time: truncate fractional seconds longer than 9 digits

Fixes #48685

Change-Id: Id246708878c2902b407ab759537f6b545a1f459f
GitHub-Last-Rev: 4d985192c5a66ae8891539f166ef88b53cd1cbea
GitHub-Pull-Request: golang/go#48750
Reviewed-on: https://go-review.googlesource.com/c/go/+/353713
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>

2 files changed, 47 insertions, 3 deletions

diff --git a/src/time/format.go b/src/time/format.go
index 7373892b97..464effdb43 100644
--- a/src/time/format.go
+++ b/src/time/format.go
@@ -1420,16 +1420,19 @@ func parseNanoseconds(value string, nbytes int) (ns int, rangeErrString string,
 		err = errBad
 		return
 	}
+	if nbytes > 10 {
+		value = value[:10]
+		nbytes = 10
+	}
 	if ns, err = atoi(value[1:nbytes]); err != nil {
 		return
 	}
-	if ns < 0 || 1e9 <= ns {
+	if ns < 0 {
 		rangeErrString = "fractional second"
 		return
 	}
 	// We need nanoseconds, which means scaling by the number
-	// of missing digits in the format, maximum length 10. If it's
-	// longer than 10, we won't scale.
+	// of missing digits in the format, maximum length 10.
 	scaleDigits := 10 - nbytes
 	for i := 0; i < scaleDigits; i++ {
 		ns *= 10
diff --git a/src/time/format_test.go b/src/time/format_test.go
index 93cbcf9401..db95536390 100644
--- a/src/time/format_test.go
+++ b/src/time/format_test.go
@@ -852,3 +852,44 @@ func TestFormatFractionalSecondSeparators(t *testing.T) {
 		}
 	}
 }
+
+// Issue 48685
+func TestParseFractionalSecondsLongerThanNineDigits(t *testing.T) {
+	tests := []struct {
+		s    string
+		want int
+	}{
+		// 9 digits
+		{"2021-09-29T16:04:33.000000000Z", 0},
+		{"2021-09-29T16:04:33.000000001Z", 1},
+		{"2021-09-29T16:04:33.100000000Z", 100_000_000},
+		{"2021-09-29T16:04:33.100000001Z", 100_000_001},
+		{"2021-09-29T16:04:33.999999999Z", 999_999_999},
+		{"2021-09-29T16:04:33.012345678Z", 12_345_678},
+		// 10 digits, truncates
+		{"2021-09-29T16:04:33.0000000000Z", 0},
+		{"2021-09-29T16:04:33.0000000001Z", 0},
+		{"2021-09-29T16:04:33.1000000000Z", 100_000_000},
+		{"2021-09-29T16:04:33.1000000009Z", 100_000_000},
+		{"2021-09-29T16:04:33.9999999999Z", 999_999_999},
+		{"2021-09-29T16:04:33.0123456789Z", 12_345_678},
+		// 11 digits, truncates
+		{"2021-09-29T16:04:33.10000000000Z", 100_000_000},
+		{"2021-09-29T16:04:33.00123456789Z", 1_234_567},
+		// 12 digits, truncates
+		{"2021-09-29T16:04:33.000123456789Z", 123_456},
+		// 15 digits, truncates
+		{"2021-09-29T16:04:33.9999999999999999Z", 999_999_999},
+	}
+
+	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)
+		}
+	}
+}