fmtsort: sort interfaces deterministically

Previously, the result of sorting a map[interface{}] containing multiple concrete types was non-deterministic. To ensure consistent results, sort first by the machine address of the value's type, then by the value itself. Sorting based on machine address results in unpredictable output, but will at least be deterministic across runs of the same program. Fixes golang#30398
lukechampine · Feb 28, 2019 · 1b07f0c · 1b07f0c
1 parent b5a68a9
commit 1b07f0c
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Showing 2 changed files with 39 additions and 5 deletions.
diff --git a/src/internal/fmtsort/sort.go b/src/internal/fmtsort/sort.go
@@ -167,7 +167,7 @@ func compare(aVal, bVal reflect.Value) int {
 		if c, ok := nilCompare(aVal, bVal); ok {
 			return c
 		}
-		c := compare(reflect.ValueOf(aType), reflect.ValueOf(bType))
+		c := compare(reflect.ValueOf(aVal.Elem().Type()), reflect.ValueOf(bVal.Elem().Type()))
 		if c != 0 {
 			return c
 		}

diff --git a/src/internal/fmtsort/sort_test.go b/src/internal/fmtsort/sort_test.go
@@ -126,10 +126,6 @@ var sortTests = []sortTest{
 		map[[2]int]string{{7, 2}: "72", {7, 1}: "71", {3, 4}: "34"},
 		"[3 4]:34 [7 1]:71 [7 2]:72",
 	},
-	{
-		map[interface{}]string{7: "7", 4: "4", 3: "3", nil: "nil"},
-		"<nil>:nil 3:3 4:4 7:7",
-	},
 }
 
 func sprint(data interface{}) string {
@@ -210,3 +206,41 @@ func TestOrder(t *testing.T) {
 		}
 	}
 }
+
+func TestInterface(t *testing.T) {
+	// A map containing multiple concrete types should be sorted by type,
+	// then value. However, the relative ordering of types is unspecified,
+	// so test this by checking the presence of sorted subgroups.
+	m := map[interface{}]string{
+		[2]int{1, 0}:             "",
+		[2]int{0, 1}:             "",
+		true:                     "",
+		false:                    "",
+		3.1:                      "",
+		2.1:                      "",
+		1.1:                      "",
+		math.NaN():               "",
+		3:                        "",
+		2:                        "",
+		1:                        "",
+		"c":                      "",
+		"b":                      "",
+		"a":                      "",
+		struct{ x, y int }{1, 0}: "",
+		struct{ x, y int }{0, 1}: "",
+	}
+	got := sprint(m)
+	typeGroups := []string{
+		"NaN: 1.1: 2.1: 3.1:", // float64
+		"false: true:",        // bool
+		"1: 2: 3:",            // int
+		"a: b: c:",            // string
+		"[0 1]: [1 0]:",       // [2]int
+		"{0 1}: {1 0}:",       // struct{ x int; y int }
+	}
+	for _, g := range typeGroups {
+		if !strings.Contains(got, g) {
+			t.Errorf("sorted map should contain %q", g)
+		}
+	}
+}