os: make use of pidfd for linux

[kolyshkin]

Based on a discussion in #51246 (in particular, #51246 (comment)), this is a proposal to implement pidfd support for process methods in the os package -- StartProcess, FindProcess, Kill, Wait.

1. Reuse the handle field of struct Process to keep pidfd. Since 0 is a valid value, use ^uintptr(0) as a default value (if pidfd is not known).

2. Instrument syscall.StartProcess os.StartProcess to get pidfd from the kernel and return it as a handle. The initial idea was to modify syscall.StartProcess as it returns a handle (which we're reusing for pidfd) but that would be backward-incompatible change and can result in e.g. leaking pidfd as callers are not expecting it.

3. Use pidfdSendSignal from (*process).Signal, if handle is known and the syscall is available; fallback to old code using syscall.Kill if syscall is not available (e.g. disabled by seccomp).

4. Use waitid(P_PIDFD, ...) in Wait, if pidfd is set; fall back to old code if P_PIDFD is not supported.

• an implementation of 1-4: CL 528438 CL 570036

5. Amend FindProcess to use pidfd_open(2), optionally returning ErrProcessGone, and modify the documentation accordingly (adding that it can return ErrProcessGone on Linux).

• an implementation of 5: CL 528798 CL 570681

Attention doc team: items 1-5 above ^^^ are fully implemented as of Go 1.23rc1, and need to be part of go 1.23 release notes.

6. (Optional) Add (*Process).Handle() uintptr method to return process handle on Windows and pidfd on Linux. This might be useful for low-level operations that require handle/pidfd (e.g. pidfd_getfd on Linux), or to ensure that pidfd (rather than pid) is being used for kill/wait.

[gopherbot]

Change https://go.dev/cl/528436 mentions this issue: internal/syscall/unix: add PidFDSendSignal for Linux

[ianlancetaylor]

I don't think there is an API change here (before optional item 7) so taking this out of the proposal process.

CC @golang/runtime

[kolyshkin]

I don't think there is an API change here (before optional item 7) so taking this out of the proposal process.

@ianlancetaylor can you please elaborate? Is this about FindProcess returning ErrProcessGone, or something else?

If this is about FindProcess returning an error, as I pointed out in 4 above, this is entirely optional. We can make it succeed every time.

[ianlancetaylor]

What I mean is that we only need a proposal for an API change or a significant functionality change. This issue doesn't seem to be either, so we should treat it as an ordinary feature request issue, rather than running it through the proposal process. I hope that makes sense. Thanks.

[kolyshkin]

Thanks for the explanation, makes sense.I will keep working on implementation.

[gopherbot]

Change https://go.dev/cl/528438 mentions this issue: syscall: make StartProcess return pidfd on Linux

[gopherbot]

Change https://go.dev/cl/528798 mentions this issue: os: make use of pidfd on linux

[metux]

I'm a bit unhappy w/ #4:
API shouldn't behave different on individual OS. So I'd put it this way:

ErrProcessGone could happen on any OS. Just practically only happens on Linux for now. So, developers always need to be prepared for this.