Memoryleak when dynamically mocked interface is passed as result of function for the same mock

[Deeem2031]

Tested with https://bitbucket.org/sglienke/spring4d/commits/121f07661f77e8e14581c5bc1d4089e4cf2cd95c

A memory leak occurs when setting up a mock to return itself as a result of a function call.

parentMock.Setup.Returns(TValue.From<IParent>(parentMock.AsType<IParent>.Instance)).When.GetChild;

I attached a patch which contains a test demonstrating the problem.

[Stefan Glienke]

Well, yes, duh - circular reference.

I think you meant parentMock.AsType<IChild> because that code would fail anyway.

I don’t see the use case of this - mocks automatically return mocks for interface returning methods so this is not necessary at all. They are different instances though but I doubt the code under test would fail in that case.

[Deeem2031]

Yes, parentMock.AsType<IChild> would be correct.

I also was quite surprised this didn’t result in a memory leak with the old develop version from 05.08.2020 (https://bitbucket.org/sglienke/spring4d/commits/c336b8dd39bfc30cbcf2fa1f6d8323e663bdcc05 - the link seems to be broken?!)

I wouldn’t have created the issue if this didn’t work before.

[Stefan Glienke]

Not sure what commit that should be (maybe some that got lost due to a force push) but I went back to bba6f47 and it leaks there as well.

FWIW call Reset on the mock to clear its expectations and resolve the circular reference.

[Deeem2031]

Calling Reset does only help if you didn’t call parentMock.AsType<IChild>() before. Not sure why. So this still results in a memory leak:

‌

var
  parentMock: Mock<IParent>;
begin
  parentMock.AsType<IChild>();
  parentMock.Setup.Returns(TValue.From<IChild>(parentMock.AsType<IChild>.Instance)).When.GetChild;
  parentMock.Reset;
  Check(True); // no fail condition; check for memory leaks
end;

‌

Btw. the use case is for a workaround of multiple inheritance on interfaces. I’m not quite sure if this still satisfies good code architecture but we have a codebase with interfaces inheriting multiple interfaces (which is not possible natively in Delphi) by using functions.

type
  IBase1 = interface
    function BaseFunction: Boolean;
  end;

  IBase2 = interface
    function BaseFunction2: Boolean;
  end;

  IImplementBothBases = interface(IBase1)
    function AsBase2: IBase2;
  end;

If you have an instance of IImplementBothBases you can now access IBase1 and IBase2 without having to deal with the situation the class might not implement IBase2, as these are always implemented by simply returning Self.

And this is exactly the behavior we replicated with parentMock.Setup.Returns(TValue.From<IChild>(parentMock.AsType<IChild>.Instance)).When.GetChild; or implementBothBases.Setup.Returns(TValue.From<IBase2>(implementBothBases.AsType<IBase2>.Instance)).When.AsBase2;

[Stefan Glienke]

‌

var
  parentMock: Mock<IParent>;
  childMock: Mock<IChild>;
begin
  childMock := parentMock.AsType<IChild>;
  parentMock.Setup.Returns(childMock.Instance).When.GetChild;
  parentMock.Reset;

‌

[Stefan Glienke]

fixed #362

→ <<cset dbbe1568529a>>