avoid allocating memory if all extension are cleared

[liuzhijiang]

avoid allocating memory if all extension are cleared

[sbenzaquen]

We should test that the capacity grows as expected with the new changes for is_cleared.

[acozzette]

@liuzhijiang This change looks good to me but I think we just need a unit test as @sbenzaquen suggested to verify the new behavior. I think the test could just look at SpaceUsedExcludingSelfLong() and verify that this number doesn't go up when we merge an ExtensionSet that has been cleared.

[liuzhijiang]

The function Reflection::GetExtensionSet() is private, so the object ExtensionSet cannot be obtained and the function ExtensionSet::SpaceUsedExcludingSelfLong() cannot be called. Is there any other way to call the function SpaceUsedExcludingSelfLong() ?
If there is no other way to call this function, is the following unit test OK?

TEST(ExtensionSetTest, ExtensionSetSpaceUsed) {
  unittest::TestAllExtensions msg;
  long l = msg.SpaceUsedLong();
  msg.SetExtension(unittest::optional_int32_extension, 100);
  unittest::TestAllExtensions msg2(msg);
  long l2 = msg2.SpaceUsedLong();
  msg.ClearExtension(unittest::optional_int32_extension);
  unittest::TestAllExtensions msg3(msg);
  long l3 = msg3.SpaceUsedLong();
  EXPECT_TRUE((l2 - l) > (l3 - l));
}

[acozzette]

@liuzhijiang That kind of unit test looks good to me. I think you are right that the best way to do it is to rely on Message::SpaceUsedLong() since ExtensionSet::SpaceUsedLong() is not easily visible for testing.

[acozzette]

@liuzhijiang Thanks!