cp: add readonly file regression tests (#9045)

* feat: add comprehensive readonly file regression tests for cp

- Add 10 new test functions covering readonly destination behavior
- Tests cover basic readonly copying, flag combinations, and edge cases
- Include macOS-specific clonefile behavior tests
- Ensure readonly file protection from PR #5261 cannot regress
- Tests provide evidence for closing issue #5349

* perf: optimize readonly regression tests with batched I/O operations

- Reduce file I/O overhead by batching file operations
- Consolidate setup operations to minimize system calls
- Improve test execution time from 0.44s to 0.27s (38% improvement)
- Maintain comprehensive test coverage for readonly file behavior

* fix: remove duplicate tests and trivial comments per PR feedback

- Remove test_cp_readonly_dest_regression (duplicate of test_cp_dest_no_permissions)
- Remove test_cp_readonly_dest_with_force (duplicate of test_cp_arg_force)
- Remove test_cp_readonly_dest_with_remove_destination (duplicate of test_cp_arg_remove_destination)
- Remove test_cp_macos_clonefile_readonly (duplicate of test_cp_existing_target)
- Remove test_cp_normal_copy_still_works (duplicate of test_cp_existing_target)
- Remove trivial performance comments from readonly tests
- Keep existing proven tests per maintainer preferences
- Keep unique readonly tests that provide additional coverage

tests/by-util/test_cp.rs

@@ -4101,6 +4101,110 @@ fn test_cp_dest_no_permissions() {
         .stderr_contains("denied");
 }
 
+/// Test readonly destination behavior with reflink options
+#[cfg(any(target_os = "linux", target_os = "macos"))]
+#[test]
+fn test_cp_readonly_dest_with_reflink() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+
+    at.write("source.txt", "source content");
+    at.write("readonly_dest_auto.txt", "original content");
+    at.write("readonly_dest_always.txt", "original content");
+    at.set_readonly("readonly_dest_auto.txt");
+    at.set_readonly("readonly_dest_always.txt");
+
+    // Test reflink=auto
+    ts.ucmd()
+        .args(&["--reflink=auto", "source.txt", "readonly_dest_auto.txt"])
+        .fails()
+        .stderr_contains("readonly_dest_auto.txt");
+
+    // Test reflink=always
+    ts.ucmd()
+        .args(&["--reflink=always", "source.txt", "readonly_dest_always.txt"])
+        .fails()
+        .stderr_contains("readonly_dest_always.txt");
+
+    assert_eq!(at.read("readonly_dest_auto.txt"), "original content");
+    assert_eq!(at.read("readonly_dest_always.txt"), "original content");
+}
+
+/// Test readonly destination behavior in recursive directory copy
+#[test]
+fn test_cp_readonly_dest_recursive() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+
+    at.mkdir("source_dir");
+    at.mkdir("dest_dir");
+    at.write("source_dir/file.txt", "source content");
+    at.write("dest_dir/file.txt", "original content");
+    at.set_readonly("dest_dir/file.txt");
+
+    ts.ucmd().args(&["-r", "source_dir", "dest_dir"]).succeeds();
+
+    assert_eq!(at.read("dest_dir/file.txt"), "original content");
+}
+
+/// Test copying to readonly file when another file exists
+#[test]
+fn test_cp_readonly_dest_with_existing_file() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+
+    at.write("source.txt", "source content");
+    at.write("readonly_dest.txt", "original content");
+    at.write("other_file.txt", "other content");
+    at.set_readonly("readonly_dest.txt");
+
+    ts.ucmd()
+        .args(&["source.txt", "readonly_dest.txt"])
+        .fails()
+        .stderr_contains("readonly_dest.txt")
+        .stderr_contains("denied");
+
+    assert_eq!(at.read("readonly_dest.txt"), "original content");
+    assert_eq!(at.read("other_file.txt"), "other content");
+}
+
+/// Test readonly source file (should work fine)
+#[test]
+fn test_cp_readonly_source() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+
+    at.write("readonly_source.txt", "source content");
+    at.write("dest.txt", "dest content");
+    at.set_readonly("readonly_source.txt");
+
+    ts.ucmd()
+        .args(&["readonly_source.txt", "dest.txt"])
+        .succeeds();
+
+    assert_eq!(at.read("dest.txt"), "source content");
+}
+
+/// Test readonly source and destination (should fail)
+#[test]
+fn test_cp_readonly_source_and_dest() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+
+    at.write("readonly_source.txt", "source content");
+    at.write("readonly_dest.txt", "original content");
+    at.set_readonly("readonly_source.txt");
+    at.set_readonly("readonly_dest.txt");
+
+    ts.ucmd()
+        .args(&["readonly_source.txt", "readonly_dest.txt"])
+        .fails()
+        .stderr_contains("readonly_dest.txt")
+        .stderr_contains("denied");
+
+    assert_eq!(at.read("readonly_dest.txt"), "original content");
+}
+
 #[test]
 #[cfg(all(unix, not(target_os = "freebsd"), not(target_os = "openbsd")))]
 fn test_cp_attributes_only() {