runtime: AddCleanup does not panic if arg equals ptr

[cherrymui]

Go version

tip (go1.24-40b3c0e58a Fri Jan 17 08:40:47 2025 -0800 darwin/arm64)

Output of go env in your module/workspace:

any

What did you do?

https://go.dev/play/p/2SwFBThx4xJ?v=gotip

In particular,runtime.AddCleanup(t, func(a *T) { println("cleanup", a.x); ch <- 1 }, t) where the first and third arguments are equal.

What did you see happen?

It does not panic at the AddCleanup call. The cleanup never runs.

What did you expect to see?

runtime.AddCleanup is documented that it panics if arg is equal to ptr. E.g.
the code above should panic at the AddCleanup call.

The source code of AddCleanup includes

	// Check that arg is not equal to ptr.
	// TODO(67535) this does not cover the case where T and *S are the same
	// type and ptr and arg are equal.
	if unsafe.Pointer(&arg) == unsafe.Pointer(ptr) {
		throw("runtime.AddCleanup: ptr is equal to arg, cleanup will never run")
	}

But it is checking the address of arg not equal to ptr, not arg itself. And &arg cannot equal to ptr, because it's the address of an argument of a new frame. So this condition can never trigger. Also, it is a throw, not a panic as documented.

Is this what the TODO is for?

cc @cagedmantis @mknyszek

[mknyszek]

I think this is just an oversight and a bug. What we should do here is use interface-typed locals to grab the types, check if they're equal, and if they are, do an unsafe conversion and a comparison. (The types can only be equal if they're both pointer types, so the comparison is trivial.)

[cherrymui]

Sounds good, thanks!
I guess we don't have to limit it to the types being equal? As long as arg is a pointer type, and the pointer value is equal to ptr, it will never clean up, right? E.g. ptr is a pointer to a struct, arg is a pointer to the struct's first field.

[gopherbot]

Change https://go.dev/cl/643655 mentions this issue: runtime: fix the equality check in AddCleanup