runtime: runtime.SetFinalizer: pointer not at beginning of allocated block

[emidoots]

What does 'go version' print?
go version devel +ed3e89f781c1 Thu Mar 27 00:29:55 2014 -0400 linux/amd64

What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.
1. (reproducable on playground and tip linux/amd64)
2. http://play.golang.org/p/o61E8TfkcV

What happened?
runtime.SetFinalizer: pointer not at beginning of allocated block
fatal error: runtime.SetFinalizer

goroutine 16 [running]:
runtime.throw(0x45cd03)
    /home/stephen/go/src/pkg/runtime/panic.c:520 +0x69 fp=0x7ff689f8ee98
runtime.SetFinalizer(0x423b80, 0x45eee0, 0x0, 0x0)
    /home/stephen/go/src/pkg/runtime/malloc.goc:936 +0x40 fp=0x7ff689f8ef18
main.main()
    /home/stephen/Desktop/godev/finalizer.go:12 +0x51 fp=0x7ff689f8ef50
runtime.main()
    /home/stephen/go/src/pkg/runtime/proc.c:243 +0x11a fp=0x7ff689f8efa8
runtime.goexit()
    /home/stephen/go/src/pkg/runtime/proc.c:1426 fp=0x7ff689f8efb0
created by _rt0_go
    /home/stephen/go/src/pkg/runtime/asm_amd64.s:97 +0x120

goroutine 17 [runnable]:
runtime.MHeap_Scavenger()
    /home/stephen/go/src/pkg/runtime/mheap.c:507
runtime.goexit()
    /home/stephen/go/src/pkg/runtime/proc.c:1426
exit status 2

What should have happened instead?
The finalizer should be set (or it should be documented that finalizers do not work with
pointers declared not in a function's scope).

Please provide any additional information below.

[emidoots]

Comment 1:

Possibly related? If I change the data type of the struct member "Something" from []byte
to uint32, a different error is produced (with Go tip but NOT on playground):
fatal error: removespecial on invalid pointer
goroutine 16 [running]:
runtime.throw(0x45c83d)
    /home/stephen/go/src/pkg/runtime/panic.c:520 +0x69 fp=0x7f1b5da3fe40
removespecial(0x465e04, 0x1)
    /home/stephen/go/src/pkg/runtime/mheap.c:699 +0x4d fp=0x7f1b5da3fe78
runtime.removefinalizer(0x465e04)
    /home/stephen/go/src/pkg/runtime/mheap.c:756 +0x30 fp=0x7f1b5da3fe98
runtime.SetFinalizer(0x423b20, 0x465e04, 0x0, 0x0)
    /home/stephen/go/src/pkg/runtime/malloc.goc:929 +0x347 fp=0x7f1b5da3ff18
main.main()
    /home/stephen/Desktop/godev/finalizer.go:12 +0x51 fp=0x7f1b5da3ff50
runtime.main()
    /home/stephen/go/src/pkg/runtime/proc.c:243 +0x11a fp=0x7f1b5da3ffa8
runtime.goexit()
    /home/stephen/go/src/pkg/runtime/proc.c:1426 fp=0x7f1b5da3ffb0
created by _rt0_go
    /home/stephen/go/src/pkg/runtime/asm_amd64.s:97 +0x120
goroutine 17 [runnable]:
runtime.MHeap_Scavenger()
    /home/stephen/go/src/pkg/runtime/mheap.c:507
runtime.goexit()
    /home/stephen/go/src/pkg/runtime/proc.c:1426
exit status 2

[ianlancetaylor]

Comment 2:

Labels changed: added repo-main, release-go1.3.

[dvyukov]

Comment 3:

Russ,
I do not see how we can properly resolve this w/o making all such objects fair heap
allocations. For user it looks like a heap object (SetFinalizer docs say: "allocated by
calling new or by taking the address of a composite literal"), but we can't distinguish
it from "real" global objects at runtime.
Making more allocations for the sake of making SetFinalizer better does not look like a
good trade-off, so I've prepared the following change (which removes surprising crashes):
https://golang.org/cl/82560043/diff/20001/src/pkg/runtime/malloc.goc
The downsides are:
1. No "pointer not at beginning of allocated block" diagnostics for "real" global
objects, they are silently ignored.
2. There is no crash, but the finalizer does not run as well.
Note that this can also cause false retention in:
var Foo = &Object{}
type Object struct {
    Something []byte
}
func main() {
    Foo.Something = append(Foo.Something, 0)
    Foo = &Object{}
    // Oops, the previous array is never collected
    // because the global anonymous object has a pointer to it...
}
What do you think?

[rsc]

Comment 4:

I think we cannot do any better before Go 1.3, so please hg mail the CL.

[dvyukov]

Comment 5:

This issue was updated by revision f4ef697.

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/82560043

[dvyukov]

Comment 6:

The immediate crash is fixed.
Retagging as 1.4 because finalizers for such objects do not run, and the new check
disables diagnostics for setting finalizer for a real global object.

Labels changed: added release-go1.4, removed release-go1.3.

[gopherbot]

Comment 7:

CL https://golang.org/cl/145930043 mentions this issue.

[rsc]

Comment 8:

This issue was closed by revision 9a5b055.

Status changed to Fixed.