Add unix::process::CommandExt::arg0

This allows argv[0] to be overridden on the executable's command-line. This also makes the program
executed independent of argv[0].

Does Fuchsia have the same semantics?

Addresses: #66510

src/libstd/sys/unix/ext/process.rs

@@ -2,6 +2,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use crate::ffi::OsStr;
 use crate::io;
 use crate::os::unix::io::{FromRawFd, RawFd, AsRawFd, IntoRawFd};
 use crate::process;
@@ -103,6 +104,14 @@ pub trait CommandExt {
     /// cross-platform `spawn` instead.
     #[stable(feature = "process_exec2", since = "1.9.0")]
     fn exec(&mut self) -> io::Error;
+
+    /// Set executable argument
+    ///
+    /// Set the first process argument, `argv[0]`, to something other than the
+    /// default executable path.
+    #[unstable(feature = "process_set_argv0", issue = "66510")]
+    fn arg0<S>(&mut self, arg: S) -> &mut process::Command
+        where S: AsRef<OsStr>;
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -127,6 +136,13 @@ impl CommandExt for process::Command {
     fn exec(&mut self) -> io::Error {
         self.as_inner_mut().exec(sys::process::Stdio::Inherit)
     }
+
+    fn arg0<S>(&mut self, arg: S) -> &mut process::Command
+        where S: AsRef<OsStr>
+    {
+        self.as_inner_mut().set_arg_0(arg.as_ref());
+        self
+    }
 }
 
 /// Unix-specific extensions to [`process::ExitStatus`].

src/libstd/sys/unix/process/process_common.rs

@@ -135,8 +135,8 @@ impl Command {
         let program = os2c(program, &mut saw_nul);
         Command {
             argv: Argv(vec![program.as_ptr(), ptr::null()]),
+            args: vec![program.clone()],
             program,
-            args: Vec::new(),
             env: Default::default(),
             cwd: None,
             uid: None,
@@ -149,11 +149,19 @@ impl Command {
         }
     }
 
+    pub fn set_arg_0(&mut self, arg: &OsStr) {
+        // Set a new arg0
+        let arg = os2c(arg, &mut self.saw_nul);
+        debug_assert!(self.argv.0.len() > 1);
+        self.argv.0[0] = arg.as_ptr();
+        self.args[0] = arg;
+    }
+
     pub fn arg(&mut self, arg: &OsStr) {
         // Overwrite the trailing NULL pointer in `argv` and then add a new null
         // pointer.
         let arg = os2c(arg, &mut self.saw_nul);
-        self.argv.0[self.args.len() + 1] = arg.as_ptr();
+        self.argv.0[self.args.len()] = arg.as_ptr();
         self.argv.0.push(ptr::null());
 
         // Also make sure we keep track of the owned value to schedule a
@@ -178,6 +186,10 @@ impl Command {
         &self.argv.0
     }
 
+    pub fn get_program(&self) -> &CStr {
+        &*self.program
+    }
+
     #[allow(dead_code)]
     pub fn get_cwd(&self) -> &Option<CString> {
         &self.cwd

src/libstd/sys/unix/process/process_fuchsia.rs

@@ -110,7 +110,7 @@ impl Command {
             ZX_HANDLE_INVALID,
             FDIO_SPAWN_CLONE_JOB | FDIO_SPAWN_CLONE_LDSVC | FDIO_SPAWN_CLONE_NAMESPACE
             | FDIO_SPAWN_CLONE_ENVIRON,  // this is ignored when envp is non-null
-            self.get_argv()[0], self.get_argv().as_ptr(), envp,
+            self.get_program().as_ptr(), self.get_argv().as_ptr(), envp,
             actions.len() as size_t, actions.as_ptr(),
             &mut process_handle,
             ptr::null_mut(),

src/libstd/sys/unix/process/process_unix.rs

@@ -248,7 +248,7 @@ impl Command {
             *sys::os::environ() = envp.as_ptr();
         }
 
-        libc::execvp(self.get_argv()[0], self.get_argv().as_ptr());
+        libc::execvp(self.get_program().as_ptr(), self.get_argv().as_ptr());
         Err(io::Error::last_os_error())
     }
 
@@ -373,7 +373,7 @@ impl Command {
                 .unwrap_or_else(|| *sys::os::environ() as *const _);
             let ret = libc::posix_spawnp(
                 &mut p.pid,
-                self.get_argv()[0],
+                self.get_program().as_ptr(),
                 file_actions.0.as_ptr(),
                 attrs.0.as_ptr(),
                 self.get_argv().as_ptr() as *const _,

src/test/ui/command-argv0.rs

@@ -0,0 +1,33 @@
+// run-pass
+
+// ignore-windows - this is a unix-specific test
+// ignore-cloudabi no processes
+// ignore-emscripten no processes
+// ignore-sgx no processes
+#![feature(process_set_argv0)]
+
+use std::env;
+use std::os::unix::process::CommandExt;
+use std::process::Command;
+
+fn main() {
+    let args: Vec<_> = env::args().collect();
+
+    if args.len() > 1 {
+        assert_eq!(args[1], "doing-test");
+        assert_eq!(args[0], "i have a silly name");
+
+        println!("passed");
+        return;
+    }
+
+    let output =
+        Command::new(&args[0]).arg("doing-test").arg0("i have a silly name").output().unwrap();
+    assert!(
+        output.stderr.is_empty(),
+        "Non-empty stderr: {}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+    assert!(output.status.success());
+    assert_eq!(output.stdout, b"passed\n");
+}