id: Handle NULL pointer gracefully within cstr2cow macro (#7810)

* id: Handle NULL pointer gracefully within `cstr2cow` macro

> getlogin() returns a pointer to a string containing the name of the user logged in on the controlling terminal of the process, or a NULL pointer if this information cannot be determined.

Ref: https://linux.die.net/man/3/getlogin

* id: Remove redundant std::ffi:: prefix from CStr

* id: Add comment for the null check

Co-authored-by: Sylvestre Ledru <sylvestre@debian.org>

* id: Remove skip for test that should not segfault anymore

Segfault fixed by 292fb9242342d47412a789f4d0002b811568bd41

---------

Co-authored-by: Sylvestre Ledru <sylvestre@debian.org>

src/uu/id/src/id.rs

@@ -47,7 +47,15 @@ use uucore::{format_usage, help_about, help_section, help_usage, show_error};
 
 macro_rules! cstr2cow {
     ($v:expr) => {
-        unsafe { CStr::from_ptr($v).to_string_lossy() }
+        unsafe {
+            let ptr = $v;
+            // Must be not null to call cstr2cow
+            if ptr.is_null() {
+                None
+            } else {
+                Some({ CStr::from_ptr(ptr) }.to_string_lossy())
+            }
+        }
     };
 }
 
@@ -451,8 +459,8 @@ fn pretty(possible_pw: Option<Passwd>) {
         let login = cstr2cow!(getlogin().cast_const());
         let rid = getuid();
         if let Ok(p) = Passwd::locate(rid) {
-            if login == p.name {
-                println!("login\t{login}");
+            if let Some(user_name) = login {
+                println!("login\t{user_name}");
             }
             println!("uid\t{}", p.name);
         } else {

tests/by-util/test_id.rs

@@ -152,19 +152,8 @@ fn test_id_real() {
 fn test_id_pretty_print() {
     // `-p` is BSD only and not supported on GNU's `id`
     let username = whoami();
-
     let result = new_ucmd!().arg("-p").run();
-    if result.stdout_str().trim().is_empty() {
-        // this fails only on: "MinRustV (ubuntu-latest, feat_os_unix)"
-        // `rustc 1.40.0 (73528e339 2019-12-16)`
-        // run: /home/runner/work/coreutils/coreutils/target/debug/coreutils id -p
-        // thread 'test_id::test_id_pretty_print' panicked at 'Command was expected to succeed.
-        // stdout =
-        // stderr = ', tests/common/util.rs:157:13
-        println!("test skipped:");
-    } else {
     result.success().stdout_contains(username);
-    }
 }
 
 #[test]