runtime: GOTRACEBACK=crash doesn't print user stack for threads on system stack

[aclements]

What version of Go are you using (go version)?

go version go1.8 linux/amd64

What did you do?

Run a program with GOTRACEBACK=crash where some of the threads are on the system stack. This was observed in the go1.8.txt traceback from https://groups.google.com/d/topic/golang-dev/PVwDFD7gDuk/discussion. It can be reproduced by running the following program with GOGC=1 GOTRACEBACK=crash and sending it SIGQUIT (it may take a few tries since it's timing-dependent):

package main

import (
	"runtime"
	"time"
)

var slice = make([]*byte, 16<<20)
var ballast = mkTree(10)
var sink interface{}

func main() {
	const count = 4
	runtime.GOMAXPROCS(count + 1)

	// Get the garbage collector going so typedslicecopy switches
	// to the system stack. We have to do this before starting
	// loop since GC won't be able to start once we're in loop.
	go func() {
		for {
			sink = make([]byte, 16<<20)
		}
	}()
	time.Sleep(10 * time.Millisecond)

	for i := 0; i < count; i++ {
		go loop(i)
	}
	select {}
}

func loop(i int) {
	for {
		for j := 0; j < 0x7fffffff; j++ {
			// This runs on the system stack when GC is active.
			copy(slice, slice[1:])
		}
	}
}

type node struct {
	l, r *node
}

func mkTree(depth int) *node {
	if depth <= 0 {
		return nil
	}
	return &node{mkTree(depth - 1), mkTree(depth - 1)}
}

What did you expect to see?

A traceback with the full stacks of all goroutines in loop.

What did you see instead?

When a loop goroutine is on the system stack when the traceback happens, we only see the system stack part:

SIGQUIT: quit
PC=0x44a0d8 m=4 sigcode=0

goroutine 0 [idle]:
runtime.memmove(0xc421459f30, 0xc421459f38, 0x8)
	/home/austin/.cache/gover/1.8/src/runtime/memmove_amd64.s:168 +0x6a8 fp=0xc420081f20 sp=0xc420081f18
runtime.typedmemmove(0x459a00, 0xc421459f30, 0xc421459f38)
	/home/austin/.cache/gover/1.8/src/runtime/mbarrier.go:246 +0x3c fp=0xc420081f58 sp=0xc420081f20
runtime.typedslicecopy.func1()
	/home/austin/.cache/gover/1.8/src/runtime/mbarrier.go:362 +0x220 fp=0xc420081fb8 sp=0xc420081f58
runtime.systemstack(0xc42003a600)
	/home/austin/.cache/gover/1.8/src/runtime/asm_amd64.s:327 +0x79 fp=0xc420081fc0 sp=0xc420081fb8
runtime.mstart()
	/home/austin/.cache/gover/1.8/src/runtime/proc.go:1132 fp=0xc420081fc8 sp=0xc420081fc0
goroutine 36 [running]:
rax    0x459a00
rbx    0x8
rcx    0xc421459f30
rdx    0xc421459f38
rdi    0xc421459f30
rsi    0xc421459f38
rbp    0xc420081f48
rsp    0xc420081f18
r8     0x3
r9     0x0
r10    0xc421459f30
r11    0x474762
r12    0x0
r13    0x34
r14    0x0
r15    0xf3
rip    0x44a0d8
rflags 0x246
cs     0x33
fs     0x0
gs     0x0

[gopherbot]

CL https://golang.org/cl/38012 mentions this issue.

[gopherbot]

CL https://golang.org/cl/39600 mentions this issue.