Golang slice append and reallocation

The GC does not collect the backing array until there are no references to the backing array. There are no races in the program.

Consider the scenario with no goroutines:

  a := make([]*A, 0, 100)
  for i := 0; i < 100; i++ {
    a = append(a, &A{i})
  }
  b := a
  for i := 0; i < 100; i++ {
    b = append(b, &A{i+100})
  }

The slice a will continue to reference the backing array with the first 100 pointers when append to b allocates a new backing array. The slice a is not left with a dangling reference to a backing array.

Now add the goroutine to the scenario:

  a := make([]*A, 0, 100)
  for i := 0; i < 100; i++ {
    a = append(a, &A{i})
  }
  b := a
  go read_slice(a, ch)
  for i := 0; i < 100; i++ {
    b = append(b, &A{i+100})
  }

The goroutine can happily use slice a. There's no dangling reference.

Now consider the program in the question. It's functionaly identical to the last snippet here.