Go: time: mockable time support

[jmhodges]

With https://github.com/jmhodges/clock and the broader https://github.com/benbjohnson/clock API,
there's a desire for the timing systems in Go to be fakeable[1]. Using a fake clock
instead of time.Now directly is useful when testing code that stores timestamps, caches
data, etc.

The benbjohnson clock package attempts to make Ticker and Timer calls (as well as their
related AfterFunc and Sleep methods) work against a fake time that can be set and
updated in tests. However, it relies on some micro-sleeps and runtime.Gosched calls that
are obviously going to be flaky. But there is a desire to able to test code that uses
Tickers and Timers, not by adjusting the durations they work for (which can induce flaky
testing), but by adjusting when they think they need to wake up.

To do that, we seem to need more runtime magic to help developers out. In fact, it might
be best if a clock package like these lived in the stdlib so that it could be tied more
carefully and thoughtfully to the scheduler.

 [1] or "mockable", whatever language you prefer. The summary of this issue is to distinguish it from issue #5356.

[ianlancetaylor]

Comment 1:

I'm not entirely sure this is a good idea.  I'm also not sure what it would look like. 
If the goal is simply to provide Ticker and Timer calls for testing, I think that could
be done entirely independently of the time package, along the lines of the old
playground code.  The testing package would keep its own queue of events, and step
forward to the next event without actually waiting.

_Labels changed: added repo-main, release-none._

[dvyukov]

Comment 2:

I am not sure playground approach will work in large apps. In playground we advice time
when the process in completely out of other work. But what if we have a goroutine in a
syscall/cgo, should we advice time? Or wait for it to return from syscall/cgo first?

[sougou]

Comment 3:

The main challenge we faced was related to the original code importing the standard time
package, which is not mockable. This forces the tests to also use actual clock time.
We've mostly managed to work around these. So, it's not a big issue now. But it's still
a nice-to-have.

[sougou]

Comment 4:

What if the time package accelerated the clock (just for the process) by an order of
magnitude? We can consider restricting this ability to the testing package.
There are still some downsides: it won't work for integration tests, but it may satisfy
the needs of simple in-process tests.

[jmhodges]

Any chance of something happening for this?

[jmhodges]

(I'm okay with hearing no! It's just an open cycle in my head I'm trying to clear out.)

[adg]

It's unlikely to happen any time soon.

[flimzy]

One possible step toward this, for the common time.Now() case, would be to add two exported symbols to the time package:

type Clock interface {
    Now() time.Time
}

var DefaultClock = /* an (unexported?) default implementation of Clock */

Then the existing time.Now() function (and possibly others) would become a de facto alias for time.DefaultClock.Now().

Whether the ticker/timer implementations could take advantage of such a Clock interface, or if it would need to be expanded, I don't know.

The primary motivation here, of course, is to provide a standard place to mock out a clock, while honoring the Go compatibility promise. It would, obviously, require altering any existing code that needs mocks, but that's already happening, so this would just provide a defined, supported way to do such mocks.

One other (possible) implication of such a Clock interface, is that it would be possible to alter the default timezone from the system default, by configuring a custom Clock instance. There may be other non-mocking uses as well, which I haven't considered yet.

[tv42]

Having a caller-replaceable time.DefaultClock sounds both racy and like a step in the wrong direction. If this is about testability, we should live in a world where tests are parallelizable and don't muck with globals. (I'm ok with eating the costs of needing to write code to be testable; that's going to be true anyway, regardless of this yet another reason to write "hermetic" libraries.)

My reading of this issue is to provide better means for such a mock-clock library to simulate the effects of time passing "until next interesting event" in the scheduler.

[flimzy]

Having a caller-replaceable time.DefaultClock sounds both racy

Perhaps so, but it's a pattern commonly used elsewhere in the Go standard library.

If this is about testability, we should live in a world where tests are parallelizable and don't muck with globals.

Code which relies on this functionality for testing shouldn't mess with globals (the same holds for the other places in the stdlib where this is done), and should instead use dependency injection.

The goal isn't to make it possible to modify time.DefaultClock in tests, but rather to provide a standard way to inject a clock into code that needs to be tested, while allowing legacy code, or code which doesn't need to be tested, to rely on the time.DefaultClock implementation.

