chroot-realpath: init

Now it's placed between initrd-switch-root.target and
initrd-switch-root.service, meaning it is truly the last thing to
happen before switch-root, as it should be.

pkgs/by-name/ch/chroot-realpath/package.nix

@@ -0,0 +1,21 @@
+{
+  lib,
+  rustPlatform,
+}:
+
+let
+  cargo = lib.importTOML ./src/Cargo.toml;
+in
+rustPlatform.buildRustPackage {
+  pname = cargo.package.name;
+  version = cargo.package.version;
+
+  src = ./src;
+
+  cargoLock.lockFile = ./src/Cargo.lock;
+
+  meta = {
+    description = "Output a path's realpath within a chroot.";
+    maintainers = [ lib.maintainers.elvishjerricco ];
+  };
+}

pkgs/by-name/ch/chroot-realpath/src/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "chroot-realpath"
+version = "0.1.0"

pkgs/by-name/ch/chroot-realpath/src/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "chroot-realpath"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+
+[profile.release]
+opt-level = "z"

pkgs/by-name/ch/chroot-realpath/src/src/main.rs

@@ -0,0 +1,24 @@
+use std::env;
+use std::io::{stdout, Error, ErrorKind, Write};
+use std::os::unix::ffi::OsStrExt;
+use std::os::unix::fs;
+
+fn main() -> std::io::Result<()> {
+    let args: Vec<String> = env::args().collect();
+
+    if args.len() != 3 {
+        return Err(Error::new(
+            ErrorKind::InvalidInput,
+            format!("Usage: {} <chroot> <path>", args[0]),
+        ));
+    }
+
+    fs::chroot(&args[1])?;
+    std::env::set_current_dir("/")?;
+
+    let path = std::fs::canonicalize(&args[2])?;
+
+    stdout().write_all(path.into_os_string().as_bytes())?;
+
+    Ok(())
+}