50,
+				Format:     imgcache.FormatJPEG,
+				ExpiresAt:  tt.expiresAt,
+			})
+			if err != nil {
+				t.Fatalf("Generate() error = %v", err)
+			}
+
+			maxAge := getMaxAge(t, srv, "/v1/e/"+token+"/img.jpg")
+			if maxAge < tt.wantAtLeast || maxAge > tt.wantAtMost {
+				t.Errorf("max-age = %d, want %d to %d",
+					maxAge, tt.wantAtLeast, tt.wantAtMost)
+			}
+		})
+	}
+}