Change UtunIfname sockopt type to CString (nix-rust#2329)

* Implement GetCString for use with sockopt

* Change UtunIfname sockopt type to CString

src/sys/socket/sockopt.rs

@@ -5,7 +5,7 @@ use crate::sys::time::TimeVal;
 use crate::Result;
 use cfg_if::cfg_if;
 use libc::{self, c_int, c_void, socklen_t};
-use std::ffi::{OsStr, OsString};
+use std::ffi::{CStr, CString, OsStr, OsString};
 use std::mem::{self, MaybeUninit};
 use std::os::unix::ffi::OsStrExt;
 use std::os::unix::io::{AsFd, AsRawFd};
@@ -1073,8 +1073,8 @@ sockopt_impl!(
     GetOnly,
     libc::SYSPROTO_CONTROL,
     libc::UTUN_OPT_IFNAME,
-    OsString,
-    GetOsString<[u8; libc::IFNAMSIZ]>
+    CString,
+    GetCString<[u8; libc::IFNAMSIZ]>
 );
 
 #[allow(missing_docs)]
@@ -1579,3 +1579,32 @@ impl<'a> Set<'a, OsString> for SetOsString<'a> {
     }
 }
 
+/// Getter for a `CString` value.
+struct GetCString<T: AsMut<[u8]>> {
+    len: socklen_t,
+    val: MaybeUninit<T>,
+}
+
+impl<T: AsMut<[u8]>> Get<CString> for GetCString<T> {
+    fn uninit() -> Self {
+        GetCString {
+            len: mem::size_of::<T>() as socklen_t,
+            val: MaybeUninit::uninit(),
+        }
+    }
+
+    fn ffi_ptr(&mut self) -> *mut c_void {
+        self.val.as_mut_ptr().cast()
+    }
+
+    fn ffi_len(&mut self) -> *mut socklen_t {
+        &mut self.len
+    }
+
+    unsafe fn assume_init(self) -> CString {
+        let mut v = unsafe { self.val.assume_init() };
+        CStr::from_bytes_until_nul(v.as_mut())
+            .expect("string should be null-terminated")
+            .to_owned()
+    }
+}

test/sys/test_sockopt.rs

@@ -856,6 +856,6 @@ fn test_utun_ifname() {
     let name = getsockopt(&fd, sockopt::UtunIfname)
         .expect("getting UTUN_OPT_IFNAME on a utun interface should succeed");
 
-    let expected_name = format!("utun{}\0", unit - 1);
+    let expected_name = format!("utun{}", unit - 1);
     assert_eq!(name.into_string(), Ok(expected_name));
 }