Support for running with specific user id and group id

Also some improvements related to typesafety. Note that with this
change, it makes it feature wise complete with the Haskell's pid1.

Author: psibi

README.md

@@ -62,6 +62,26 @@ RUN chmod +x /usr/bin/pid1
 ENTRYPOINT [ "pid1" ]
 ```
 
+Various options supported by the binary:
+
+``` shellsession
+❯ pid1 --help
+Usage:
+
+Arguments:
+  <COMMAND>  Process to run
+  [ARGS]...  Arguments to the process
+
+Options:
+  -w, --workdir <DIR>        Specify working direcory
+  -t, --timeout <TIMEOUT>    Timeout (in seconds) to wait for child proess to exit [default: 2]
+  -v, --verbose              Turn on verbose output
+  -e, --env <ENV>            Override environment variables. Can specify multiple times
+  -u, --user-id <USER_ID>    Run command with user ID
+  -g, --group-id <GROUP_ID>  Run command with group ID
+  -h, --help                 Print help
+```
+
 ## Development
 
 The testing steps are documented in [Development.md](./Development.md). We only have

pid1-exe/src/cli.rs

@@ -26,19 +26,34 @@ pub(crate) struct Pid1App {
     /// Override environment variables. Can specify multiple times.
     #[arg(short, long, value_parser=parse_key_val::<OsString, OsString>)]
     pub(crate) env: Vec<(OsString, OsString)>,
-    /// Process arguments
+    /// Run command with user ID
+    #[arg(short, long, value_name = "USER_ID")]
+    user_id: Option<u32>,
+    /// Run command with group ID
+    #[arg(short, long, value_name = "GROUP_ID")]
+    group_id: Option<u32>,
+    /// Process to run
     #[arg(required = true)]
-    child_process: Vec<String>,
+    pub(crate) command: String,
+    /// Arguments to the process
+    #[arg(required = false)]
+    pub(crate) args: Vec<String>,
 }
 
 impl Pid1App {
     #[cfg(target_family = "unix")]
     pub(crate) fn run(self) -> ! {
-        let mut child = std::process::Command::new(&self.child_process[0]);
-        let child = child.args(&self.child_process[1..]);
+        let mut child = std::process::Command::new(&self.command);
+        let child = child.args(&self.args[..]);
         if let Some(workdir) = &self.workdir {
             child.current_dir(workdir);
         }
+        if let Some(user_id) = &self.user_id {
+            child.uid(*user_id);
+        }
+        if let Some(group_id) = &self.group_id {
+            child.gid(*group_id);
+        }
         for (key, value) in &self.env {
             child.env(key, value);
         }
@@ -55,10 +70,7 @@ impl Pid1App {
             let child = match child {
                 Ok(child) => child,
                 Err(err) => {
-                    eprintln!(
-                        "pid1: {} spawn failed. Got error: {err}",
-                        self.child_process[0]
-                    );
+                    eprintln!("pid1: {} spawn failed. Got error: {err}", self.command);
                     std::process::exit(1);
                 }
             };