Add std::os::unix::process::CommandExt::chroot to safely chroot a child process

[joshtriplett]

This adds a chroot method to the CommandExt extension trait for the
Command builder, to set a directory to chroot into. This will chroot
the child process into that directory right before calling chdir for the
Command's working directory.

To avoid allowing a process to have a working directory outside of the
chroot, if the Command does not yet have a working directory set,
chroot will set its working directory to "/".

---

ACP: rust-lang/libs-team#551

This PR currently has the tracking issue set to "none"; if the ACP is approved,
I'll file a tracking issue and update the PR.

[rustbot]

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[bors]

☔ The latest upstream changes (presumably #136929) made this pull request unmergeable. Please resolve the merge conflicts.

[joshtriplett]

Will rebase shortly.

[Amanieu]

@bors r+

[bors]

📌 Commit 33d3376 has been approved by Amanieu

It is now in the queue for this repository.

[joshtriplett]

Sigh. This is the kind of thing that makes me hesitate to try to use links in doc comments.

[bors]

⌛ Testing commit 33d3376 with merge 0424534...

[joshtriplett]

@bors r-

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

 Documenting std v0.0.0 (/checkout/library/std)
error: unresolved link to `Command::current_dir`
   --> library/std/src/os/unix/process.rs:205:72
    |
205 |     /// This happens before changing to the directory specified with [`Command::current_dir`], and
    |                                                                        ^^^^^^^^^^^^^^^^^^^^ no item named `Command` in scope
    |
    = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`

error: unresolved link to `Command::current_dir`
   --> library/std/src/os/unix/process.rs:208:51
    |
208 |     /// If no directory has been specified with [`Command::current_dir`], this will set the directory
    |                                                   ^^^^^^^^^^^^^^^^^^^^ no item named `Command` in scope

error: could not document `std`
warning: build failed, waiting for other jobs to finish...
[RUSTC-TIMING] std test:false 3.146
Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:00:23
  local time: Wed May 21 05:48:59 UTC 2025
  network time: Wed, 21 May 2025 05:48:59 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

[joshtriplett]

@bors r=Amanieu

[bors]

📌 Commit 348c1b0 has been approved by Amanieu

It is now in the queue for this repository.

[joshtriplett]

@bors rollup