ensure diff output includes the portion that differs

staging/src/k8s.io/apimachinery/pkg/util/diff/diff.go

@@ -89,20 +89,52 @@ func ObjectReflectDiff(a, b interface{}) string {
 	}
 	out := []string{""}
 	for _, d := range diffs {
+		elidedA, elidedB := limit(d.a, d.b, 80)
 		out = append(out,
 			fmt.Sprintf("%s:", d.path),
-			limit(fmt.Sprintf("  a: %#v", d.a), 80),
-			limit(fmt.Sprintf("  b: %#v", d.b), 80),
+			fmt.Sprintf("  a: %s", elidedA),
+			fmt.Sprintf("  b: %s", elidedB),
 		)
 	}
 	return strings.Join(out, "\n")
 }
 
-func limit(s string, max int) string {
-	if len(s) > max {
-		return s[:max]
+// limit:
+// 1. stringifies aObj and bObj
+// 2. elides identical prefixes if either is too long
+// 3. elides remaining content from the end if either is too long
+func limit(aObj, bObj interface{}, max int) (string, string) {
+	elidedPrefix := ""
+	elidedASuffix := ""
+	elidedBSuffix := ""
+	a, b := fmt.Sprintf("%#v", aObj), fmt.Sprintf("%#v", bObj)
+	for {
+		switch {
+		case len(a) > max && len(a) > 4 && len(b) > 4 && a[:4] == b[:4]:
+			// a is too long, b has data, and the first several characters are the same
+			elidedPrefix = "..."
+			a = a[2:]
+			b = b[2:]
+
+		case len(b) > max && len(b) > 4 && len(a) > 4 && a[:4] == b[:4]:
+			// b is too long, a has data, and the first several characters are the same
+			elidedPrefix = "..."
+			a = a[2:]
+			b = b[2:]
+
+		case len(a) > max:
+			a = a[:max]
+			elidedASuffix = "..."
+
+		case len(b) > max:
+			b = b[:max]
+			elidedBSuffix = "..."
+
+		default:
+			// both are short enough
+			return elidedPrefix + a + elidedASuffix, elidedPrefix + b + elidedBSuffix
+		}
 	}
-	return s
 }
 
 func public(s string) bool {

staging/src/k8s.io/apimachinery/pkg/util/diff/diff_test.go

@@ -94,3 +94,50 @@ func TestStringDiff(t *testing.T) {
 		t.Errorf("diff returned %v", diff)
 	}
 }
+
+func TestLimit(t *testing.T) {
+	testcases := []struct {
+		a       interface{}
+		b       interface{}
+		expectA string
+		expectB string
+	}{
+		{
+			a:       `short a`,
+			b:       `short b`,
+			expectA: `"short a"`,
+			expectB: `"short b"`,
+		},
+		{
+			a:       `short a`,
+			b:       `long b needs truncating`,
+			expectA: `"short a"`,
+			expectB: `"long b ne...`,
+		},
+		{
+			a:       `long a needs truncating`,
+			b:       `long b needs truncating`,
+			expectA: `...g a needs ...`,
+			expectB: `...g b needs ...`,
+		},
+		{
+			a:       `long common prefix with different stuff at the end of a`,
+			b:       `long common prefix with different stuff at the end of b`,
+			expectA: `...end of a"`,
+			expectB: `...end of b"`,
+		},
+		{
+			a:       `long common prefix with different stuff at the end of a`,
+			b:       `long common prefix with different stuff at the end of b which continues`,
+			expectA: `...of a"`,
+			expectB: `...of b which...`,
+		},
+	}
+
+	for _, tc := range testcases {
+		a, b := limit(tc.a, tc.b, 10)
+		if a != tc.expectA || b != tc.expectB {
+			t.Errorf("limit(%q, %q)\n\texpected: %s, %s\n\tgot:      %s, %s", tc.a, tc.b, tc.expectA, tc.expectB, a, b)
+		}
+	}
+}