allow overriding cache directives in responses

Add a new `-minCacheDuration` flag to specify a minimum duration to
cache images for.

Updates #28
Updates #144
Fixes #207
Fixes #208

.gitignore

@@ -0,0 +1,2 @@
+.cache
+imageproxy

README.md

@@ -184,6 +184,18 @@ first check an in-memory cache for an image, followed by a gcs bucket:
 
 [tiered fashion]: https://godoc.org/github.com/die-net/lrucache/twotier
 
+#### Cache Duration
+
+By default, images are cached for the duration specified in response headers.
+If an image has no cache directives, or an explicit `Cache-Control: no-cache` header,
+then the response is not cached.
+
+To override the response cache directives, set a minimum time that response should be cached for.
+This will ignore `no-cache` and `no-store` directives, and will set `max-age`
+to the specified value if it is greater than the original `max-age` value.
+
+    imageproxy -cache /tmp/imageproxy -minCacheDuration 5m
+
 ### Allowed Referrer List
 
 You can limit images to only be accessible for certain hosts in the HTTP

cmd/imageproxy/main.go

@@ -46,6 +46,7 @@ var verbose = flag.Bool("verbose", false, "print verbose logging messages")
 var _ = flag.Bool("version", false, "Deprecated: this flag does nothing")
 var contentTypes = flag.String("contentTypes", "image/*", "comma separated list of allowed content types")
 var userAgent = flag.String("userAgent", "willnorris/imageproxy", "specify the user-agent used by imageproxy when fetching images from origin website")
+var minCacheDuration = flag.Duration("minCacheDuration", 0, "minimum duration to cache remote images")
 
 func init() {
 	flag.Var(&cache, "cache", "location to cache images (see https://github.com/willnorris/imageproxy#cache)")
@@ -87,6 +88,7 @@ func main() {
 	p.ScaleUp = *scaleUp
 	p.Verbose = *verbose
 	p.UserAgent = *userAgent
+	p.MinimumCacheDuration = *minCacheDuration
 
 	server := &http.Server{
 		Addr:    *addr,

imageproxy.go

@@ -28,6 +28,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	tphttp "willnorris.com/go/imageproxy/third_party/http"
+	tphc "willnorris.com/go/imageproxy/third_party/httpcache"
 )
 
 // Maximum number of redirection-followings allowed.
@@ -91,6 +92,10 @@ type Proxy struct {
 	// PassRequestHeaders identifies HTTP headers to pass from inbound
 	// requests to the proxied server.
 	PassRequestHeaders []string
+
+	// MinimumCacheDuration is the minimum duration to cache remote images.
+	// This will override cache-control instructions from the remote server.
+	MinimumCacheDuration time.Duration
 }
 
 // NewProxy constructs a new proxy.  The provided http RoundTripper will be
@@ -118,6 +123,7 @@ func NewProxy(transport http.RoundTripper, cache Cache) *Proxy {
 					proxy.logf(format, v...)
 				}
 			},
+			updateCacheHeaders: proxy.updateCacheHeaders,
 		},
 		Cache:               cache,
 		MarkCachedResponses: true,
@@ -128,6 +134,39 @@ func NewProxy(transport http.RoundTripper, cache Cache) *Proxy {
 	return proxy
 }
 
+// updateCacheHeaders updates the cache-control headers in the provided headers.
+// It sets the cache-control max-age value to the maximum of the minimum cache
+// duration, the expires header, and the max-age header.  It also removes the
+// expires header.
+func (p *Proxy) updateCacheHeaders(hdr http.Header) {
+	if p.MinimumCacheDuration == 0 {
+		return
+	}
+	cc := tphc.ParseCacheControl(hdr)
+
+	var expiresDuration time.Duration
+	var maxAgeDuration time.Duration
+
+	if maxAge, ok := cc["max-age"]; ok {
+		maxAgeDuration, _ = time.ParseDuration(maxAge + "s")
+	}
+	if date, err := httpcache.Date(hdr); err == nil {
+		if expiresHeader := hdr.Get("Expires"); expiresHeader != "" {
+			if expires, err := time.Parse(time.RFC1123, expiresHeader); err == nil {
+				expiresDuration = expires.Sub(date)
+			}
+		}
+	}
+
+	maxAge := max(p.MinimumCacheDuration, expiresDuration, maxAgeDuration)
+	cc["max-age"] = fmt.Sprintf("%d", int(maxAge.Seconds()))
+	delete(cc, "no-cache")
+	delete(cc, "no-store")
+
+	hdr.Set("Cache-Control", cc.String())
+	hdr.Del("Expires")
+}
+
 // ServeHTTP handles incoming requests.
 func (p *Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if r.URL.Path == "/favicon.ico" {
@@ -475,6 +514,8 @@ type TransformingTransport struct {
 	CachingClient *http.Client
 
 	log func(format string, v ...any)
+
+	updateCacheHeaders func(hdr http.Header)
 }
 
