runtime: make gcTestMoveStackOnNextCall not double the stack

Currently, gcTestMoveStackOnNextCall doubles the stack allocation on
each call because stack movement always doubles the stack. That's
rather unfortunate if you're doing a bunch of stack movement tests in
a row that don't actually have to grow the stack because you'll
quickly hit the stack size limit even though you're hardly using any
of the stack.

Fix this by adding a special stack poison value for
gcTestMoveStackOnNextCall that newstack recognizes and inhibits the
allocation doubling.

Change-Id: Iace7055a0f33cb48dc97b8f4b46e45304bee832c
Reviewed-on: https://go-review.googlesource.com/c/go/+/306672
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/runtime/stack.go

@@ -132,6 +132,10 @@ const (
 	// Stored into g->stackguard0 to cause split stack check failure.
 	// Must be greater than any real sp.
 	stackFork = uintptrMask & -1234
+
+	// Force a stack movement. Used for debugging.
+	// 0xfffffeed in hex.
+	stackForceMove = uintptrMask & -275
 )
 
 // Global pool of spans that have free stacks.
@@ -1054,11 +1058,18 @@ func newstack() {
 	// recheck the bounds on return.)
 	if f := findfunc(gp.sched.pc); f.valid() {
 		max := uintptr(funcMaxSPDelta(f))
-		for newsize-oldsize < max+_StackGuard {
+		for newsize-gp.sched.sp < max+_StackGuard {
 			newsize *= 2
 		}
 	}
 
+	if gp.stackguard0 == stackForceMove {
+		// Forced stack movement used for debugging.
+		// Don't double the stack (or we may quickly run out
+		// if this is done repeatedly).
+		newsize = oldsize
+	}
+
 	if newsize > maxstacksize || newsize > maxstackceiling {
 		if maxstacksize < maxstackceiling {
 			print("runtime: goroutine stack exceeds ", maxstacksize, "-byte limit\n")