2 files changed, 186 insertions, 0 deletions
diff --git a/users/Profpatsch/execline/default.nix b/users/Profpatsch/execline/default.nix
new file mode 100644
index 0000000000..752774e6ad
--- /dev/null
+++ b/users/Profpatsch/execline/default.nix
@@ -0,0 +1,37 @@
+{ depot, pkgs, lib, ... }:
+
+let
+  exec-helpers = depot.nix.writers.rustSimpleLib
+    {
+      name = "exec-helpers";
+    }
+    (builtins.readFile ./exec_helpers.rs);
+
+  print-one-env = depot.nix.writers.rustSimple
+    {
+      name = "print-one-env";
+      dependencies = [
+        depot.users.Profpatsch.execline.exec-helpers
+      ];
+    } ''
+    extern crate exec_helpers;
+    use std::os::unix::ffi::OsStrExt;
+    use std::io::Write;
+
+    fn main() {
+      let args = exec_helpers::args("print-one-env", 1);
+      let valname = std::ffi::OsStr::from_bytes(&args[0]);
+      match std::env::var_os(&valname) {
+        None => exec_helpers::die_user_error("print-one-env", format!("Env variable `{:?}` is not set", valname)),
+        Some(val) => std::io::stdout().write_all(&val.as_bytes()).unwrap()
+      }
+    }
+  '';
+
+in
+depot.nix.readTree.drvTargets {
+  inherit
+    exec-helpers
+    print-one-env
+    ;
+}
diff --git a/users/Profpatsch/execline/exec_helpers.rs b/users/Profpatsch/execline/exec_helpers.rs
new file mode 100644
index 0000000000..a57cbca353
--- /dev/null
+++ b/users/Profpatsch/execline/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)
+}