1943217 - Result's `unwrap()` will affect its value, make descendants `unwrap()` return wrong value

Status: RESOLVED WONTFIX

(Core :: MFBT, defect)

Description

[Alastor Wu [:alwu]]

I'm not sure if this is intended or a bug, and I discover this issue in this code.

For the debug purpose, I added following logs.

  uint8_t numSPS = config1.unwrap().NumSPS();
  auto numSPS2 = config2.unwrap().NumSPS();
  if (numSPS == 0 || numSPS != numSPS2) {
    return false;
  }

  printf_stderr("DD | ### num1=%u, num2=%u\n", numSPS, numSPS2);
  for (auto idx = 0; idx < 3; idx++) {
    printf_stderr("DD | loop %u, num1=%u, num2=%u\n", idx,
                  config1.unwrap().NumSPS(), config2.unwrap().NumSPS());
  }

What I expect is calling unwrap() won't affect the successful value in the Result. The NumSPS() in the first printf should be identical with other printf in the loop. However, after running above codes, I got the result like below,

DD | ### num1=1, num2=1
DD | loop 0, num1=0, num2=0
DD | loop 1, num1=0, num2=0
DD | loop 2, num1=0, num2=0

After calling unwrap(), it alters the successful value of HVCCConfig. Make the following unwrap() calls return an incorrect value. I can reproduce this issue by playing this file on Windows.

Comment 1

[Mike Hommey [:glandium]]

This is intended. Once you unwrap, the Result is not valid anymore. Of course, this being C++, you still can use the variable, leading to your mistake.