tests: make xattr test more robust

Currently the xattr test asserts that listxattr() on Cargo.toml returns
a length of 0.

This causes a spurious failure when Cargo.toml does have xattrs, and
this can happen in fairly normal situations like when a Linux
distribution enables selinux (as it does on an AlmaLinux 10 test system,
for example):

$ cat /etc/almalinux-release
AlmaLinux release 10.0 (Purple Lion)

$ getfattr -d -m - Cargo.toml
security.selinux=[...]

$ cargo test --features=fs,stdio xattr
[...]
thread 'xattr::xattr_basic' panicked at tests/fs/xattr.rs:88:5:
assertion `left == right` failed
  left: 17
 right: 0

This change fixes the failure by comparing the output of rustix
*listxattr() with an external implementation, the libc crate's
implementation of listxattr(). Rather than asserting a specific value
for rustix *listxattr(), it should match the value produced by libc
listxattr().

Author: bertschingert

tests/fs/xattr.rs

@@ -85,8 +85,14 @@ fn xattr_basic() {
             .raw_os_error(),
         enodata
     );
-    assert_eq!(rustix::fs::listxattr("Cargo.toml", &mut empty).unwrap(), 0);
-    assert_eq!(rustix::fs::llistxattr("Cargo.toml", &mut empty).unwrap(), 0);
+    assert_eq!(
+        rustix::fs::listxattr("Cargo.toml", &mut empty).unwrap(),
+        libc_listxattr("Cargo.toml")
+    );
+    assert_eq!(
+        rustix::fs::llistxattr("Cargo.toml", &mut empty).unwrap(),
+        libc_listxattr("Cargo.toml")
+    );
     assert_eq!(
         rustix::fs::removexattr("Cargo.toml", "user.test")
             .unwrap_err()
@@ -113,11 +119,26 @@ fn xattr_basic() {
             .raw_os_error(),
         enodata
     );
-    assert_eq!(rustix::fs::flistxattr(&file, &mut empty).unwrap(), 0);
+    assert_eq!(
+        rustix::fs::flistxattr(&file, &mut empty).unwrap(),
+        libc_listxattr("Cargo.toml"),
+    );
     assert_eq!(
         rustix::fs::fremovexattr(&file, "user.test")
             .unwrap_err()
             .raw_os_error(),
         enodata
     );
 }
+
+/// To check the correctness of the tested implementations of *listxattr(), their output can be
+/// compared to an external implementation, in this case listxattr() from the libc crate.
+fn libc_listxattr(path: &str) -> usize {
+    let path = std::ffi::CString::new(path).unwrap();
+    let path: *const _ = path.as_ptr();
+
+    let list = std::ffi::CString::new("").unwrap();
+    let list = list.as_ptr() as *mut _;
+
+    (unsafe { libc::listxattr(path, list, 0) }) as usize
+}