Allow REST APIs to return HTTP redirects

Eventually we can expose avatar links under /accounts/{id}/avatar
instead of /avatar/{id}. This better fits with the path space of
the server.

Change-Id: I461ea540310622b5d769bf1951a72cf34ad60a0e

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

@@ -35,6 +35,11 @@ public abstract class Response<T> {
     return NONE;
   }
 
+  /** HTTP 302 Found: temporary redirect to another URL. */
+  public static Redirect redirect(String location) {
+    return new Redirect(location);
+  }
+
   @SuppressWarnings({"unchecked", "rawtypes"})
   public static <T> T unwrap(T obj) {
     while (obj instanceof Response) {
@@ -90,4 +95,33 @@ public abstract class Response<T> {
       return "[204 No Content] None";
     }
   }
+
+  /** An HTTP redirect to another location. */
+  public static final class Redirect {
+    private final String location;
+
+    private Redirect(String url) {
+      this.location = url;
+    }
+
+    public String location() {
+      return location;
+    }
+
+    @Override
+    public int hashCode() {
+      return location.hashCode();
+    }
+
+    @Override
+    public boolean equals(Object o) {
+      return o instanceof Redirect
+        && ((Redirect) o).location.equals(location);
+    }
+
+    @Override
+    public String toString() {
+      return String.format("[302 Redirect] %s", location);
+    }
+  }
 }

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

@@ -43,12 +43,12 @@ import com.google.gerrit.extensions.restapi.AcceptsCreate;
 import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.extensions.restapi.BadRequestException;
 import com.google.gerrit.extensions.restapi.BinaryResult;
-import com.google.gerrit.extensions.restapi.Response;
 import com.google.gerrit.extensions.restapi.DefaultInput;
 import com.google.gerrit.extensions.restapi.MethodNotAllowedException;
 import com.google.gerrit.extensions.restapi.PutInput;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
 import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
+import com.google.gerrit.extensions.restapi.Response;
 import com.google.gerrit.extensions.restapi.RestCollection;
 import com.google.gerrit.extensions.restapi.RestModifyView;
 import com.google.gerrit.extensions.restapi.RestReadView;
@@ -258,6 +258,9 @@ public class RestApiServlet extends HttpServlet {
         @SuppressWarnings("rawtypes")
         Response r = (Response) result;
         status = r.statusCode();
+      } else if (result instanceof Response.Redirect) {
+        res.sendRedirect(((Response.Redirect) result).location());
+        return;
       }
       res.setStatus(status);