fix: return a more-complete 304 from TransformingTransport.RoundTrip

to address the segfault we're seeing in production. The necessary
conditions for the issue are:

1. The 303 must return a location URI that changes every time, so that
   the real location of the image is never cached. As an example, if the
   remove service redirects to S3, the presence of the
   `X-Amz-Security-Token` accomplishes this.

2. The image responses must match by Etag, so that
   imageProxy.should304() returns true causing
   TranformingTransport.RoundTrip() to return a bare 304 response.

If those conditions are met, then the bare 304 Response returned will
be used and read by one of the callers. Specifically, the lack of a
Body causes a segfault.

So let's make it more like a real Response and use http.NoBody so when
it's used it doesn't cause things to explode.

imageproxy_test.go

@@ -23,6 +23,10 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/die-net/lrucache"
+	"github.com/google/uuid"
+	"github.com/gregjones/httpcache"
 )
 
 func TestPeekContentType(t *testing.T) {
@@ -332,9 +336,11 @@ func TestShould304(t *testing.T) {
 
 // testTransport is an http.RoundTripper that returns certained canned
 // responses for particular requests.
-type testTransport struct{}
+type testTransport struct {
+	replyNotModified bool
+}
 
-func (t testTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+func (t *testTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	var raw string
 
 	switch req.URL.Path {
@@ -352,6 +358,19 @@ func (t testTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 		_ = png.Encode(img, m)
 
 		raw = fmt.Sprintf("HTTP/1.1 200 OK\nContent-Length: %d\nContent-Type: image/png\n\n%s", len(img.Bytes()), img.Bytes())
+	case "/redirect-to-notmodified":
+		parts := []string{
+			"HTTP/1.1 303\nLocation: http://notmodified.test/notmodified?X-Security-Token=",
+			uuid.NewString(),
+			"#_=_\nCache-Control: no-store\n\n",
+		}
+		raw = strings.Join(parts, "")
+	case "/notmodified":
+		if t.replyNotModified {
+			raw = "HTTP/1.1 304 Not modified\nEtag: \"abcdef\"\n\n"
+		} else {
+			raw = "HTTP/1.1 200 OK\nEtag: \"abcdef\"\n\nOriginal response\n"
+		}
 	default:
 		redirectRegexp := regexp.MustCompile(`/redirects-(\d+)`)
 		if redirectRegexp.MatchString(req.URL.Path) {
@@ -526,7 +545,7 @@ func TestProxy_UpdateCacheHeaders(t *testing.T) {
 func TestProxy_ServeHTTP(t *testing.T) {
 	p := &Proxy{
 		Client: &http.Client{
-			Transport: testTransport{},
+			Transport: &testTransport{},
 		},
 		AllowHosts:   []string{"good.test"},
 		ContentTypes: []string{"image/*"},
@@ -564,7 +583,7 @@ func TestProxy_ServeHTTP(t *testing.T) {
 func TestProxy_ServeHTTP_is304(t *testing.T) {
 	p := &Proxy{
 		Client: &http.Client{
-			Transport: testTransport{},
+			Transport: &testTransport{},
 		},
 	}
 
@@ -581,10 +600,57 @@ func TestProxy_ServeHTTP_is304(t *testing.T) {
 	}
 }
 
+func TestProxy_ServeHTTP_cached304(t *testing.T) {
+	cache := lrucache.New(1024*1024*8, 0)
+	client := new(http.Client)
+	tt := testTransport{}
+	client.Transport = &httpcache.Transport{
+		Transport: &TransformingTransport{
+			Transport:     &tt,
+			CachingClient: client,
+		},
+		Cache:               cache,
+		MarkCachedResponses: true,
+	}
+
+	p := &Proxy{
+		Client:          client,
+		FollowRedirects: true,
+	}
+
+	// prime the cache
+	req := httptest.NewRequest("GET", "http://localhost//http://good.test/redirect-to-notmodified", nil)
+	recorder := httptest.NewRecorder()
+	p.ServeHTTP(recorder, req)
+
+	resp := recorder.Result()
+	if got, want := resp.StatusCode, http.StatusOK; got != want {
+		t.Errorf("ServeHTTP(%v) returned status %d, want %d", req, got, want)
+	}
+	if _, found := cache.Get("http://good.test/redirect-to-notmodified#0x0"); !found {
+		t.Errorf("Response to http://good.test/redirect-to-notmodified#0x0 should be cached")
+	}
+
+	// now make the same request again, but this time make sure the server responds with a 304
+	tt.replyNotModified = true
+	req = httptest.NewRequest("GET", "http://localhost//http://good.test/redirect-to-notmodified", nil)
+	recorder = httptest.NewRecorder()
+	p.ServeHTTP(recorder, req)
+
+	resp = recorder.Result()
+	if got, want := resp.StatusCode, http.StatusOK; got != want {
+		t.Errorf("ServeHTTP(%v) returned status %d, want %d", req, got, want)
+	}
+
+	if recorder.Body.String() != "Original response\n" {
+		t.Errorf("Response isn't what we expected: %v", recorder.Body.String())
+	}
+}
+
 func TestProxy_ServeHTTP_maxRedirects(t *testing.T) {
 	p := &Proxy{
 		Client: &http.Client{
-			Transport: testTransport{},
+			Transport: &testTransport{},
 		},
 		FollowRedirects: true,
 	}
@@ -658,7 +724,7 @@ func TestProxy_log_default(t *testing.T) {
 func TestTransformingTransport(t *testing.T) {
 	client := new(http.Client)
 	tr := &TransformingTransport{
-		Transport:     testTransport{},
+		Transport:     &testTransport{},
 		CachingClient: client,
 	}
 	client.Transport = tr