use getgroups(2) for apple macos

Although nix-rust/nix mentioned that this feature should be implemented
based on opendirectoryd, CPython implemented it based on getgroups(2)
for apple macOS.
So I ported the code from nix-rust/nix and it works well.

ref0: https://github.com/nix-rust/nix/blob/500036a206188dc8f57bf95a7c61616db417038e/src/unistd.rs#L1320
ref1: https://github.com/python/cpython/blob/c4cacc8c5eab50db8da3140353596f38a01115ca/Modules/posixmodule.c#L6852

.travis.yml

@@ -12,8 +12,22 @@ before_cache:
 matrix:
   fast_finish: true
   include:
-    - name: Run Rust tests
+    - name: Run Rust tests(linux)
       language: rust
+      os: linux
+      rust: stable
+      cache: cargo
+      script:
+        - cargo build --verbose --all
+        - cargo test --verbose --all
+      env:
+        # Prevention of cache corruption.
+        # See: https://docs.travis-ci.com/user/caching/#caches-and-build-matrices
+        - JOBCACHE=1
+
+    - name: Run Rust tests(osx)
+      language: rust
+      os: osx
       rust: stable
       cache: cargo
       script:

vm/src/stdlib/os.rs

@@ -262,7 +262,7 @@ fn get_right_permission(
     let others_permissions = get_permissions(others_mode);
 
     let user_id = nix::unistd::getuid();
-    let groups_ids = nix::unistd::getgroups()?;
+    let groups_ids = getgroups()?;
 
     if file_owner == user_id {
         Ok(owner_permissions)
@@ -273,6 +273,32 @@ fn get_right_permission(
     }
 }
 
+#[cfg(target_os = "macos")]
+fn getgroups() -> nix::Result<Vec<Gid>> {
+    use libc::{c_int, gid_t};
+    use std::ptr;
+    let ret = unsafe { libc::getgroups(0, ptr::null_mut()) };
+    let mut groups = Vec::<Gid>::with_capacity(Errno::result(ret)? as usize);
+    loop {
+        let ret = unsafe {
+            libc::getgroups(
+                groups.capacity() as c_int,
+                groups.as_mut_ptr() as *mut gid_t,
+            )
+        };
+
+        return Errno::result(ret).map(|s| {
+            unsafe { groups.set_len(s as usize) };
+            groups
+        });
+    }
+}
+
+#[cfg(target_os = "linux")]
+fn getgroups() -> nix::Result<Vec<Gid>> {
+    nix::unistd::getgroups()
+}
+
 #[cfg(unix)]
 fn os_access(path: PyStringRef, mode: u8, vm: &VirtualMachine) -> PyResult<bool> {
     use std::os::unix::fs::MetadataExt;