diff: detect cycles in the values being compared.

diff.go

@@ -41,7 +41,12 @@ type Printfer interface {
 // It calls Printf once for each difference, with no trailing newline.
 // The standard library log.Logger is a Printfer.
 func Pdiff(p Printfer, a, b interface{}) {
-	diffPrinter{w: p}.diff(reflect.ValueOf(a), reflect.ValueOf(b))
+	d := diffPrinter{
+		w:        p,
+		aVisited: make(map[visit]visit),
+		bVisited: make(map[visit]visit),
+	}
+	d.diff(reflect.ValueOf(a), reflect.ValueOf(b))
 }
 
 type Logfer interface {
@@ -66,6 +71,9 @@ func Ldiff(l Logfer, a, b interface{}) {
 type diffPrinter struct {
 	w Printfer
 	l string // label
+
+	aVisited map[visit]visit
+	bVisited map[visit]visit
 }
 
 func (w diffPrinter) printf(f string, a ...interface{}) {
@@ -96,6 +104,28 @@ func (w diffPrinter) diff(av, bv reflect.Value) {
 		return
 	}
 
+	if av.CanAddr() && bv.CanAddr() {
+		avis := visit{av.UnsafeAddr(), at}
+		bvis := visit{bv.UnsafeAddr(), bt}
+		var cycle bool
+
+		// Have we seen this value before?
+		if vis, ok := w.aVisited[avis]; ok {
+			cycle = true
+			if vis != bvis {
+				w.printf("%# v (previously visited) != %# v", formatter{v: av, quote: true}, formatter{v: bv, quote: true})
+			}
+		} else if _, ok := w.bVisited[bvis]; ok {
+			cycle = true
+			w.printf("%# v != %# v (previously visited)", formatter{v: av, quote: true}, formatter{v: bv, quote: true})
+		}
+		w.aVisited[avis] = bvis
+		w.bVisited[bvis] = avis
+		if cycle {
+			return
+		}
+	}
+
 	switch kind := at.Kind(); kind {
 	case reflect.Bool:
 		if a, b := av.Bool(), bv.Bool(); a != b {

diff_test.go

@@ -130,20 +130,23 @@ var diffs = []difftest{
 
 func TestDiff(t *testing.T) {
 	for _, tt := range diffs {
-		got := Diff(tt.a, tt.b)
-		eq := len(got) == len(tt.exp)
-		if eq {
-			for i := range got {
-				eq = eq && got[i] == tt.exp[i]
-			}
-		}
-		if !eq {
-			t.Errorf("diffing % #v", tt.a)
-			t.Errorf("with    % #v", tt.b)
-			diffdiff(t, got, tt.exp)
-			continue
+		expectDiffOutput(t, tt.a, tt.b, tt.exp)
+	}
+}
+
+func expectDiffOutput(t *testing.T, a, b interface{}, exp []string) {
+	got := Diff(a, b)
+	eq := len(got) == len(exp)
+	if eq {
+		for i := range got {
+			eq = eq && got[i] == exp[i]
 		}
 	}
+	if !eq {
+		t.Errorf("diffing % #v", a)
+		t.Errorf("with    % #v", b)
+		diffdiff(t, got, exp)
+	}
 }
 
 func TestKeyEqual(t *testing.T) {
@@ -193,6 +196,47 @@ func TestFdiff(t *testing.T) {
 	}
 }
 
+func TestDiffCycle(t *testing.T) {
+	// Diff two cyclic structs
+	a := &I{i: 1, R: nil}
+	a.R = a
+	b := &I{i: 2, R: nil}
+	b.R = b
+	expectDiffOutput(t, a, b, []string{
+		`i: 1 != 2`,
+	})
+
+	// Diff two equal cyclic structs
+	b.i = 1
+	expectDiffOutput(t, a, b, []string{})
+
+	// Diff two structs with different cycles
+	b2 := &I{i: 1, R: b}
+	b.R = b2
+	expectDiffOutput(t, a, b, []string{`R: pretty.I{
+    i:  1,
+    R:  &pretty.I{(CYCLIC REFERENCE)},
+} (previously visited) != pretty.I{
+    i:  1,
+    R:  &pretty.I{
+        i:  1,
+        R:  &pretty.I{(CYCLIC REFERENCE)},
+    },
+}`})
+
+	// ... and the same in the other direction
+	expectDiffOutput(t, b, a, []string{`R: pretty.I{
+    i:  1,
+    R:  &pretty.I{
+        i:  1,
+        R:  &pretty.I{(CYCLIC REFERENCE)},
+    },
+} != pretty.I{
+    i:  1,
+    R:  &pretty.I{(CYCLIC REFERENCE)},
+} (previously visited)`})
+}
+
 func diffdiff(t *testing.T, got, exp []string) {
 	minus(t, "unexpected:", got, exp)
 	minus(t, "missing:", exp, got)