Validate R_HOME path early and surface env discovery failures

[Copilot]

The kernel should fail fast when R_HOME is invalid and log when R env discovery fails before loading libraries.

• R_HOME validation: Check existence of the resolved R_HOME path immediately after r_home_setup, panicking with a clear path-specific message if missing or unreadable.
• Env discovery logging: Wrap the R command used to fetch share/include/doc paths in a match; on failure, emit an error log, preserving existing success handling.

Example:

let r_home = r_home_setup();
match r_home.try_exists() {
    Ok(true) => (),
    Ok(false) => panic!("The `R_HOME` path '{}' does not exist.", r_home.display()),
    Err(err) => panic!("Can't check if `R_HOME` path exists: {err}"),
}

match r_command(|cmd| { /* ... */ }) {
    Ok(output) => { /* parse and set envs */ }
    Err(err) => log::error!("Failed to discover R envvars: {err}"),
};

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https /usr/lib/apt/methods/https -I /home/REDACTED/work/ark/ark/target/debug/build/zmq-sys-c862c33cd3a44fa9/out --gdwarf-4 --64 -o 2wklqb7xgg.1hztmw8.rcgu.o mxffn8yka9.1hztmw8.rcgu.o zw4ve52ou6.1hztmw8.rcgu.o 5ry69rcexk.1hztmw8.rcgu.o 0phvhlm4b5.1hztmw8.rcgu.o xfri8wg97u.1hztmw8.rcgu.o a7d053c6gw.1hztmw8.rcgu.o 3qle9k0cmd.1hztmw8.rcgu.o r4el06n8il.1hztmw8.rcgu.o qdyyu0aiiz.1hztmw8.rcgu.o 65yysuh2hs.1hztmw8.rcgu.o jij157dqkn.1hztmw8.rcgu.o 2mvn4118p5.1hztmw8.rcgu.o f2coryg2l5.1hztmw8.rcgu.o (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

/copilot Please send a PR addressing these two bullet points:

Around https://github.com/posit-dev/ark/blob/main/crates/ark/src/interface.rs#L555-L561

We should check that the r_home path exists before trying to find the library. This way we can give a more accurate error message. This should happen before running the R command to get the paths.

If the R command fails altogether, we should log an error message.

---

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[lionel-]

Context is at: #997

The goal of this PR is to improve error reporting when the R_HOME install path is invalid.

[github-actions]

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[DavisVaughan]

Approved! But I would like to see the check move into r_home_setup()

[DavisVaughan]

I somewhat strongly think this should just move into r_home_setup().

i.e. make an invariant of r_home_setup() a guarantee that it has checked that the R_HOME it found does exist.

[lionel-]

Alright but I'll have r_home_setup() return a Result, I don't think it should be the one to panic

[DavisVaughan]

So IIUC all this does is handle the error path of r_command() better, rather than just ignoring it. That seems nice

[lionel-]

We're now also checking that R can be called