time: revert strict parsing of RFC 3339

CL 444277 fixed Time.UnmarshalText and Time.UnmarshalJSON to properly
unmarshal timestamps according to RFC 3339 instead of according
to Go's bespoke time syntax that is a superset of RFC 3339.

However, this change seems to have broken an AWS S3 unit test
that relies on parsing timestamps with single digit hours.
It is unclear whether S3 emits these timestamps in production or
whether this is simply a testing artifact that has been cargo culted
across many code bases. Either way, disable strict parsing for now
and re-enable later with better GODEBUG support.

Updates #54580

Change-Id: Icced2c7f9a6b2fc06bbd9c7e90f90edce24c2306
Reviewed-on: https://go-review.googlesource.com/c/go/+/462286
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Joseph Tsai <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

doc/go1.20.html

@@ -1200,9 +1200,8 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
     </p>
 
     <p><!-- CL 444277 -->
-      The <a href="/pkg/time/#Time.MarshalJSON"><code>Time.MarshalJSON</code></a> and
-      <a href="/pkg/time/#Time.UnmarshalJSON"><code>Time.UnmarshalJSON</code></a> methods
-      are now more strict about adherence to RFC 3339.
+      The <a href="/pkg/time/#Time.MarshalJSON"><code>Time.MarshalJSON</code></a> method
+      is now more strict about adherence to RFC 3339.
     </p>
   </dd>
 </dl><!-- time -->

src/time/format_rfc3339.go

@@ -155,7 +155,8 @@ func parseRFC3339[bytes []byte | string](s bytes, local *Location) (Time, bool)
 func parseStrictRFC3339(b []byte) (Time, error) {
 	t, ok := parseRFC3339(b, Local)
 	if !ok {
-		if _, err := Parse(RFC3339, string(b)); err != nil {
+		t, err := Parse(RFC3339, string(b))
+		if err != nil {
 			return Time{}, err
 		}
 
@@ -164,6 +165,10 @@ func parseStrictRFC3339(b []byte) (Time, error) {
 		// See https://go.dev/issue/54580.
 		num2 := func(b []byte) byte { return 10*(b[0]-'0') + (b[1] - '0') }
 		switch {
+		// TODO(https://go.dev/issue/54580): Strict parsing is disabled for now.
+		// Enable this again with a GODEBUG opt-out.
+		case true:
+			return t, nil
 		case b[len("2006-01-02T")+1] == ':': // hour must be two digits
 			return Time{}, &ParseError{RFC3339, string(b), "15", string(b[len("2006-01-02T"):][:1]), ""}
 		case b[len("2006-01-02T15:04:05")] == ',': // sub-second separator must be a period

src/time/time_test.go

@@ -830,10 +830,10 @@ func TestUnmarshalInvalidTimes(t *testing.T) {
 	}{
 		{`{}`, "Time.UnmarshalJSON: input is not a JSON string"},
 		{`[]`, "Time.UnmarshalJSON: input is not a JSON string"},
-		{`"2000-01-01T1:12:34Z"`, `parsing time "2000-01-01T1:12:34Z" as "2006-01-02T15:04:05Z07:00": cannot parse "1" as "15"`},
-		{`"2000-01-01T00:00:00,000Z"`, `parsing time "2000-01-01T00:00:00,000Z" as "2006-01-02T15:04:05Z07:00": cannot parse "," as "."`},
-		{`"2000-01-01T00:00:00+24:00"`, `parsing time "2000-01-01T00:00:00+24:00": timezone hour out of range`},
-		{`"2000-01-01T00:00:00+00:60"`, `parsing time "2000-01-01T00:00:00+00:60": timezone minute out of range`},
+		{`"2000-01-01T1:12:34Z"`, `<nil>`},
+		{`"2000-01-01T00:00:00,000Z"`, `<nil>`},
+		{`"2000-01-01T00:00:00+24:00"`, `<nil>`},
+		{`"2000-01-01T00:00:00+00:60"`, `<nil>`},
 		{`"2000-01-01T00:00:00+123:45"`, `parsing time "2000-01-01T00:00:00+123:45" as "2006-01-02T15:04:05Z07:00": cannot parse "+123:45" as "Z07:00"`},
 	}
 
@@ -842,13 +842,13 @@ func TestUnmarshalInvalidTimes(t *testing.T) {
 
 		want := tt.want
 		err := json.Unmarshal([]byte(tt.in), &ts)
-		if err == nil || err.Error() != want {
+		if fmt.Sprint(err) != want {
 			t.Errorf("Time.UnmarshalJSON(%s) = %v, want %v", tt.in, err, want)
 		}
 
 		if strings.HasPrefix(tt.in, `"`) && strings.HasSuffix(tt.in, `"`) {
 			err = ts.UnmarshalText([]byte(strings.Trim(tt.in, `"`)))
-			if err == nil || err.Error() != want {
+			if fmt.Sprint(err) != want {
 				t.Errorf("Time.UnmarshalText(%s) = %v, want %v", tt.in, err, want)
 			}
 		}