Fix #931: Support no-change in setresuid/setresgid

- Change signature to accept Option<Uid/Gid>
- None maps to -1 via wrapping arithmetic
- Add comprehensive tests for both Some and None cases
- Fix setresgid documentation link (was pointing to setresuid)

Author: naoNao89

src/unistd.rs

@@ -3228,16 +3228,23 @@ mod setres {
     /// * returns: Ok or libc error code.
     ///
     /// Err is returned if the user doesn't have permission to set this UID.
+    ///
+    /// If one of the arguments equals None, the corresponding value is not changed.
     #[inline]
-    pub fn setresuid(ruid: Uid, euid: Uid, suid: Uid) -> Result<()> {
+    pub fn setresuid(ruid: Option<Uid>, euid: Option<Uid>, suid: Option<Uid>) -> Result<()> {
+        let ruid = ruid.map(Into::into).unwrap_or((0 as libc::uid_t).wrapping_sub(1));
+        let euid = euid.map(Into::into).unwrap_or((0 as libc::uid_t).wrapping_sub(1));
+        let suid = suid.map(Into::into).unwrap_or((0 as libc::uid_t).wrapping_sub(1));
         let res =
-            unsafe { libc::setresuid(ruid.into(), euid.into(), suid.into()) };
+            unsafe { libc::setresuid(ruid, euid, suid) };
 
         Errno::result(res).map(drop)
     }
 
     /// Sets the real, effective, and saved gid.
-    /// ([see setresuid(2)](https://man7.org/linux/man-pages/man2/setresuid.2.html))
+    ///
+    /// If one of the arguments equals None, the corresponding value is not changed.
+    /// ([see setresgid(2)](https://man7.org/linux/man-pages/man2/setresgid.2.html))
     ///
     /// * `rgid`: real group id
     /// * `egid`: effective group id
@@ -3246,9 +3253,12 @@ mod setres {
     ///
     /// Err is returned if the user doesn't have permission to set this GID.
     #[inline]
-    pub fn setresgid(rgid: Gid, egid: Gid, sgid: Gid) -> Result<()> {
+    pub fn setresgid(rgid: Option<Gid>, egid: Option<Gid>, sgid: Option<Gid>) -> Result<()> {
+        let rgid = rgid.map(Into::into).unwrap_or((0 as libc::gid_t).wrapping_sub(1));
+        let egid = egid.map(Into::into).unwrap_or((0 as libc::gid_t).wrapping_sub(1));
+        let sgid = sgid.map(Into::into).unwrap_or((0 as libc::gid_t).wrapping_sub(1));
         let res =
-            unsafe { libc::setresgid(rgid.into(), egid.into(), sgid.into()) };
+            unsafe { libc::setresgid(rgid, egid, sgid) };
 
         Errno::result(res).map(drop)
     }

test/test_unistd.rs

@@ -1430,3 +1430,40 @@ fn test_group_from() {
     assert_eq!(group.gid, group_id);
     assert_eq!(group.name, "wheel");
 }
+
+// Tests for PR #1305: setresuid/setresgid with Option support
+#[test]
+#[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"))]
+fn test_setresuid_with_option() {
+    let uid = getuid();
+
+    // Test 1: Set all to current values
+    assert!(setresuid(Some(uid), Some(uid), Some(uid)).is_ok());
+    assert_eq!(geteuid(), uid);
+
+    // Test 2: No-change (None for all)
+    let euid_before = geteuid();
+    assert!(setresuid(None, None, None).is_ok());
+    assert_eq!(geteuid(), euid_before, "UID should not change with None");
+
+    // Test 3: Selective change (only effective)
+    assert!(setresuid(None, Some(uid), None).is_ok());
+}
+
+#[test]
+#[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "openbsd"))]
+fn test_setresgid_with_option() {
+    let gid = getgid();
+
+    // Test 1: Set all to current values
+    assert!(setresgid(Some(gid), Some(gid), Some(gid)).is_ok());
+    assert_eq!(getegid(), gid);
+
+    // Test 2: No-change (None for all)
+    let egid_before = getegid();
+    assert!(setresgid(None, None, None).is_ok());
+    assert_eq!(getegid(), egid_before, "GID should not change with None");
+
+    // Test 3: Selective change (only effective)
+    assert!(setresgid(None, Some(gid), None).is_ok());
+}