runtime.AddCleanup and C struct hierarchies: cleanup order?

[twp...@gmail.com]

The documentation for runtime.AddCleanup says:

> There is no specified order in which cleanups will run.

Given the following types:

type Parent struct {

    parentResource int

}

type Child struct {

    parent *Parent

    childResource int

}

and the following code:

parentResource := 0

parent := &Parent{

    parentResource: parentResource,

}

runtime.AddCleanup(parent, func (int) {}, parentResource)

childResource := 1

child := &Child{

    parent: parent,

    childResource: childResource

}

runtime.AddCleanup(child, func(int) {}, childResource)

is it guaranteed that the cleanup for childResource will run before the cleanup for parentResource?

Notes:

* I know that the structs here are small enough to be "tiny", but Child contains a pointer to Parent so the cleanups should run.

* The example here is for a specific case where the resources are allocated by a C library, the child resource is "owned" by the parent, and so it's important that the child resource cleanup runs before the parent resource cleanup.

Many thanks for any insight!

Tom

The actual code I'm worried about is here: https://github.com/twpayne/go-geos/pull/183/commits/8ea732a7c65f873d48526cb1574e3c137dd66f74

[Ian Lance Taylor]

On Wed, Feb 19, 2025 at 1:12 PM twp...@gmail.com <twp...@gmail.com> wrote:
>
> The documentation for runtime.AddCleanup says:
>
> > There is no specified order in which cleanups will run.
>
> Given the following types:
>
> type Parent struct {
> parentResource int
> }
>
> type Child struct {
> parent *Parent
> childResource int
> }
>
> and the following code:
>
> parentResource := 0
> parent := &Parent{
> parentResource: parentResource,
> }
> runtime.AddCleanup(parent, func (int) {}, parentResource)
> childResource := 1
> child := &Child{
> parent: parent,
> childResource: childResource
> }
> runtime.AddCleanup(child, func(int) {}, childResource)
>
> is it guaranteed that the cleanup for childResource will run before the cleanup for parentResource?

No. There is no specified order in which cleanups will run.

The fact that child points to parent doesn't really matter here. They
can both be collected as garbage in the same GC pass, and that GC pass
may schedule both cleanups. There is no guarantee as to which cleanup
will be scheduler first or which will run first.

Ian

[twp...@gmail.com]

Thank you for the super-fast and authoritative response!

OK, I will work on an alternative solution for cleaning up hierarchies in bottom-up order and will report back here if/when I find a good solution.

Regards,

Tom

[Jason E. Aten]

Usually depth-first-search works just fine.