Re: [go-nuts] Changing wall clock in unit test

[David Finkel]

On Fri, Jul 30, 2021 at 12:13 AM Carl <carl...@gmail.com> wrote:

Hi,

Is there a way to write a unit test that simulates a change in the wall clock for a time.Time value?

I use a combination of two approaches to simulate the passage of time (often both work together):

• have functions a `now` argument so the caller supplies the timestamp (this avoids timestamp-skew for cases where the timestamps should be close together anyway)
• Use a package like https://github.com/vimeo/go-clocks (https://pkg.go.dev/github.com/vimeo/go-clocks) to use a real clock in production, but have a fake clock you can advance/set however you want without impinging on the main code

Full disclosure: I wrote github.com/vimeo/go-clocks, so I might be a bit biased -- it is based on one of Google's internal packages for C++, though.

One additional benefit of using a fake clock is that it allows for fine-grained synchronization between tests and code if you're sleeping in goroutines, as the fake clock in go-clocks has methods for awaiting a certain number of sleepers, etc.

 

Cheers,

Carl

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[Kurtis Rader]

On Thu, Jul 29, 2021 at 9:13 PM Carl <carl...@gmail.com> wrote:

Is there a way to write a unit test that simulates a change in the wall clock for a time.Time value?

Here is an example of how the Elvish shell stubs the time.After() function to verify its builtin "sleep" command behaves correctly without having to sleep for arbitrary intervals. It doesn't exactly do what you asked but illustrates one approach for doing such things.

https://github.com/elves/elvish/blob/659bd5fa68d4b61a6c916f2cafca360c9e37482b/pkg/eval/builtin_fn_misc.go#L356-L364 

https://github.com/elves/elvish/blob/659bd5fa68d4b61a6c916f2cafca360c9e37482b/pkg/eval/builtin_fn_misc_test.go#L100-L131

https://github.com/elves/elvish/blob/master/pkg/eval/builtin_fn_misc_unix_test.go

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Kurtis Rader

Caretaker of the exceptional canines Junior and Hank