x/playground: exec.CommandContext(...).Run() does not honor context timeout

[hlin117]

What version of Go are you using (go version)?

1.14.2, as stated on the Go Playground (as of May 4th, 2020).

Does this issue reproduce with the latest release?

Yes, 1.14.2 is the latest release.

What operating system and processor architecture are you using (go env)?

Environment is the Go Playground.

Note that I cannot reproduce this error on my 2019 Macbook, running 1.14.2.

What did you do?

I ran the demo on the Go documentation with os/exec:CommandContext, I discovered that the timeout example seemed to have strange behavior.

It seemed the demo was hitting the timeout limit of the Go Playground, ignoring the 100ms context timeout.

Modifying the demo a little bit, I discovered that the timeout on the Go Playground was being ignored completely. See below screenshot.

See Go Playground link: https://play.golang.org/p/ckZbgZ7n1tM

What did you expect to see?

The 100ms timeout from the context should have been honored when running exec.CommandContext(...). It seems based on #21880 and me running the demo on my laptop that this behavior worked in previous releases, and this bug might exist on different environments.

What did you see instead?

The 100ms timeout is being ignored on the Go Playground environment.

[ianlancetaylor]

Does the playground really let you exec other programs?

[hlin117]

I'm assuming from the documentation (whose code hasn't changed for several releases) that this has been the case for several years (based on #21880, since 2017).

[ianlancetaylor]

issue #21880 is not about the Go playground. I read your issue as saying that your program works correctly on your local system, but fails on the Go playground. Is that right? Thanks.

[hlin117]

Yes that's correct. It works on my local system, but not on the Go playground.

[ianlancetaylor]

Thanks. I am saying: does using the os/exec package ever work on the Go playground at all? The Go playground is a very limited execution environment. It doesn't support running arbitrary programs.

[prattmic]

cc @toothrot

The playground is now running on gVisor, so it will allow executing other programs unless the Go build itself prevents calling execve [1]. I could be wrong, but I thought faketime was the only change from a standard build now.

I'd imagine it is faketime that would break timeouts.

[1] That said, I wouldn't necessarily expect other binaries, like sleep, to exist in the filesystem.

[toothrot]

@prattmic That is correct, as of the changes in #25224. The container is based on busybox:glibc, so there are some built in programs.

[toothrot]

I believe that this issue is related to faketime. I can't think of any easy way to resolve that.

In order to test timeouts with faketime, you'll have to add code that advances the fake clock: https://play.golang.org/p/9ERCtbudekx

I'm going to close this as it seems to be working as intended. Also, I believe we're generally fine with the ability to exec inside the gvisor'd sandbox docker container. Please let me know if I am mistaken.