reflect: handling flagIndir in DeepEqual potential cycles

Fixes #39607

Change-Id: Ia7e597e0da8a193a25382cc633a1c6080b4f7cbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/238361
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/reflect/all_test.go

@@ -824,6 +824,11 @@ var loop1, loop2 Loop
 var loopy1, loopy2 Loopy
 var cycleMap1, cycleMap2, cycleMap3 map[string]interface{}
 
+type structWithSelfPtr struct {
+	p *structWithSelfPtr
+	s string
+}
+
 func init() {
 	loop1 = &loop2
 	loop2 = &loop1
@@ -880,6 +885,7 @@ var deepEqualTests = []DeepEqualTest{
 	{[]float64{math.NaN()}, self{}, true},
 	{map[float64]float64{math.NaN(): 1}, map[float64]float64{1: 2}, false},
 	{map[float64]float64{math.NaN(): 1}, self{}, true},
+	{&structWithSelfPtr{p: &structWithSelfPtr{s: "a"}}, &structWithSelfPtr{p: &structWithSelfPtr{s: "b"}}, false},
 
 	// Nil vs empty: not the same.
 	{[]int{}, []int(nil), false},

src/reflect/deepequal.go

@@ -45,8 +45,20 @@ func deepValueEqual(v1, v2 Value, visited map[visit]bool, depth int) bool {
 	}
 
 	if hard(v1, v2) {
-		addr1 := v1.ptr
+		// For a Ptr or Map value, we need to check flagIndir,
-		addr2 := v2.ptr
+		// which we do by calling the pointer method.
+		// For Slice or Interface, flagIndir is always set,
+		// and using v.ptr suffices.
+		ptrval := func(v Value) unsafe.Pointer {
+			switch v.Kind() {
+			case Ptr, Map:
+				return v.pointer()
+			default:
+				return v.ptr
+			}
+		}
+		addr1 := ptrval(v1)
+		addr2 := ptrval(v2)
 		if uintptr(addr1) > uintptr(addr2) {
 			// Canonicalize order to reduce number of entries in visited.
 			// Assumes non-moving garbage collector.