useradd: implemented useradd -K/--key KEY=VALUE (override /etc/login.defs defaults)

[jlesage]

This implements the useradd -K/--key KEY=VALUE (override /etc/login.defs defaults).

[pierre-warnier]

Thanks for this @jlesage, and sorry for the slow first response — the maintainers were away over August.

This is a well-put-together contribution: the LoginDefs::set/apply_overrides API is a clean place for the logic, overrides are applied before the defaults are read (so CREATE_HOME/SKEL behave correctly), and you covered it at three levels — unit tests on the parser, allocation tests for UID/GID/SYS ranges, and root-only integration tests. I ran the suite as root in the Debian container: 23/23 test_useradd tests pass, and fmt + clippy -D warnings + the full workspace suite are green.

One thing to fix before merge, then I'm happy to land it:

useradd -D ignores -K

cmd_defaults() is the third LoginDefs::load site and it doesn't get the overrides, so -K is silently dropped in -D mode. Verified on your branch:

$ useradd -D -K HOME=/OVERRIDDEN -K SHELL=/bin/zsh
GROUP=100
HOME=/home        # expected /OVERRIDDEN
INACTIVE=-1
EXPIRE=
SHELL=            # expected /bin/zsh

parse_options and do_useradd both apply them correctly — it's just this one path. Something like:

fn cmd_defaults(matches: &clap::ArgMatches) -> UResult<()> {
    let root = SysRoot::default();
    let mut defs = LoginDefs::load(&root.login_defs_path())
        .map_err(|e| UseraddError::CannotUpdatePasswd(format!("{e}")))?;
    apply_login_defs_overrides(&mut defs, &parse_login_defs_overrides(matches)?);

(the _matches parameter is already there, just unused). A small -D -K assertion in the integration tests would lock it in.

Non-blocking, for me to follow up

groupadd already has its own -K handling in allocate_gid, which hand-patches only GID_MIN/GID_MAX/SYS_GID_* and silently ignores unparseable values and every other key. Your LoginDefs::apply_overrides is strictly better — I'll open a separate PR migrating groupadd onto it after this lands, so the two tools behave the same. Nothing for you to do here.

[jlesage]

@pierre-warnier thank you for the review. I pushed the requested fix. Thank you!

[pierre-warnier]

Verified and merging — thanks @jlesage.

The -D path now honours the overrides:

$ useradd -D
HOME=/home
$ useradd -D -K HOME=/OVERRIDDEN -K SHELL=/bin/zsh
HOME=/OVERRIDDEN
SHELL=/bin/zsh

You also went past what I asked for, and both additions are good ones:

• cmd_defaults was hardcoding SysRoot::default(), so useradd -D -R /some/root silently read the host's login.defs. That was a separate bug and you fixed it.
• Splitting out write_defaults(matches, out) makes the report testable — it previously wrote straight to stdout with no seam.

Full gate green on Debian: fmt, clippy -D warnings, cargo test --workspace, and 25/25 test_useradd integration tests as root.

Sorry again for the four-week wait on the first review. Follow-up on my side: groupadd has its own hand-rolled -K handling that only understands the GID-range keys and swallows parse errors; I'll migrate it onto your LoginDefs::apply_overrides so both tools behave identically (#223).