uucore: add getsid for process.rs (#6600)

* uucore: Add `getsid` for `process.rs`

* uucore: Changing the return type of the `getsid` function to `Result`

From the manpage of `getsid` we can find that the syscall may failed and set errno, so we have to process those case.

* uucore: Drop support of `getsid` for RedoxOS

* uucore: Add `getpid` for get pid of calling process.

* uucore: Add tests for `getsid` and `getpid`

* uucore: Fix tests failure for `test_getsid`

uucore: Fix tests failure for `test_getsid`

uucore: Fix tests failure for `test_getsid`

* uucore: Fix tests failure on FreeBSD

* uucore: Apply suggestions for `process.rs::tests::test_getsid`

Suggestions from @BenWiederhake , very thanks!

src/uucore/src/lib/features/process.rs

@@ -3,11 +3,12 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
-// spell-checker:ignore (vars) cvar exitstatus cmdline kworker
+// spell-checker:ignore (vars) cvar exitstatus cmdline kworker getsid getpid
-// spell-checker:ignore (sys/unix) WIFSIGNALED
+// spell-checker:ignore (sys/unix) WIFSIGNALED ESRCH
 // spell-checker:ignore pgrep pwait snice
 
 use libc::{gid_t, pid_t, uid_t};
+use nix::errno::Errno;
 use std::io;
 use std::process::Child;
 use std::process::ExitStatus;
@@ -36,6 +37,37 @@ pub fn getuid() -> uid_t {
     unsafe { libc::getuid() }
 }
 
+/// `getpid()` returns the pid of the calling process.
+pub fn getpid() -> pid_t {
+    unsafe { libc::getpid() }
+}
+
+/// `getsid()` returns the session ID of the process with process ID pid.
+///
+/// If pid is 0, getsid() returns the session ID of the calling process.
+///
+/// # Error
+///
+/// - [Errno::EPERM] A process with process ID pid exists, but it is not in the same session as the calling process, and the implementation considers this an error.
+/// - [Errno::ESRCH] No process with process ID pid was found.
+///
+///
+/// # Platform
+///
+/// This function only support standard POSIX implementation platform,
+/// so some system such as redox doesn't supported.
+#[cfg(not(target_os = "redox"))]
+pub fn getsid(pid: i32) -> Result<pid_t, Errno> {
+    unsafe {
+        let result = libc::getsid(pid);
+        if Errno::last() == Errno::UnknownErrno {
+            Ok(result)
+        } else {
+            Err(Errno::last())
+        }
+    }
+}
+
 /// Missing methods for Child objects
 pub trait ChildExt {
     /// Send a signal to a Child process.
@@ -99,3 +131,25 @@ impl ChildExt for Child {
         Ok(None)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[cfg(not(target_os = "redox"))]
+    fn test_getsid() {
+        assert_eq!(
+            getsid(getpid()).expect("getsid(getpid)"),
+            // zero is a special value for SID.
+            // https://pubs.opengroup.org/onlinepubs/9699919799/functions/getsid.html
+            getsid(0).expect("getsid(0)")
+        );
+
+        // SID never be 0.
+        assert!(getsid(getpid()).expect("getsid(getpid)") > 0);
+
+        // This might caused tests failure but the probability is low.
+        assert!(getsid(999999).is_err());
+    }
+}