fix: wait for activation before doing things

flake.nix

@@ -50,6 +50,7 @@
 
           devShell = pkgs.mkShell {
             RUST_SRC_PATH = "${pkgs.rust.packages.stable.rustPlatform.rustLibSrc}";
+            RUSTC_BOOTSTRAP = "1";
 
             nativeBuildInputs = with pkgs; [
               nixpkgs-fmt

scripts/native-systemd-shim/shim.nix

@@ -5,4 +5,6 @@ rustPlatform.buildRustPackage {
 
   src = ./.;
   cargoLock.lockFile = ./Cargo.lock;
+
+  RUSTC_BOOTSTRAP = "1";
 }

scripts/native-systemd-shim/src/main.rs

@@ -1,11 +1,13 @@
+#![feature(linux_pidfd)]
+
 use anyhow::Context;
 use nix::mount::{mount, MsFlags};
-use nix::unistd::execv;
+use nix::sys::wait::{waitid, Id, WaitPidFlag};
 use std::env;
-use std::ffi::CString;
 use std::fs::OpenOptions;
-use std::os::unix::ffi::OsStrExt;
+use std::os::linux::process::{ChildExt, CommandExt};
 use std::os::unix::io::{FromRawFd, IntoRawFd};
+use std::os::unix::process::CommandExt as _;
 use std::process::{Command, Stdio};
 
 fn real_main() -> anyhow::Result<()> {
@@ -37,30 +39,27 @@ fn real_main() -> anyhow::Result<()> {
         .context("When opening /dev/kmsg")?
         .into_raw_fd();
 
-    Command::new("/nix/var/nix/profiles/system/activate")
+    let mut child = Command::new("/nix/var/nix/profiles/system/activate")
         .env("LANG", "C.UTF-8")
         // SAFETY: we just opened this
         .stdout(unsafe { Stdio::from_raw_fd(kmsg_fd) })
         .stderr(unsafe { Stdio::from_raw_fd(kmsg_fd) })
+        .create_pidfd(true)
         .spawn()
         .context("When activating")?;
 
-    log::trace!("Spawning real systemd...");
+    let pidfd = child.take_pidfd().context("When getting pidfd")?;
+    waitid(Id::PIDFd(pidfd.into_raw_fd()), WaitPidFlag::WEXITED).context("When waiting")?;
 
-    let args = env::args_os()
-        .map(|x| CString::new(x.as_bytes()))
-        .collect::<Result<Vec<_>, _>>()
-        .context("When preparing systemd arguments")?;
+    log::trace!("Spawning real systemd...");
 
-    execv(
-        CString::new("/nix/var/nix/profiles/system/systemd/lib/systemd/systemd")?.as_c_str(),
-        &args,
+    // if things go right, we will never return from here
+    Err(
+        Command::new("/nix/var/nix/profiles/system/systemd/lib/systemd/systemd")
+            .args(env::args_os())
+            .exec()
+            .into(),
     )
-    .context("When spawning systemd")?;
-
-    // this should never be reached, but the ! type is experimental
-    // so we can't just return that
-    Ok(())
 }
 
 fn main() {