runtime: fix conflict between lfstack and checkptr

lfstack does very unsafe things. In particular, it will not
work with nodes that live on the heap. In normal use by the runtime,
that is the case (it is only used for gc work bufs). But the lfstack
test does use heap objects. It goes through some hoops to prevent
premature deallocation, but those hoops are not enough to convince
-d=checkptr that everything is ok.

Instead, allocate the test objects outside the heap, like the runtime
does for all of its lfstack usage. Remove the lifetime workaround
from the test.

Reported in https://groups.google.com/g/golang-nuts/c/psjrUV2ZKyI

Change-Id: If611105eab6c823a4d6c105938ce145ed731781d
Reviewed-on: https://go-review.googlesource.com/c/go/+/448899
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>

src/runtime/export_test.go

@@ -69,6 +69,9 @@ func LFStackPush(head *uint64, node *LFNode) {
 func LFStackPop(head *uint64) *LFNode {
 	return (*LFNode)(unsafe.Pointer((*lfstack)(head).pop()))
 }
+func LFNodeValidate(node *LFNode) {
+	lfnodeValidate((*lfnode)(unsafe.Pointer(node)))
+}
 
 func Netpoll(delta int64) {
 	systemstack(func() {
@@ -1709,3 +1712,9 @@ func BlockUntilEmptyFinalizerQueue(timeout int64) bool {
 func FrameStartLine(f *Frame) int {
 	return f.startLine
 }
+
+// PersistentAlloc allocates some memory that lives outside the Go heap.
+// This memory will never be freed; use sparingly.
+func PersistentAlloc(n uintptr) unsafe.Pointer {
+	return persistentalloc(n, 0, &memstats.other_sys)
+}