runtime: add GC testing helpers for regabi signature fuzzer

This CL adds a set of helper functions for testing GC interactions.
These are intended for use in the regabi signature fuzzer, but are
generally useful for GC tests, so we make them generally available to
runtime tests.

These provide:

1. An easy way to force stack movement, for testing stack copying.

2. A simple and robust way to check the reachability of a set of
pointers.

3. A way to check what general category of memory a pointer points to,
mostly so tests can make sure they're testing what they mean to.

For #40724, but generally useful.

Change-Id: I15d33ccb3f5a792c0472a19c2cc9a8b4a9356a66
Reviewed-on: https://go-review.googlesource.com/c/go/+/305330
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/runtime/mheap.go b/src/runtime/mheap.go
index d7f6a88cc9..da3772cdb6 100644
--- a/src/runtime/mheap.go
+++ b/src/runtime/mheap.go
@@ -212,6 +212,7 @@ type mheap struct {
 	cachealloc            fixalloc // allocator for mcache*
 	specialfinalizeralloc fixalloc // allocator for specialfinalizer*
 	specialprofilealloc   fixalloc // allocator for specialprofile*
+	specialReachableAlloc fixalloc // allocator for specialReachable
 	speciallock           mutex    // lock for special record allocators.
 	arenaHintAlloc        fixalloc // allocator for arenaHints
 
@@ -703,6 +704,7 @@ func (h *mheap) init() {
 	h.cachealloc.init(unsafe.Sizeof(mcache{}), nil, nil, &memstats.mcache_sys)
 	h.specialfinalizeralloc.init(unsafe.Sizeof(specialfinalizer{}), nil, nil, &memstats.other_sys)
 	h.specialprofilealloc.init(unsafe.Sizeof(specialprofile{}), nil, nil, &memstats.other_sys)
+	h.specialReachableAlloc.init(unsafe.Sizeof(specialReachable{}), nil, nil, &memstats.other_sys)
 	h.arenaHintAlloc.init(unsafe.Sizeof(arenaHint{}), nil, nil, &memstats.other_sys)
 
 	// Don't zero mspan allocations. Background sweeping can
@@ -1649,6 +1651,9 @@ func (list *mSpanList) takeAll(other *mSpanList) {
 const (
 	_KindSpecialFinalizer = 1
 	_KindSpecialProfile   = 2
+	// _KindSpecialReachable is a special used for tracking
+	// reachability during testing.
+	_KindSpecialReachable = 3
 	// Note: The finalizer special must be first because if we're freeing
 	// an object, a finalizer special will cause the freeing operation
 	// to abort, and we want to keep the other special records around
@@ -1854,6 +1859,14 @@ func setprofilebucket(p unsafe.Pointer, b *bucket) {
 	}
 }
 
+// specialReachable tracks whether an object is reachable on the next
+// GC cycle. This is used by testing.
+type specialReachable struct {
+	special   special
+	done      bool
+	reachable bool
+}
+
 // specialsIter helps iterate over specials lists.
 type specialsIter struct {
 	pprev **special
@@ -1898,6 +1911,10 @@ func freeSpecial(s *special, p unsafe.Pointer, size uintptr) {
 		lock(&mheap_.speciallock)
 		mheap_.specialprofilealloc.free(unsafe.Pointer(sp))
 		unlock(&mheap_.speciallock)
+	case _KindSpecialReachable:
+		sp := (*specialReachable)(unsafe.Pointer(s))
+		sp.done = true
+		// The creator frees these.
 	default:
 		throw("bad special kind")
 		panic("not reached")