tstime/mono: fix Time.Unmarshal (#8480)

Calling both mono.Now() and time.Now() is slow and
leads to unnecessary precision errors.
Instead, directly compute mono.Time relative to baseMono and baseWall.
This is the opposite calculation as mono.Time.WallTime.

Updates tailscale/corp#8427

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

tstime/mono/mono.go

@@ -104,7 +104,8 @@ func (t Time) WallTime() time.Time {
 
 // MarshalJSON formats t for JSON as if it were a time.Time.
 // We format Time this way for backwards-compatibility.
-// This is best-effort only. Time does not survive a MarshalJSON/UnmarshalJSON round trip unchanged.
+// Time does not survive a MarshalJSON/UnmarshalJSON round trip unchanged
+// across different invocations of the Go process. This is best-effort only.
 // Since t is a monotonic time, it can vary from the actual wall clock by arbitrary amounts.
 // Even in the best of circumstances, it may vary by a few milliseconds.
 func (t Time) MarshalJSON() ([]byte, error) {
@@ -113,7 +114,8 @@ func (t Time) MarshalJSON() ([]byte, error) {
 }
 
 // UnmarshalJSON sets t according to data.
-// This is best-effort only. Time does not survive a MarshalJSON/UnmarshalJSON round trip unchanged.
+// Time does not survive a MarshalJSON/UnmarshalJSON round trip unchanged
+// across different invocations of the Go process. This is best-effort only.
 func (t *Time) UnmarshalJSON(data []byte) error {
 	var tt time.Time
 	err := tt.UnmarshalJSON(data)
@@ -124,6 +126,6 @@ func (t *Time) UnmarshalJSON(data []byte) error {
 		*t = 0
 		return nil
 	}
-	*t = Now().Add(-time.Since(tt))
+	*t = baseMono.Add(tt.Sub(baseWall))
 	return nil
 }

tstime/mono/mono_test.go

@@ -33,6 +33,21 @@ func TestUnmarshalZero(t *testing.T) {
 	}
 }
 
+func TestJSONRoundtrip(t *testing.T) {
+	want := Now()
+	b, err := want.MarshalJSON()
+	if err != nil {
+		t.Errorf("MarshalJSON error: %v", err)
+	}
+	var got Time
+	if err := got.UnmarshalJSON(b); err != nil {
+		t.Errorf("UnmarshalJSON error: %v", err)
+	}
+	if got != want {
+		t.Errorf("got %v, want %v", got, want)
+	}
+}
+
 func BenchmarkMonoNow(b *testing.B) {
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {