uucore: use symlink_metadata for backup lookup

Avoids Path::exists() returning false for a live symlink whose target
stat fails (observed under wasmtime), and also keeps a dangling .~N~
symlink from being overwritten by a --backup=numbered rename.

Author: DePasqualeOrg

src/uucore/src/lib/features/backup_control.rs

@@ -91,6 +91,7 @@ use std::{
     error::Error,
     ffi::{OsStr, OsString},
     fmt::{Debug, Display},
+    fs,
     path::{Path, PathBuf},
 };
 
@@ -442,7 +443,11 @@ fn numbered_backup_path(path: &Path) -> PathBuf {
     let mut i: u64 = 1;
     loop {
         let new_path = simple_backup_path(path, OsString::from(format!(".~{i}~")));
-        if !new_path.exists() {
+        // Use `symlink_metadata` rather than `exists()` so that a dangling
+        // symlink still counts as an existing backup (avoiding a silent
+        // overwrite), and so we do not report a live symlink as missing when
+        // the target cannot be stat'd.
+        if fs::symlink_metadata(&new_path).is_err() {
             return new_path;
         }
         i += 1;
@@ -451,7 +456,7 @@ fn numbered_backup_path(path: &Path) -> PathBuf {
 
 fn existing_backup_path<S: AsRef<OsStr>>(path: &Path, suffix: S) -> PathBuf {
     let test_path = simple_backup_path(path, OsString::from(".~1~"));
-    if test_path.exists() {
+    if fs::symlink_metadata(&test_path).is_ok() {
         return numbered_backup_path(path);
     }
     simple_backup_path(path, suffix.as_ref())