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6072 :: Is this a gc:arc memory leak bug or not?

• Ward (orginal) [2020-03-15T03:47:14+01:00] view original

  Example

  type
    Foo = ref object of RootObj
      data: int
      parent: Foo
  
  proc `=destroy`(self: var type(Foo()[])) =
    echo "destroy foo ", self.data
    for i in self.fields: i.reset
  
  proc getParent(self: Foo): Foo = self.parent
  
  var foo1 = Foo(data: 1)
  var foo2 = Foo(data: 2, parent: foo1)
  
  foo2.getParent.data = 1
  
  # output
  destroy foo 2
  Run

  Possible Solution 1

  proc getParent(self: Foo): lent Foo = self.parent
  
  # output
  destroy foo 2
  destroy foo 1
  Run

  Possible Solution 2

  var foo = foo2.getParent
  foo.data = 1
  
  # output
  destroy foo 2
  destroy foo 1
  Run

  To my understanding, lent type for ref object is meaningless, it shouldn't change the result for the code.

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• Sixte (orginal) [2020-03-15T15:05:03+01:00] view original

  This is a subtle problem. In your original code you created an intermediate ref with getParent and the refcount of "foo1" got incremented accordingly. But the intermediate ref is not maintained, so no decrementation happens afterwards. Solution 1: The lent annotation prohibits the update of the refcounter. Solution 2 : With introduction of an additional variable, decrementation happens when the variable leaves the scope. That works. It is expensive though. So, i would prefer the lent annotation.

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• Araq (orginal) [2020-03-18T12:15:17+01:00] view original

  Fixed with Nim devel.

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