time: fix misleading error with the leading zero format

When the leading zero format is used, we currently don't handle the
month and year properly.

For the month, we were reporting an out of range error when getnum
returns zero of its own, as it also returns the month 0. That's
confusing, so only check the range when getnum returns a nil error.

For the year, we don't restore the value when parsing error occurs. For
example, with the incorrect input "111-01", "01" will be used to report
an error. So restore the value when an error occurs fix the problem.

Fixes golang#29918
Fixes golang#29916

Change-Id: I3145f8c46813a0457766b7c302482e6b56f94ed6
Reviewed-on: https://go-review.googlesource.com/c/go/+/160338
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/format.go

@@ -865,9 +865,12 @@ func parse(layout, value string, defaultLocation, local *Location) (Time, error)
 				err = errBad
 				break
 			}
+			hold := value
 			p, value = value[0:2], value[2:]
 			year, err = atoi(p)
-			if year >= 69 { // Unix time starts Dec 31 1969 in some time zones
+			if err != nil {
+				value = hold
+			} else if year >= 69 { // Unix time starts Dec 31 1969 in some time zones
 				year += 1900
 			} else {
 				year += 2000
@@ -887,7 +890,7 @@ func parse(layout, value string, defaultLocation, local *Location) (Time, error)
 			month++
 		case stdNumMonth, stdZeroMonth:
 			month, value, err = getnum(value, std == stdZeroMonth)
-			if month <= 0 || 12 < month {
+			if err == nil && (month <= 0 || 12 < month) {
 				rangeErrString = "month"
 			}
 		case stdWeekDay:

src/time/format_test.go

@@ -710,3 +710,49 @@ func TestUnderscoreTwoThousand(t *testing.T) {
 		t.Errorf("Incorrect minute, got %d", m)
 	}
 }
+
+// Issue 29918, 29916
+func TestStd0xParseError(t *testing.T) {
+	tests := []struct {
+		format, value, valueElemPrefix string
+	}{
+		{"01 MST", "0 MST", "0"},
+		{"01 MST", "1 MST", "1"},
+		{RFC850, "Thursday, 04-Feb-1 21:00:57 PST", "1"},
+	}
+	for _, tt := range tests {
+		_, err := Parse(tt.format, tt.value)
+		if err == nil {
+			t.Errorf("Parse(%q, %q) did not fail as expected", tt.format, tt.value)
+		} else if perr, ok := err.(*ParseError); !ok {
+			t.Errorf("Parse(%q, %q) returned error type %T, expected ParseError", tt.format, tt.value, perr)
+		} else if !strings.Contains(perr.Error(), "cannot parse") || !strings.HasPrefix(perr.ValueElem, tt.valueElemPrefix) {
+			t.Errorf("Parse(%q, %q) returned wrong parsing error message: %v", tt.format, tt.value, perr)
+		}
+	}
+}
+
+var monthOutOfRangeTests = []struct {
+	value string
+	ok    bool
+}{
+	{"00-01", false},
+	{"13-01", false},
+	{"01-01", true},
+}
+
+func TestParseMonthOutOfRange(t *testing.T) {
+	for _, test := range monthOutOfRangeTests {
+		_, err := Parse("01-02", test.value)
+		switch {
+		case !test.ok && err != nil:
+			if !strings.Contains(err.Error(), "month out of range") {
+				t.Errorf("%q: expected 'month' error, got %v", test.value, err)
+			}
+		case test.ok && err != nil:
+			t.Errorf("%q: unexpected error: %v", test.value, err)
+		case !test.ok && err == nil:
+			t.Errorf("%q: expected 'month' error, got none", test.value)
+		}
+	}
+}