cat: return the same error message as GNU with loop symlink (#5466)

* cat: return the same error message as GNU with loop symlink

Should fix tests/du/long-sloop.sh because it is using cat as a ref
for error messages

Co-authored-by: Daniel Hofstetter <daniel.hofstetter@42dh.com>

src/uu/cat/src/cat.rs

@@ -3,7 +3,7 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
-// spell-checker:ignore (ToDO) nonprint nonblank nonprinting
+// spell-checker:ignore (ToDO) nonprint nonblank nonprinting ELOOP
 use clap::{crate_version, Arg, ArgAction, Command};
 use std::fs::{metadata, File};
 use std::io::{self, IsTerminal, Read, Write};
@@ -50,6 +50,8 @@ enum CatError {
     IsDirectory,
     #[error("input file is output file")]
     OutputIsInput,
+    #[error("Too many levels of symbolic links")]
+    TooManySymlinks,
 }
 
 type CatResult<T> = Result<T, CatError>;
@@ -401,7 +403,23 @@ fn get_input_type(path: &str) -> CatResult<InputType> {
         return Ok(InputType::StdIn);
     }
 
-    let ft = metadata(path)?.file_type();
+    let ft = match metadata(path) {
+        Ok(md) => md.file_type(),
+        Err(e) => {
+            if let Some(raw_error) = e.raw_os_error() {
+                // On Unix-like systems, the error code for "Too many levels of symbolic links" is 40 (ELOOP).
+                // we want to provide a proper error message in this case.
+                #[cfg(not(target_os = "macos"))]
+                let too_many_symlink_code = 40;
+                #[cfg(target_os = "macos")]
+                let too_many_symlink_code = 62;
+                if raw_error == too_many_symlink_code {
+                    return Err(CatError::TooManySymlinks);
+                }
+            }
+            return Err(CatError::Io(e));
+        }
+    };
     match ft {
         #[cfg(unix)]
         ft if ft.is_block_device() => Ok(InputType::BlockDevice),

tests/by-util/test_cat.rs

@@ -540,3 +540,15 @@ fn test_write_to_self() {
         "first_file_content.second_file_content."
     );
 }
+
+#[test]
+#[cfg(unix)]
+fn test_error_loop() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.symlink_file("2", "1");
+    at.symlink_file("3", "2");
+    at.symlink_file("1", "3");
+    ucmd.arg("1")
+        .fails()
+        .stderr_is("cat: 1: Too many levels of symbolic links\n");
+}