Allow user agents to cache /diff responses

The /diff response is expensive to compute at the server side, and is
rather large to transfer to the client.  Allow user agents to cache
the resource locally for up to 7 days.

This should eventually allow navigation within a patch set to be
quicker as the browser will be able to load previously viewed files
from cache, saving round-trip time to the server.

Change-Id: I1c2f3e237a0c4c802564e5d5d87cd13d0d1b358f

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/restapi/Response.java

@@ -19,6 +19,10 @@ public abstract class Response<T> {
   @SuppressWarnings({"rawtypes"})
   private static final Response NONE = new None();
 
+  public enum CacheControl {
+    NONE, PUBLIC, PRIVATE;
+  }
+
   /** HTTP 200 OK: pointless wrapper for type safety. */
   public static <T> Response<T> ok(T value) {
     return new Impl<T>(200, value);
@@ -50,11 +54,14 @@ public abstract class Response<T> {
 
   public abstract int statusCode();
   public abstract T value();
+  public abstract CacheControl caching();
+  public abstract Response<T> caching(CacheControl c);
   public abstract String toString();
 
   private static final class Impl<T> extends Response<T> {
     private final int statusCode;
     private final T value;
+    private CacheControl caching = CacheControl.NONE;
 
     private Impl(int sc, T val) {
       statusCode = sc;
@@ -71,6 +78,17 @@ public abstract class Response<T> {
       return value;
     }
 
+    @Override
+    public CacheControl caching() {
+      return caching;
+    }
+
+    @Override
+    public Response<T> caching(CacheControl c) {
+      caching = c;
+      return this;
+    }
+
     @Override
     public String toString() {
       return "[" + statusCode() + "] " + value();
@@ -90,6 +108,16 @@ public abstract class Response<T> {
       throw new UnsupportedOperationException();
     }
 
+    @Override
+    public CacheControl caching() {
+      return CacheControl.NONE;
+    }
+
+    @Override
+    public Response<Object> caching(CacheControl c) {
+      throw new UnsupportedOperationException();
+    }
+
     @Override
     public String toString() {
       return "[204 No Content] None";