Summary

Add a process_group method to std::os::unix::process::CommandExt that allows setting the process group id (i.e. calling setpgid) in the child, thus enabling users to set process groups while leveraging the posix_spawn fast path.

Motivation

The Unix process spawn code has two paths: a fast path that uses posix_spawn, and a slow path that uses fork and exec.

The performance between the two APIs has been shown to be very noticeable:

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Currently, users can set the process group on the commands they spawn via:

let pre_exec = || nix::unistd::setpgid( ... );
unsafe {
    cmd.pre_exec(pre_exec)
};

This approach forces the slow path because of the usage of pre_exec.

However, posix_spawn supports setting the process group (posix_spawnattr_setpgroup). This RFC proposes exposing that functionality, which allows users to set the process group id without forcing the slow path.

Guide-level explanation

std::os::unix::process::CommandExt::process_group allows you to set the process group ID of the child process. This translates to a setpgid call in the child.

Reference-level explanation

The changes needed are:

Expose a CommandExt a process_group method on CommandExt that takes a PID as argument.
Add a call to posix_spawnattr_setpgroup on the fast path.
Add a call to setpgid on the slow path.

Drawbacks

This marginally expands the API surface on CommandExt.

Rationale and alternatives

Using pre_exec this is a viable alternative for programs where fork is either sufficiently fast or infrequent.
Not using std::process, and rolling your own instead, is an alternative. This would however break interoperability with e.g. Tokio's tokio::process::Command, which currently can be created using a Command from the std lib.

Prior art

The primary prior art here is all the other calls that already exist on CommandExt that translate to parameterizing posix_spawn, such as configuring groups, signal mask, current working directory, open pipes.

Unresolved questions

None known at this point.

Future possibilities

There are a few other posix_spawn options that are not supported, such as setsid (which is a GNU extension). Those might warrant inclusion as well.

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