runtime: fix dumpgoroutine() to deal with open-coded defers

_defer.fn can be nil, so we need to add a check when dumping
_defer.fn.fn.

Fixes #35172

Change-Id: Ic1138be5ec9dce915a87467cfa51ff83acc6e3a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/203697
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/heapdump.go

@@ -371,7 +371,12 @@ func dumpgoroutine(gp *g) {
 		dumpint(uint64(d.sp))
 		dumpint(uint64(d.pc))
 		dumpint(uint64(uintptr(unsafe.Pointer(d.fn))))
-		dumpint(uint64(uintptr(unsafe.Pointer(d.fn.fn))))
+		if d.fn == nil {
+			// d.fn can be nil for open-coded defers
+			dumpint(uint64(0))
+		} else {
+			dumpint(uint64(uintptr(unsafe.Pointer(d.fn.fn))))
+		}
 		dumpint(uint64(uintptr(unsafe.Pointer(d.link))))
 	}
 	for p := gp._panic; p != nil; p = p.link {

src/runtime/runtime2.go

@@ -824,10 +824,10 @@ type _defer struct {
 	// defers. We have only one defer record for the entire frame (which may
 	// currently have 0, 1, or more defers active).
 	openDefer bool
-	sp        uintptr // sp at time of defer
-	pc        uintptr // pc at time of defer
-	fn        *funcval
-	_panic    *_panic // panic that is running defer
+	sp        uintptr  // sp at time of defer
+	pc        uintptr  // pc at time of defer
+	fn        *funcval // can be nil for open-coded defers
+	_panic    *_panic  // panic that is running defer
 	link      *_defer
 
 	// If openDefer is true, the fields below record values about the stack