rust: library: Add setsid method to CommandExt trait

Add a setsid method to the CommandExt trait so that callers can create
a process in a new session and process group whilst still using the
POSIX spawn fast path.

Co-Authored-By: Harvey Hunt <harveyhunt@fb.com>

Author: LevitatingBusinessMan

library/std/src/os/unix/process.rs

@@ -210,6 +210,9 @@ pub trait CommandExt: Sealed {
     /// intentional difference from the underlying `chroot` system call.)
     #[unstable(feature = "process_chroot", issue = "141298")]
     fn chroot<P: AsRef<Path>>(&mut self, dir: P) -> &mut process::Command;
+
+    #[unstable(feature = "process_setsid", issue = "105376")]
+    fn setsid(&mut self, setsid: bool) -> &mut process::Command;
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -260,6 +263,11 @@ impl CommandExt for process::Command {
         self.as_inner_mut().chroot(dir.as_ref());
         self
     }
+
+    fn setsid(&mut self, setsid: bool) -> &mut process::Command {
+        self.as_inner_mut().setsid(setsid);
+        self
+    }
 }
 
 /// Unix-specific extensions to [`process::ExitStatus`] and

library/std/src/sys/process/unix/common.rs

@@ -98,6 +98,7 @@ pub struct Command {
     #[cfg(target_os = "linux")]
     create_pidfd: bool,
     pgroup: Option<pid_t>,
+    setsid: bool,
 }
 
 // passed back to std::process with the pipes connected to the child, if any
@@ -185,6 +186,7 @@ impl Command {
             #[cfg(target_os = "linux")]
             create_pidfd: false,
             pgroup: None,
+            setsid: false,
         }
     }
 
@@ -220,6 +222,9 @@ impl Command {
             self.cwd(&OsStr::new("/"));
         }
     }
+    pub fn setsid(&mut self, setsid: bool) {
+        self.setsid = setsid;
+    }
 
     #[cfg(target_os = "linux")]
     pub fn create_pidfd(&mut self, val: bool) {
@@ -298,6 +303,10 @@ impl Command {
     pub fn get_chroot(&self) -> Option<&CStr> {
         self.chroot.as_deref()
     }
+    #[allow(dead_code)]
+    pub fn get_setsid(&self) -> bool {
+        self.setsid
+    }
 
     pub fn get_closures(&mut self) -> &mut Vec<Box<dyn FnMut() -> io::Result<()> + Send + Sync>> {
         &mut self.closures

library/std/src/sys/process/unix/common/tests.rs

@@ -134,6 +134,64 @@ fn test_process_group_no_posix_spawn() {
     }
 }
 
+#[test]
+#[cfg_attr(
+    any(
+        // See test_process_mask
+        target_os = "macos",
+        target_arch = "arm",
+        target_arch = "aarch64",
+        target_arch = "riscv64",
+    ),
+    ignore
+)]
+fn test_setsid_posix_spawn() {
+    // Spawn a cat subprocess that's just going to hang since there is no I/O.
+    let mut cmd = Command::new(OsStr::new("cat"));
+    cmd.setsid(true);
+    cmd.stdin(Stdio::MakePipe);
+    cmd.stdout(Stdio::MakePipe);
+    let (mut cat, _pipes) = t!(cmd.spawn(Stdio::Null, true));
+
+    unsafe {
+        // Setsid will create a new session and process group, so check that
+        // we can kill the process group, which means there *is* one.
+        t!(cvt(libc::kill(-(cat.id() as libc::pid_t), libc::SIGINT)));
+
+        t!(cat.wait());
+    }
+}
+
+#[test]
+#[cfg_attr(
+    any(
+        // See test_process_mask
+        target_os = "macos",
+        target_arch = "arm",
+        target_arch = "aarch64",
+        target_arch = "riscv64",
+    ),
+    ignore
+)]
+fn test_setsid_no_posix_spawn() {
+    let mut cmd = Command::new(OsStr::new("cat"));
+    cmd.setsid(true);
+    cmd.stdin(Stdio::MakePipe);
+    cmd.stdout(Stdio::MakePipe);
+
+    unsafe {
+        // Same as above, create hang-y cat. This time, force using the non-posix_spawn path.
+        cmd.pre_exec(Box::new(|| Ok(()))); // pre_exec forces fork + exec rather than posix spawn.
+        let (mut cat, _pipes) = t!(cmd.spawn(Stdio::Null, true));
+
+        // Setsid will create a new session and process group, so check that
+        // we can kill the process group, which means there *is* one.
+        t!(cvt(libc::kill(-(cat.id() as libc::pid_t), libc::SIGINT)));
+
+        t!(cat.wait());
+    }
+}
+
 #[test]
 fn test_program_kind() {
     let vectors = &[

library/std/src/sys/process/unix/unix.rs

@@ -340,6 +340,10 @@ impl Command {
             cvt(libc::setpgid(0, pgroup))?;
         }
 
+        if self.get_setsid() {
+            cvt(libc::setsid())?;
+        }
+
         // emscripten has no signal support.
         #[cfg(not(target_os = "emscripten"))]
         {
@@ -741,6 +745,16 @@ impl Command {
                 flags |= libc::POSIX_SPAWN_SETSIGDEF;
             }
 
+            if self.get_setsid() {
+                cfg_if::cfg_if! {
+                    if #[cfg(all(target_os = "linux", target_env = "gnu"))] {
+                        flags |= libc::POSIX_SPAWN_SETSID;
+                    } else {
+                        return Ok(None);
+                    }
+                }
+            }
+
             cvt_nz(libc::posix_spawnattr_setflags(attrs.0.as_mut_ptr(), flags as _))?;
 
             // Make sure we synchronize access to the global `environ` resource