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>

1 files changed, 21 insertions, 0 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() {
 	})