Allow empty request body for POST/PUT/DELETE REST endpoints that expect List/Map as input

If a REST endpoint expects a List or a Map as input, but the user
provides an empty request body we currently fail with 500 Internal
Server Error, e.g.: POST /accounts/<account-id>/watched.projects

Instead just create an empty List/Map and provide it to the REST
endpoint. This is consistent with instantiating empty Input objects when
the REST endpoint expects a non-generic input type and the user provides
no body.

Change-Id: I78fe66b2d0f63a6078411dd50a55196424d88490
Signed-off-by: Edwin Kempin <ekempin@google.com>

java/com/google/gerrit/httpd/restapi/RestApiServlet.java

@@ -846,16 +846,25 @@ public class RestApiServlet extends HttpServlet {
     throw new BadRequestException("Expected JSON object");
   }
 
+  @SuppressWarnings("unchecked")
   private static Object createInstance(Type type)
       throws NoSuchMethodException, InstantiationException, IllegalAccessException,
           InvocationTargetException {
     if (type instanceof Class) {
-      @SuppressWarnings("unchecked")
       Class<Object> clazz = (Class<Object>) type;
       Constructor<Object> c = clazz.getDeclaredConstructor();
       c.setAccessible(true);
       return c.newInstance();
     }
+    if (type instanceof ParameterizedType) {
+      Type rawType = ((ParameterizedType) type).getRawType();
+      if (rawType instanceof Class && List.class.isAssignableFrom((Class<Object>) rawType)) {
+        return new ArrayList<>();
+      }
+      if (rawType instanceof Class && Map.class.isAssignableFrom((Class<Object>) rawType)) {
+        return new HashMap<>();
+      }
+    }
     throw new InstantiationException("Cannot make " + type);
   }
 

javatests/com/google/gerrit/acceptance/rest/account/WatchedProjectsIT.java

@@ -235,4 +235,9 @@ public class WatchedProjectsIT extends AbstractDaemonTest {
     assertThat(persistedWatchedProjects).doesNotContain(pwi);
     assertThat(persistedWatchedProjects).containsAllIn(projectsToWatch);
   }
+
+  @Test
+  public void postWithoutBody() throws Exception {
+    adminRestSession.post("/accounts/" + admin.username + "/watched.projects").assertOK();
+  }
 }