I think of this as the same as http.DefaultClient. While it's _possible_ to override http.DefaultClient in tests, I've never seen anyone advocate this. I wouldn't advocate this for time.DefaultClock, either.

[flimzy]

Note that the suggestion of var DefaultClock could easily be removed, and my proposal would still satisfy my purpose, without adding the "scary" race opportunities. It just wouldn't be as transparent (I suppose) that the package level Now() is the equivalent of using a default implementation of the Clock interface.

[tv42]

Perhaps so, but it's a pattern commonly used elsewhere in the Go standard library.

If you dig around the issues enough, you will find core devs sometimes expressing regret about those decisions. Just because it was done in the early days doesn't mean it should be the standard going forward.

[flimzy]

If you dig around the issues enough, you will find core devs sometimes expressing regret about those decisions.

That's fair. I've often wondered about the wisdom of that myself.

But I still don't think that renders my (entire) suggestion untenable. I'd welcome your feedback on the remainder of my proposal. Specifically on the Clock interface idea.

For the sake of discussion, the DefaultClock can be turned into a function which returns a default implementation. This eliminates the user-replaceable, racy aspects.

I feel like this conversation got derailed by this (IMO, minor) aspect of my proposal. I'd rather address the core of it.

[tv42]

I think the two existing & used libraries mentioned in the first comment are plenty to inform the API design; as far as I understand the challenge here is purely the integration with runtime. See mentions of "flaky".

[jmhodges]

Yeah, my first paragraph in this ticket was really to set up the latter two that describe why we want and what needs stdlib or runtime support to write better time-dependent tests. Clock interfaces are easy to implement outside of the stdlib and runtime (there's been a proliferation of these beyond just the packages listed now) but the real trick is giving people the ability to test code that involves Tickers and Timers.

There are likely lots of levels of ideas to address that Ticker and Timer problem.

One is to give Tickers and Timers new constructor funcs that take some kind of expanded Clock interface that can inform the Tickers and Timers when to run (instead of having them burn a bunch of CPU checking it in a loop). This would be similar to the addition of Context functions to database/sql.

[jmhodges]

This ticket pre-dates the Go proposal mechanisms and we might be getting to the time for a formal proposal to resolve this one.

It would be cool to hear ideas different from the rough sketch I gave above. I'm fairly sure there is one.

[maruel]

When this issue was filed, context.Context was not in the standard library; Context was added in Go 1.7 in 2016.

I like the idea of attaching mocks to the context. It may be considered a non-starter for some people as an abuse of context.

The following assumes this is reasonable:

Here's a precedent based on attaching a Clock interface on a context.Context. This is not racy and permits very well scoped tests:

• time replacement package: https://godoc.org/go.chromium.org/luci/common/clock#Clock
• time mock package: https://godoc.org/go.chromium.org/luci/common/clock/testclock

main.go

package main

import (
        "context"
        "fmt"

        "go.chromium.org/luci/common/clock"
)

func getFormattedTime(ctx context.Context) string {
        clk := clock.Get(ctx)
        return clk.Now().String()
}

func main() {
        fmt.Println(getFormattedTime(context.Background()))
}

main_test.go

package main

import (
        "context"
        "fmt"
        "time"

        "go.chromium.org/luci/common/clock/testclock"
)

func ExampleGetFormattedTime() {
        fixed := time.Date(2006, 1, 2, 15, 4, 5, 6, time.UTC)
        ctx, tm := testclock.UseTime(context.Background(), fixed)
        fmt.Println(getFormattedTime(ctx))

        later := fixed.Add(time.Hour)
        tm.Set(later)
        fmt.Println(getFormattedTime(ctx))

        // Output:
        // 2006-01-02 15:04:05.000000006 +0000 UTC
        // 2006-01-02 16:04:05.000000006 +0000 UTC
}

IMHO this is quite clean. I'm not sure the Clock interface here should be used as-is. I'm not sure it's a good idea to pass a context in to Sleep and NewTimer. Still, I think something inspired by this could be added to the standard library without any regression.