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-httpd/src/main/java/com/google/gerrit/httpd/restapi/ParameterParser.java

@@ -33,6 +33,7 @@ import com.google.gson.JsonArray;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
 import com.google.gson.JsonPrimitive;
+import com.google.gwtexpui.server.CacheHeaders;
 import com.google.inject.Inject;
 
 import org.kohsuke.args4j.CmdLineException;
@@ -79,6 +80,7 @@ class ParameterParser {
       msg.write('\n');
       clp.printUsage(msg, null);
       msg.write('\n');
+      CacheHeaders.setNotCacheable(res);
       replyBinaryResult(req, res,
           BinaryResult.create(msg.toString()).setContentType("text/plain"));
       return false;

gerrit-httpd/src/main/java/com/google/gerrit/httpd/restapi/RestApiServlet.java

@@ -112,6 +112,7 @@ import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.TimeUnit;
 import java.util.regex.Pattern;
 import java.util.zip.GZIPOutputStream;
 
@@ -184,7 +185,6 @@ public class RestApiServlet extends HttpServlet {
   protected final void service(HttpServletRequest req, HttpServletResponse res)
       throws ServletException, IOException {
     long auditStartTs = System.currentTimeMillis();
-    CacheHeaders.setNotCacheable(res);
     res.setHeader("Content-Disposition", "attachment");
     res.setHeader("X-Content-Type-Options", "nosniff");
     int status = SC_OK;
@@ -302,9 +302,13 @@ public class RestApiServlet extends HttpServlet {
         @SuppressWarnings("rawtypes")
         Response r = (Response) result;
         status = r.statusCode();
+        configureCaching(req, res, r);
       } else if (result instanceof Response.Redirect) {
+        CacheHeaders.setNotCacheable(res);
         res.sendRedirect(((Response.Redirect) result).location());
         return;
+      } else {
+        CacheHeaders.setNotCacheable(res);
       }
       res.setStatus(status);
 
@@ -349,6 +353,26 @@ public class RestApiServlet extends HttpServlet {
     }
   }
 
+  private static <T> void configureCaching(HttpServletRequest req,
+      HttpServletResponse res, Response<T> r) {
+    if ("GET".equals(req.getMethod())) {
+      switch (r.caching()) {
+        case NONE:
+        default:
+          CacheHeaders.setNotCacheable(res);
+          break;
+        case PRIVATE:
+          CacheHeaders.setCacheablePrivate(res, 7, TimeUnit.DAYS);
+          break;
+        case PUBLIC:
+          CacheHeaders.setCacheable(req, res, 7, TimeUnit.DAYS);
+          break;
+      }
+    } else {
+      CacheHeaders.setNotCacheable(res);
+    }
+  }
+
   private void checkPreconditions(HttpServletRequest req, RestResource rsrc)
       throws PreconditionFailedException {
     if ("*".equals(req.getHeader("If-None-Match"))) {
@@ -818,6 +842,7 @@ public class RestApiServlet extends HttpServlet {
   static void replyError(HttpServletResponse res, int statusCode, String msg)
       throws IOException {
     res.setStatus(statusCode);
+    CacheHeaders.setNotCacheable(res);
     replyText(null, res, msg);
   }