code review 7289047: runtime: save LR on stack when panicking to handle leaf...

runtime: save LR to stack when panicking to handle leaf function traceback

[minux1, 2013-02-03 20:02:09]

Hello golang-dev@googlegroups.com (cc: golang-dev@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go/

[rsc, 2013-02-04 04:04:33]

LGTM

https://codereview.appspot.com/7289047/diff/2001/src/pkg/runtime/signal_linux...
File src/pkg/runtime/signal_linux_arm.c (right):

https://codereview.appspot.com/7289047/diff/2001/src/pkg/runtime/signal_linux...
src/pkg/runtime/signal_linux_arm.c:80: // probably a call to a nil func: the old
link register
Have you tested this case (calling a nil func)? I worry that the unwinder will
get out of sync if we don't set the pc to 0 here.

[minux1, 2013-02-04 21:43:03]

i will write a few tests in the runtime package to verify that the backtracer
does a decent job.

my few thoughts:
0. normal panics (*nilptr = 0)
1. calling nil functions (zero.go below)
(the line number for the top most frame is wrong, but
i don't understand why)
2. panics in sync/atomic
any others?

suggestions welcome.

https://codereview.appspot.com/7289047/diff/2001/src/pkg/runtime/signal_linux...
File src/pkg/runtime/signal_linux_arm.c (right):

https://codereview.appspot.com/7289047/diff/2001/src/pkg/runtime/signal_linux...
src/pkg/runtime/signal_linux_arm.c:80: // probably a call to a nil func: the old
link register
On 2013/02/04 04:04:33, rsc wrote:
> Have you tested this case (calling a nil func)? I worry that the unwinder will
> get out of sync if we don't set the pc to 0 here.
Yes, i had tested with the following program.

$ cat zero.go 
package main
var f func() int
func nest(i int) {
	if i == 0 {
		f()
	}
	nest(i-1)
}
func main() {
	nest(5)
}
$ go run zero.go
panic: runtime error: call of nil func value
[signal 0xb code=0x1 addr=0x0 pc=0x0]

goroutine 1 [running]:
main.nest(0x0, 0x10c48)
	/root/fast/go.hg/src/zero.go:9 +0x38
main.nest(0x1, 0x10c48)
	/root/fast/go.hg/src/zero.go:9 +0x48
main.nest(0x2, 0x10c48)
	/root/fast/go.hg/src/zero.go:9 +0x48
main.nest(0x3, 0x10c7c)
	/root/fast/go.hg/src/zero.go:9 +0x48
main.nest(0x4, 0x1c984)
	/root/fast/go.hg/src/zero.go:9 +0x48
main.nest(0x5, 0x1ca54)
	/root/fast/go.hg/src/zero.go:9 +0x48
main.main()
	/root/fast/go.hg/src/zero.go:13 +0x28
exit status 2

[rsc, 2013-02-05 12:00:09]

LGTM

[minux1, 2013-02-05 17:19:53]

*** Submitted as https://code.google.com/p/go/source/detail?r=8f547c432c68 ***

runtime: save LR to stack when panicking to handle leaf function traceback

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7289047