runtime, time: don't use monotonic clock inside synctest bubbles

Don't include a monotonic time in time.Times created inside
a bubble, to avoid the confusion of different Times using
different monotonic clock epochs.

For #67434

goos: darwin
goarch: arm64
pkg: time
cpu: Apple M1 Pro
         │ /tmp/bench.0 │            /tmp/bench.1            │
         │    sec/op    │   sec/op     vs base               │
Since-10    18.42n ± 2%   18.68n ± 1%       ~ (p=0.101 n=10)
Until-10    18.28n ± 2%   18.46n ± 2%  +0.98% (p=0.009 n=10)
geomean     18.35n        18.57n       +1.20%

Change-Id: Iaf1b80d0a4df52139c5b80d4bde4410ef8a49f2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/657415
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

4 files changed, 52 insertions, 3 deletions

diff --git a/src/internal/synctest/synctest_test.go b/src/internal/synctest/synctest_test.go
index 450d5f5416..62acb42359 100644
--- a/src/internal/synctest/synctest_test.go
+++ b/src/internal/synctest/synctest_test.go
@@ -37,6 +37,27 @@ func TestNow(t *testing.T) {
 	})
 }
 
+// TestMonotonicClock exercises comparing times from within a bubble
+// with ones from outside the bubble.
+func TestMonotonicClock(t *testing.T) {
+	start := time.Now()
+	synctest.Run(func() {
+		time.Sleep(time.Until(start.Round(0)))
+		if got, want := time.Now().In(time.UTC), start.In(time.UTC); !got.Equal(want) {
+			t.Fatalf("time.Now() = %v, want %v", got, want)
+		}
+
+		wait := 1 * time.Second
+		time.Sleep(wait)
+		if got := time.Since(start); got != wait {
+			t.Fatalf("time.Since(start) = %v, want %v", got, wait)
+		}
+		if got := time.Now().Sub(start); got != wait {
+			t.Fatalf("time.Now().Sub(start) = %v, want %v", got, wait)
+		}
+	})
+}
+
 func TestRunEmpty(t *testing.T) {
 	synctest.Run(func() {
 	})
diff --git a/src/runtime/time.go b/src/runtime/time.go
index c22d39c089..3ece161cf4 100644
--- a/src/runtime/time.go
+++ b/src/runtime/time.go
@@ -18,7 +18,13 @@ func time_runtimeNow() (sec int64, nsec int32, mono int64) {
 	if sg := getg().syncGroup; sg != nil {
 		sec = sg.now / (1000 * 1000 * 1000)
 		nsec = int32(sg.now % (1000 * 1000 * 1000))
-		return sec, nsec, sg.now
+		// Don't return a monotonic time inside a synctest bubble.
+		// If we return a monotonic time based on the fake clock,
+		// arithmetic on times created inside/outside bubbles is confusing.
+		// If we return a monotonic time based on the real monotonic clock,
+		// arithmetic on times created in the same bubble is confusing.
+		// Simplest is to omit the monotonic time within a bubble.
+		return sec, nsec, 0
 	}
 	return time_now()
 }
@@ -32,6 +38,11 @@ func time_runtimeNano() int64 {
 	return nanotime()
 }
 
+//go:linkname time_runtimeIsBubbled time.runtimeIsBubbled
+func time_runtimeIsBubbled() bool {
+	return getg().syncGroup != nil
+}
+
 // A timer is a potentially repeating trigger for calling t.f(t.arg, t.seq).
 // Timers are allocated by client code, often as part of other data structures.
 // Each P has a heap of pointers to timers that it manages.
diff --git a/src/time/time.go b/src/time/time.go
index 14e79672ca..bcaeee407e 100644
--- a/src/time/time.go
+++ b/src/time/time.go
@@ -1221,7 +1221,7 @@ func subMono(t, u int64) Duration {
 // Since returns the time elapsed since t.
 // It is shorthand for time.Now().Sub(t).
 func Since(t Time) Duration {
-	if t.wall&hasMonotonic != 0 {
+	if t.wall&hasMonotonic != 0 && !runtimeIsBubbled() {
 		// Common case optimization: if t has monotonic time, then Sub will use only it.
 		return subMono(runtimeNano()-startNano, t.ext)
 	}
@@ -1231,7 +1231,7 @@ func Since(t Time) Duration {
 // Until returns the duration until t.
 // It is shorthand for t.Sub(time.Now()).
 func Until(t Time) Duration {
-	if t.wall&hasMonotonic != 0 {
+	if t.wall&hasMonotonic != 0 && !runtimeIsBubbled() {
 		// Common case optimization: if t has monotonic time, then Sub will use only it.
 		return subMono(t.ext, runtimeNano()-startNano)
 	}
@@ -1325,6 +1325,9 @@ func runtimeNow() (sec int64, nsec int32, mono int64)
 //go:linkname runtimeNano
 func runtimeNano() int64
 
+//go:linkname runtimeIsBubbled
+func runtimeIsBubbled() bool
+
 // Monotonic times are reported as offsets from startNano.
 // We initialize startNano to runtimeNano() - 1 so that on systems where
 // monotonic time resolution is fairly low (e.g. Windows 2008
diff --git a/src/time/time_test.go b/src/time/time_test.go
index ff253be46b..dcb477b658 100644
--- a/src/time/time_test.go
+++ b/src/time/time_test.go
@@ -1497,6 +1497,20 @@ func BenchmarkNowUnixMicro(b *testing.B) {
 	}
 }
 
+func BenchmarkSince(b *testing.B) {
+	start := Now()
+	for b.Loop() {
+		u = int64(Since(start))
+	}
+}
+
+func BenchmarkUntil(b *testing.B) {
+	end := Now().Add(1 * Hour)
+	for b.Loop() {
+		u = int64(Until(end))
+	}
+}
+
 func BenchmarkFormat(b *testing.B) {
 	t := Unix(1265346057, 0)
 	for i := 0; i < b.N; i++ {