 // RoundTrip implements the http.RoundTripper interface.
@@ -484,7 +525,11 @@ func (t *TransformingTransport) RoundTrip(req *http.Request) (*http.Response, er
 		if t.log != nil {
 			t.log("fetching remote URL: %v", req.URL)
 		}
-		return t.Transport.RoundTrip(req)
+		resp, err := t.Transport.RoundTrip(req)
+		if err == nil && t.updateCacheHeaders != nil {
+			t.updateCacheHeaders(resp.Header)
+		}
+		return resp, err
 	}
 
 	f := req.URL.Fragment

imageproxy_test.go

@@ -12,6 +12,7 @@ import (
 	"image"
 	"image/png"
 	"log"
+	"maps"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -21,6 +22,7 @@ import (
 	"strconv"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestPeekContentType(t *testing.T) {
@@ -368,6 +370,108 @@ func (t testTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	return http.ReadResponse(buf, req)
 }
 
+func TestProxy_UpdateCacheHeaders(t *testing.T) {
+	date := "Mon, 02 Jan 2006 15:04:05 MST"
+	exp := "Mon, 02 Jan 2006 16:04:05 MST"
+
+	tests := []struct {
+		name        string
+		minDuration time.Duration
+		headers     http.Header
+		want        http.Header
+	}{
+		{
+			name:    "zero",
+			headers: http.Header{},
+			want:    http.Header{},
+		},
+		{
+			name: "no min duration",
+			headers: http.Header{
+				"Date":          {date},
+				"Expires":       {exp},
+				"Cache-Control": {"max-age=600"},
+			},
+			want: http.Header{
+				"Date":          {date},
+				"Expires":       {exp},
+				"Cache-Control": {"max-age=600"},
+			},
+		},
+		{
+			name:        "cache control exceeds min duration",
+			minDuration: 30 * time.Second,
+			headers: http.Header{
+				"Cache-Control": {"max-age=600"},
+			},
+			want: http.Header{
+				"Cache-Control": {"max-age=600"},
+			},
+		},
+		{
+			name:        "cache control exceeds min duration, expires",
+			minDuration: 30 * time.Second,
+			headers: http.Header{
+				"Date":          {date},
+				"Expires":       {exp},
+				"Cache-Control": {"max-age=86400"},
+			},
+			want: http.Header{
+				"Date":          {date},
+				"Cache-Control": {"max-age=86400"},
+			},
+		},
+		{
+			name:        "min duration exceeds cache control",
+			minDuration: 1 * time.Hour,
+			headers: http.Header{
+				"Cache-Control": {"max-age=600"},
+			},
+			want: http.Header{
+				"Cache-Control": {"max-age=3600"},
+			},
+		},
+		{
+			name:        "min duration exceeds cache control, expires",
+			minDuration: 2 * time.Hour,
+			headers: http.Header{
+				"Date":          {date},
+				"Expires":       {exp},
+				"Cache-Control": {"max-age=600"},
+			},
+			want: http.Header{
+				"Date":          {date},
+				"Cache-Control": {"max-age=7200"},
+			},
+		},
+		{
+			name:        "expires exceeds min duration, cache control",
+			minDuration: 30 * time.Minute,
+			headers: http.Header{
+				"Date":          {date},
+				"Expires":       {exp},
+				"Cache-Control": {"max-age=600"},
+			},
+			want: http.Header{
+				"Date":          {date},
+				"Cache-Control": {"max-age=3600"},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			p := &Proxy{MinimumCacheDuration: tt.minDuration}
+			hdr := maps.Clone(tt.headers)
+			p.updateCacheHeaders(hdr)
+
+			if !reflect.DeepEqual(hdr, tt.want) {
+				t.Errorf("updateCacheHeaders(%v) returned %v, want %v", tt.headers, hdr, tt.want)
+			}
+		})
+	}
+}
+
 func TestProxy_ServeHTTP(t *testing.T) {
 	p := &Proxy{
 		Client: &http.Client{

third_party/httpcache/httpcache.go

@@ -5,10 +5,10 @@ import (
 	"strings"
 )
 
-type cacheControl map[string]string
+type CacheControl map[string]string
 
-func parseCacheControl(headers http.Header) cacheControl {
-	cc := cacheControl{}
+func ParseCacheControl(headers http.Header) CacheControl {
+	cc := CacheControl{}
 	ccHeader := headers.Get("Cache-Control")
 	for _, part := range strings.Split(ccHeader, ",") {
 		part = strings.Trim(part, " ")
@@ -24,3 +24,15 @@ func parseCacheControl(headers http.Header) cacheControl {
 	}
 	return cc
 }
+
+func (cc CacheControl) String() string {
+	parts := make([]string, 0, len(cc))
+	for k, v := range cc {
+		if v == "" {
+			parts = append(parts, k)
+		} else {
+			parts = append(parts, k+"="+v)
+		}
+	}
+	return strings.Join(parts, ", ")
+}