fix(readlink): use physical resolution for canonicalize flags to match GNU behavior

Changed ResolveMode from Logical to Physical for -f, -e, and -m flags in readlink to ensure symlinks are followed before resolving '..' (parent directory), matching GNU readlink's physical resolution order for compatibility. Added a test case to verify the symlink resolution occurs before parent directory evaluation.

Author: mattsu2020

src/uu/readlink/src/readlink.rs

@@ -37,11 +37,14 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     let silent = matches.get_flag(OPT_SILENT) || matches.get_flag(OPT_QUIET);
     let verbose = matches.get_flag(OPT_VERBOSE);
 
+    // GNU readlink -f/-e/-m follows symlinks first and then applies `..` (physical resolution).
+    // ResolveMode::Logical collapses `..` before following links, which yields the opposite order,
+    // so we choose Physical here for GNU compatibility.
     let res_mode = if matches.get_flag(OPT_CANONICALIZE)
         || matches.get_flag(OPT_CANONICALIZE_EXISTING)
         || matches.get_flag(OPT_CANONICALIZE_MISSING)
     {
-        ResolveMode::Logical
+        ResolveMode::Physical
     } else {
         ResolveMode::None
     };

tests/by-util/test_readlink.rs

@@ -68,6 +68,21 @@ fn test_canonicalize_missing() {
     assert_eq!(actual, expect);
 }
 
+#[test]
+#[cfg(unix)]
+fn test_canonicalize_symlink_before_parentdir() {
+    // GNU readlink follows the symlink first and only then evaluates `..`.
+    // Logical resolution would collapse `link/..` up front and return the current directory instead.
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.mkdir("real");
+    at.mkdir("real/sub");
+    at.relative_symlink_dir("real/sub", "link");
+
+    let actual = ucmd.args(&["-f", "link/.."]).succeeds().stdout_move_str();
+    let expect = format!("{}/real\n", at.root_dir_resolved());
+    assert_eq!(actual, expect);
+}
+
 #[test]
 fn test_long_redirection_to_current_dir() {
     let (at, mut ucmd) = at_and_ucmd!();