ensureMove usage leading to memory leak for JsonNode !

[Anubhav]

Description

I was using ensureMove with custom objects to make sure compiler is doing what i expect.
I accidentally used ensureMove with a JsonNode (with kind JObject), later i pinned memory leak to usage of ensureMove .

I am not sure if i am supposed to use ensureMove anywhere , but compiler didn't give me any warning.

Following minimal code reproduces the issue for me .

# happens both with ARC and ORC mm
import std/json
proc test_something():JsonNode=

    var final_result = JsonNode(kind:JObject)
    var a = JsonNode(kind:JArray)
    for i in 0..<10000:
        a.add(JsonNode(kind:JString, str:"random string"))
    final_result["key"] = ensureMove a
    # final_result["key"] = a

    return final_result

var temp:JsonNode
for i in 0..<100000:
    temp = test_something()

Nim Version

Nim Compiler Version 2.2.0 [Windows: amd64]
Compiled at 2024-10-02
Copyright (c) 2006-2024 by Andreas Rumpf

active boot switches: -d:release

Current Output

No response

Expected Output

I think it is not supposed to do memory leak or i don't know if `ensureMove` is supposed to be used for custom objects only!

Known Workarounds

No response

Additional Information

No response

[Jason Beetham]

This is a bug, but JsonNode is a ref object so there is very little point in using ensureMove. Seems for refs ensureMove extends the lifetime of references and they never get collected.

[Anubhav]

yeah i thought so, i.e not using ensureMove with references , but wanted to document it just in case here.
BTW, if i use JArray kind, it doesn't leak the memory ! Does ensureMove always leak memory for every ref object?

[Jason Beetham]

ensureMove should be a noop for references but it accidentally does something for some reference tree.

[ringabout]

Reduced:

type
  JsonNode = ref object

proc foo(d: JsonNode) =
  discard

proc test_something()=
  var a = JsonNode()
  foo(ensureMove(a))

test_something()