time: strip monotonic time in t.Round, t.Truncate

The original analysis of the Go corpus assumed that these
stripped monotonic time. During the design discussion we
decided to try not stripping monotonic time here, but existing
code works better if we do.

See the discussion on golang.org/issue/18991 for more details.

For #18991.

Change-Id: I04d355ffe56ca0317acdd2ca76cb3033c277f6d1
Reviewed-on: https://go-review.googlesource.com/37542Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/mono_test.go

@@ -54,8 +54,8 @@ func TestHasMonotonicClock(t *testing.T) {
 	yes("tm.In(UTC)", tm.In(UTC))
 	yes("tm.Local()", tm.Local())
 	yes("tm.UTC()", tm.UTC())
-	yes("tm.Round(2)", tm.Round(2))
-	yes("tm.Truncate(2)", tm.Truncate(2))
+	no("tm.Round(2)", tm.Round(2))
+	no("tm.Truncate(2)", tm.Truncate(2))
 }
 
 func TestMonotonicAdd(t *testing.T) {

src/time/time.go

@@ -37,13 +37,12 @@
 // to use this package.
 //
 // The Time returned by time.Now contains a monotonic clock reading.
-// If Time t has a monotonic clock reading, t.Add, t.Round, and
-// t.Truncate add the same duration to both the wall clock and
-// monotonic clock readings to compute the result. Similarly, t.In,
-// t.Local, and t.UTC, which are defined to change only the Time's
+// If Time t has a monotonic clock reading, t.Add adds the same duration to
+// both the wall clock and monotonic clock readings to compute the result.
+// Similarly, t.In, t.Local, and t.UTC, which are defined to change only the Time's
 // Location, pass any monotonic clock reading through unmodified.
-// Because t.AddDate(y, m, d) is a wall time computation, it always
-// strips any monotonic clock reading from its result.
+// Because t.AddDate(y, m, d), t.Round(d), and t.Truncate(d) are wall time
+// computations, they always strip any monotonic clock reading from their results.
 //
 // If Times t and u both contain monotonic clock readings, the operations
 // t.After(u), t.Before(u), t.Equal(u), and t.Sub(u) are carried out
@@ -172,8 +171,7 @@ func (t *Time) addSec(d int64) {
 		}
 		// Wall second now out of range for packed field.
 		// Move to ext.
-		t.ext = t.sec()
-		t.wall &= nsecMask
+		t.stripMono()
 	}
 
 	// TODO: Check for overflow.
@@ -188,6 +186,14 @@ func (t *Time) setLoc(loc *Location) {
 	t.loc = loc
 }
 
+// stripMono strips the monotonic clock reading in t.
+func (t *Time) stripMono() {
+	if t.wall&hasMonotonic != 0 {
+		t.ext = t.sec()
+		t.wall &= nsecMask
+	}
+}
+
 // setMono sets the monotonic clock reading in t.
 // If t cannot hold a monotonic clock reading,
 // because its wall time is too large,
@@ -839,8 +845,7 @@ func (t Time) Add(d Duration) Time {
 		te := t.ext + int64(d)
 		if d < 0 && te > int64(t.ext) || d > 0 && te < int64(t.ext) {
 			// Monotonic clock reading now out of range; degrade to wall-only.
-			t.ext = t.sec()
-			t.wall &= nsecMask
+			t.stripMono()
 		} else {
 			t.ext = te
 		}
@@ -1373,6 +1378,7 @@ func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) T
 // time. Thus, Truncate(Hour) may return a time with a non-zero
 // minute, depending on the time's Location.
 func (t Time) Truncate(d Duration) Time {
+	t.stripMono()
 	if d <= 0 {
 		return t
 	}
@@ -1389,6 +1395,7 @@ func (t Time) Truncate(d Duration) Time {
 // time. Thus, Round(Hour) may return a time with a non-zero
 // minute, depending on the time's Location.
 func (t Time) Round(d Duration) Time {
+	t.stripMono()
 	if d <= 0 {
 		return t
 	}