storage: Add Reader.CacheControl()

In order to proxy Google Cloud Storage efficiently, it's necessary
to have access to the Cache-Control header without making a separate
request.

Fixes #714

Change-Id: If1757c76909c78811fbe5333093c353120bb4ae9
Reviewed-on: https://code-review.googlesource.com/15330
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

storage/integration_test.go

@@ -364,6 +364,9 @@ func TestObjects(t *testing.T) {
 		if got, want := rc.ContentType(), "text/plain"; got != want {
 			t.Errorf("ContentType (%q) = %q; want %q", obj, got, want)
 		}
+		if got, want := rc.CacheControl(), "public, max-age=60"; got != want {
+			t.Errorf("CacheControl (%q) = %q; want %q", obj, got, want)
+		}
 		rc.Close()
 
 		// Check early close.
@@ -1448,6 +1451,7 @@ func TestIntegration_RequesterPays(t *testing.T) {
 func writeObject(ctx context.Context, obj *ObjectHandle, contentType string, contents []byte) error {
 	w := obj.NewWriter(ctx)
 	w.ContentType = contentType
+	w.CacheControl = "public, max-age=60"
 	if contents != nil {
 		if _, err := w.Write(contents); err != nil {
 			_ = w.Close()

storage/reader.go

@@ -28,6 +28,7 @@ type Reader struct {
 	body         io.ReadCloser
 	remain, size int64
 	contentType  string
+	cacheControl string
 	checkCRC     bool   // should we check the CRC?
 	wantCRC      uint32 // the CRC32c value the server sent in the header
 	gotCRC       uint32 // running crc
@@ -72,3 +73,8 @@ func (r *Reader) Remain() int64 {
 func (r *Reader) ContentType() string {
 	return r.contentType
 }
+
+// CacheControl returns the cache control of the object.
+func (r *Reader) CacheControl() string {
+	return r.cacheControl
+}

storage/storage.go

@@ -567,12 +567,13 @@ func (o *ObjectHandle) NewRangeReader(ctx context.Context, offset, length int64)
 		crc, checkCRC = parseCRC32c(res)
 	}
 	return &Reader{
-		body:        body,
-		size:        size,
-		remain:      remain,
-		contentType: res.Header.Get("Content-Type"),
-		wantCRC:     crc,
-		checkCRC:    checkCRC,
+		body:         body,
+		size:         size,
+		remain:       remain,
+		contentType:  res.Header.Get("Content-Type"),
+		cacheControl: res.Header.Get("Cache-Control"),
+		wantCRC:      crc,
+		checkCRC:     checkCRC,
 	}, nil
 }