chore(users/Profpatsch): move rust exec-helpers to own subdir

This is so we can use the rust language server for the file.

Change-Id: I8a2fe15ea67fd0e26814fda57bf0cace0d264cae
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7792
Autosubmit: Profpatsch <mail@profpatsch.de>
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

users/Profpatsch/execline/exec-helpers/exec_helpers.rs

@@ -0,0 +1,149 @@
+use std::ffi::OsStr;
+use std::os::unix::ffi::{OsStrExt, OsStringExt};
+use std::os::unix::process::CommandExt;
+
+pub fn no_args(current_prog_name: &str) -> () {
+    let mut args = std::env::args_os();
+    // remove argv[0]
+    let _ = args.nth(0);
+    if args.len() > 0 {
+        die_user_error(
+            current_prog_name,
+            format!("Expected no arguments, got {:?}", args.collect::<Vec<_>>()),
+        )
+    }
+}
+
+pub fn args(current_prog_name: &str, no_of_positional_args: usize) -> Vec<Vec<u8>> {
+    let mut args = std::env::args_os();
+    // remove argv[0]
+    let _ = args.nth(0);
+    if args.len() != no_of_positional_args {
+        die_user_error(
+            current_prog_name,
+            format!(
+                "Expected {} arguments, got {}, namely {:?}",
+                no_of_positional_args,
+                args.len(),
+                args.collect::<Vec<_>>()
+            ),
+        )
+    }
+    args.map(|arg| arg.into_vec()).collect()
+}
+
+pub fn args_for_exec(
+    current_prog_name: &str,
+    no_of_positional_args: usize,
+) -> (Vec<Vec<u8>>, Vec<Vec<u8>>) {
+    let mut args = std::env::args_os();
+    // remove argv[0]
+    let _ = args.nth(0);
+    let mut args = args.map(|arg| arg.into_vec());
+    let mut pos_args = vec![];
+    // get positional args
+    for i in 1..no_of_positional_args + 1 {
+        pos_args.push(args.nth(0).expect(&format!(
+            "{}: expects {} positional args, only got {}",
+            current_prog_name, no_of_positional_args, i
+        )));
+    }
+    // prog... is the rest of the iterator
+    let prog: Vec<Vec<u8>> = args.collect();
+    (pos_args, prog)
+}
+
+pub fn exec_into_args<'a, 'b, Args, Arg, Env, Key, Val>(
+    current_prog_name: &str,
+    args: Args,
+    env_additions: Env,
+) -> !
+where
+    Args: IntoIterator<Item = Arg>,
+    Arg: AsRef<[u8]>,
+    Env: IntoIterator<Item = (Key, Val)>,
+    Key: AsRef<[u8]>,
+    Val: AsRef<[u8]>,
+{
+    // TODO: is this possible without collecting into a Vec first, just leaving it an IntoIterator?
+    let args = args.into_iter().collect::<Vec<Arg>>();
+    let mut args = args.iter().map(|v| OsStr::from_bytes(v.as_ref()));
+    let prog = args.nth(0).expect(&format!(
+        "{}: first argument must be an executable",
+        current_prog_name
+    ));
+    // TODO: same here
+    let env = env_additions.into_iter().collect::<Vec<(Key, Val)>>();
+    let env = env
+        .iter()
+        .map(|(k, v)| (OsStr::from_bytes(k.as_ref()), OsStr::from_bytes(v.as_ref())));
+    let err = std::process::Command::new(prog).args(args).envs(env).exec();
+    die_missing_executable(
+        current_prog_name,
+        format!(
+            "exec failed: {}, while trying to execing into {:?}",
+            err, prog
+        ),
+    );
+}
+
+/// Exit 1 to signify a generic expected error
+/// (e.g. something that sometimes just goes wrong, like a nix build).
+pub fn die_expected_error<S>(current_prog_name: &str, msg: S) -> !
+where
+    S: AsRef<str>,
+{
+    die_with(1, current_prog_name, msg)
+}
+
+/// Exit 100 to signify a user error (“the user is holding it wrong”).
+/// This is a permanent error, if the program is executed the same way
+/// it should crash with 100 again.
+pub fn die_user_error<S>(current_prog_name: &str, msg: S) -> !
+where
+    S: AsRef<str>,
+{
+    die_with(100, current_prog_name, msg)
+}
+
+/// Exit 101 to signify an unexpected crash (failing assertion or panic).
+/// This is the same exit code that `panic!()` emits.
+pub fn die_panic<S>(current_prog_name: &str, msg: S) -> !
+where
+    S: AsRef<str>,
+{
+    die_with(101, current_prog_name, msg)
+}
+
+/// Exit 111 to signify a temporary error (such as resource exhaustion)
+pub fn die_temporary<S>(current_prog_name: &str, msg: S) -> !
+where
+    S: AsRef<str>,
+{
+    die_with(111, current_prog_name, msg)
+}
+
+/// Exit 126 to signify an environment problem
+/// (the user has set up stuff incorrectly so the program cannot work)
+pub fn die_environment_problem<S>(current_prog_name: &str, msg: S) -> !
+where
+    S: AsRef<str>,
+{
+    die_with(126, current_prog_name, msg)
+}
+
+/// Exit 127 to signify a missing executable.
+pub fn die_missing_executable<S>(current_prog_name: &str, msg: S) -> !
+where
+    S: AsRef<str>,
+{
+    die_with(127, current_prog_name, msg)
+}
+
+fn die_with<S>(status: i32, current_prog_name: &str, msg: S) -> !
+where
+    S: AsRef<str>,
+{
+    eprintln!("{}: {}", current_prog_name, msg.as_ref());
+    std::process::exit(status)
+}