time: add fuzz test for Time.appendFormatRFC3339

Time.appendFormatRFC3339 is a specialized implementation of
Time.appendFormat. We expect the two to be identical.
Add a fuzz test to ensure this property.

Updates #54093

Change-Id: I0bc41ee6e016d3dac75d1ac372d8c9c7266d0299
Reviewed-on: https://go-review.googlesource.com/c/go/+/425100
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

src/time/export_test.go

@@ -132,3 +132,6 @@ var StdChunkNames = map[int]string{
 }
 
 var Quote = quote
+
+var AppendFormatAny = Time.appendFormat
+var AppendFormatRFC3339 = Time.appendFormatRFC3339

src/time/format.go

@@ -618,6 +618,17 @@ func (t Time) Format(layout string) string {
 // AppendFormat is like Format but appends the textual
 // representation to b and returns the extended buffer.
 func (t Time) AppendFormat(b []byte, layout string) []byte {
+	switch layout {
+	case RFC3339:
+		return t.appendFormatRFC3339(b, false)
+	case RFC3339Nano:
+		return t.appendFormatRFC3339(b, true)
+	default:
+		return t.appendFormat(b, layout)
+	}
+}
+
+func (t Time) appendFormat(b []byte, layout string) []byte {
 	var (
 		name, offset, abs = t.locabs()
 
@@ -630,14 +641,6 @@ func (t Time) AppendFormat(b []byte, layout string) []byte {
 		sec   int
 	)
 
-	// Handle most frequent layouts separately.
-	switch layout {
-	case RFC3339:
-		return t.appendFormatRFC3339(b, abs, offset, false)
-	case RFC3339Nano:
-		return t.appendFormatRFC3339(b, abs, offset, true)
-	}
-
 	// Each iteration generates one std value.
 	for layout != "" {
 		prefix, std, suffix := nextStdChunk(layout)
@@ -793,7 +796,9 @@ func (t Time) AppendFormat(b []byte, layout string) []byte {
 	return b
 }
 
-func (t Time) appendFormatRFC3339(b []byte, abs uint64, offset int, nanos bool) []byte {
+func (t Time) appendFormatRFC3339(b []byte, nanos bool) []byte {
+	_, offset, abs := t.locabs()
+
 	// Format date.
 	year, month, day, _ := absDate(abs, true)
 	b = appendInt(b, year, 4)

src/time/format_test.go

@@ -5,7 +5,9 @@
 package time_test
 
 import (
+	"bytes"
 	"fmt"
+	"math"
 	"strconv"
 	"strings"
 	"testing"
@@ -911,3 +913,45 @@ func TestParseFractionalSecondsLongerThanNineDigits(t *testing.T) {
 		}
 	}
 }
+
+func FuzzFormatRFC3339(f *testing.F) {
+	for _, ts := range [][2]int64{
+		{math.MinInt64, math.MinInt64}, // 292277026304-08-26T15:42:51Z
+		{-62167219200, 0},              // 0000-01-01T00:00:00Z
+		{1661201140, 676836973},        // 2022-08-22T20:45:40.676836973Z
+		{253402300799, 999999999},      // 9999-12-31T23:59:59.999999999Z
+		{math.MaxInt64, math.MaxInt64}, // -292277022365-05-08T08:17:07Z
+	} {
+		f.Add(ts[0], ts[1], true, false, 0)
+		f.Add(ts[0], ts[1], false, true, 0)
+		for _, offset := range []int{0, 60, 60 * 60, 99*60*60 + 99*60, 123456789} {
+			f.Add(ts[0], ts[1], false, false, -offset)
+			f.Add(ts[0], ts[1], false, false, +offset)
+		}
+	}
+
+	f.Fuzz(func(t *testing.T, sec, nsec int64, useUTC, useLocal bool, tzOffset int) {
+		var loc *Location
+		switch {
+		case useUTC:
+			loc = UTC
+		case useLocal:
+			loc = Local
+		default:
+			loc = FixedZone("", tzOffset)
+		}
+		ts := Unix(sec, nsec).In(loc)
+
+		got := AppendFormatRFC3339(ts, nil, false)
+		want := AppendFormatAny(ts, nil, RFC3339)
+		if !bytes.Equal(got, want) {
+			t.Errorf("Format(%s, RFC3339) mismatch:\n\tgot:  %s\n\twant: %s", ts, got, want)
+		}
+
+		gotNanos := AppendFormatRFC3339(ts, nil, true)
+		wantNanos := AppendFormatAny(ts, nil, RFC3339Nano)
+		if !bytes.Equal(got, want) {
+			t.Errorf("Format(%s, RFC3339Nano) mismatch:\n\tgot:  %s\n\twant: %s", ts, gotNanos, wantNanos)
+		}
+	})
+}