unistd::getgroups: respond to comments

Author: JayH5

src/unistd.rs

@@ -954,9 +954,11 @@ pub fn getgroups() -> Result<Vec<Gid>> {
     // First get the number of groups so we can size our Vec
     use std::ptr;
     let ret = unsafe { libc::getgroups(0, ptr::null_mut()) };
-    let mut size = try!(Errno::result(ret));
+    let mut size = Errno::result(ret)?;
 
-    // Now actually get the groups
+    // Now actually get the groups. We try multiple times in case the number of
+    // groups has changed since the first call to getgroups() and the buffer is
+    // now too small
     let mut groups = Vec::<Gid>::with_capacity(size as usize);
     loop {
         // We can coerce a pointer to some `Gid`s as a pointer to some `gid_t`s