runtime: don't crash in SetFinalizer if sizeof *x is zero

And document it explicitly, even though it already said
it wasn't guaranteed.

Fixes #6857

R=golang-dev, khr
CC=golang-dev
https://golang.org/cl/43580043

src/pkg/runtime/extern.go

@@ -160,6 +160,9 @@ func funcentry_go(*Func) uintptr
 // to depend on a finalizer to flush an in-memory I/O buffer such as a
 // bufio.Writer, because the buffer would not be flushed at program exit.
 //
+// It is not guaranteed that a finalizer will run if the size of *x is
+// zero bytes.
+//
 // A single goroutine runs all finalizers for a program, sequentially.
 // If a finalizer must run for a long time, it should do so by starting
 // a new goroutine.

src/pkg/runtime/malloc.goc

@@ -760,12 +760,15 @@ func SetFinalizer(obj Eface, finalizer Eface) {
 		runtime·printf("runtime.SetFinalizer: first argument is %S, not pointer\n", *obj.type->string);
 		goto throw;
 	}
+	ot = (PtrType*)obj.type;
+	if(ot->elem != nil && ot->elem->size == 0) {
+		return;
+	}
 	if(!runtime·mlookup(obj.data, &base, &size, nil) || obj.data != base) {
 		runtime·printf("runtime.SetFinalizer: pointer not at beginning of allocated block\n");
 		goto throw;
 	}
 	nret = 0;
-	ot = (PtrType*)obj.type;
 	fint = nil;
 	if(finalizer.type != nil) {
 		if(finalizer.type->kind != KindFunc)

src/pkg/runtime/mfinal_test.go

@@ -100,6 +100,13 @@ func TestFinalizerInterfaceBig(t *testing.T) {
 func fin(v *int) {
 }
 
+// Verify we don't crash at least. golang.org/issue/6857
+func TestFinalizerZeroSizedStruct(t *testing.T) {
+	type Z struct{}
+	z := new(Z)
+	runtime.SetFinalizer(z, func(*Z) {})
+}
+
 func BenchmarkFinalizer(b *testing.B) {
 	const CallsPerSched = 1000
 	procs := runtime.GOMAXPROCS(-1)