time: fix Parse to ignore extra sub-nanosecond digits

This modifies the code to match the comment such that
the behavior truly is identical to stdSecond case.
Also, it modifies the behavior to match the documented
behavior where:

    Fractional seconds are truncated to nanosecond precision.

Fixes #54567
Updates #48685

Change-Id: Ie64549e4372ab51624c105ad8ab4cc99b9b5a0b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/425037
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>

src/time/format.go

@@ -1278,7 +1278,7 @@ func parse(layout, value string, defaultLocation, local *Location) (Time, error)
 			// Take any number of digits, even more than asked for,
 			// because it is what the stdSecond case would do.
 			i := 0
-			for i < 9 && i+1 < len(value) && '0' <= value[i+1] && value[i+1] <= '9' {
+			for i+1 < len(value) && '0' <= value[i+1] && value[i+1] <= '9' {
 				i++
 			}
 			nsec, rangeErrString, err = parseNanoseconds(value, 1+i)

src/time/format_test.go

@@ -863,7 +863,7 @@ func TestFormatFractionalSecondSeparators(t *testing.T) {
 	}
 }
 
-// Issue 48685
+// Issue 48685 and 54567.
 func TestParseFractionalSecondsLongerThanNineDigits(t *testing.T) {
 	tests := []struct {
 		s    string
@@ -893,13 +893,15 @@ func TestParseFractionalSecondsLongerThanNineDigits(t *testing.T) {
 	}
 
 	for _, tt := range tests {
-		tm, err := Parse(RFC3339, tt.s)
-		if err != nil {
-			t.Errorf("Unexpected error: %v", err)
-			continue
-		}
-		if got := tm.Nanosecond(); got != tt.want {
-			t.Errorf("Parse(%q) = got %d, want %d", tt.s, got, tt.want)
+		for _, format := range []string{RFC3339, RFC3339Nano} {
+			tm, err := Parse(format, tt.s)
+			if err != nil {
+				t.Errorf("Parse(%q, %q) error: %v", format, tt.s, err)
+				continue
+			}
+			if got := tm.Nanosecond(); got != tt.want {
+				t.Errorf("Parse(%q, %q) = got %d, want %d", format, tt.s, got, tt.want)
+			}
 		}
 	}
 }