Add a process_group method to UNIX CommandExt

Author: krallin

text/3228-process-process_group.md

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+- Feature Name: `process_set_process_group`
+- Start Date: 02-02-2022
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/3228)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/3228)
+
+# Summary
+[summary]: #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
+[motivation]: #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:
+
+https://github.com/rust-lang/rust/commit/8fe61546696b626ecf68ef838d5d82e393719e80
+
+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
+[guide-level-explanation]: #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
+[reference-level-explanation]: #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
+[drawbacks]: #drawbacks
+
+This marginally expands the API surface on `CommandExt`.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #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
+[prior-art]: #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
+[unresolved-questions]: #unresolved-questions
+
+- None known at this point.
+
+# Future possibilities
+[future-possibilities]: #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.