install: implement option -C

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

src/uu/install/src/install.rs

@@ -1000,6 +1000,30 @@ fn copy(from: &Path, to: &Path, b: &Behavior) -> UResult<()> {
     Ok(())
 }
 
+/// Check if a file needs to be copied due to ownership differences when no explicit group is specified.
+/// Returns true if the destination file's ownership would differ from what it should be after installation.
+fn needs_copy_for_ownership(to: &Path, to_meta: &fs::Metadata) -> bool {
+    use std::os::unix::fs::MetadataExt;
+
+    // Check if the destination file's owner differs from the effective user ID
+    if to_meta.uid() != geteuid() {
+        return true;
+    }
+
+    // For group, we need to determine what the group would be after installation
+    // If no group is specified, the behavior depends on the directory:
+    // - If the directory has setgid bit, the file inherits the directory's group
+    // - Otherwise, the file gets the user's effective group
+    let expected_gid = to
+        .parent()
+        .and_then(|parent| metadata(parent).ok())
+        .filter(|parent_meta| parent_meta.mode() & 0o2000 != 0)
+        .map(|parent_meta| parent_meta.gid())
+        .unwrap_or(getegid());
+
+    to_meta.gid() != expected_gid
+}
+
 /// Return true if a file is necessary to copy. This is the case when:
 ///
 /// - _from_ or _to_ is nonexistent;
@@ -1030,6 +1054,13 @@ fn need_copy(from: &Path, to: &Path, b: &Behavior) -> UResult<bool> {
         return Ok(true);
     };
 
+    // Check if the destination is a symlink (should always be replaced)
+    if let Ok(to_symlink_meta) = fs::symlink_metadata(to) {
+        if to_symlink_meta.file_type().is_symlink() {
+            return Ok(true);
+        }
+    }
+
     // Define special file mode bits (setuid, setgid, sticky).
     let extra_mode: u32 = 0o7000;
     // Define all file mode bits (including permissions).
@@ -1038,7 +1069,7 @@ fn need_copy(from: &Path, to: &Path, b: &Behavior) -> UResult<bool> {
 
     // Check if any special mode bits are set in the specified mode,
     // source file mode, or destination file mode.
-    if b.specified_mode.unwrap_or(0) & extra_mode != 0
+    if b.mode() & extra_mode != 0
         || from_meta.mode() & extra_mode != 0
         || to_meta.mode() & extra_mode != 0
     {
@@ -1079,14 +1110,9 @@ fn need_copy(from: &Path, to: &Path, b: &Behavior) -> UResult<bool> {
         if group_id != to_meta.gid() {
             return Ok(true);
         }
-    } else {
+    } else if needs_copy_for_ownership(to, &to_meta) {
-        #[cfg(not(target_os = "windows"))]
-        // Check if the destination file's owner or group
-        // differs from the effective user/group ID of the process.
-        if to_meta.uid() != geteuid() || to_meta.gid() != getegid() {
         return Ok(true);
     }
-    }
 
     // Check if the contents of the source and destination files differ.
     if !diff(from.to_str().unwrap(), to.to_str().unwrap()) {

tests/by-util/test_install.rs

@@ -1694,6 +1694,193 @@ fn test_install_compare_option() {
         .stderr_contains("Options --compare and --strip are mutually exclusive");
 }
 
+#[test]
+#[cfg(not(target_os = "openbsd"))]
+fn test_install_compare_basic() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    let source = "source_file";
+    let dest = "dest_file";
+
+    at.write(source, "test content");
+
+    // First install should copy
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m644", source, dest])
+        .succeeds()
+        .stdout_contains(format!("'{source}' -> '{dest}'"));
+
+    // Second install with same mode should be no-op (compare works)
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m644", source, dest])
+        .succeeds()
+        .no_stdout();
+
+    // Test that compare works correctly when content actually differs
+    let source2 = "source2";
+    at.write(source2, "different content");
+
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m644", source2, dest])
+        .succeeds()
+        .stdout_contains("removed")
+        .stdout_contains(format!("'{source2}' -> '{dest}'"));
+
+    // Second install should be no-op since content is now identical
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m644", source2, dest])
+        .succeeds()
+        .no_stdout();
+}
+
+#[test]
+#[cfg(not(any(target_os = "openbsd", target_os = "freebsd")))]
+fn test_install_compare_special_mode_bits() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    let source = "source_file";
+    let dest = "dest_file";
+
+    at.write(source, "test content");
+
+    // Special mode bits - setgid (tests the core bug fix)
+    // When setgid bit is set, -C should be ignored (always copy)
+    // This tests the bug where b.specified_mode.unwrap_or(0) was used instead of b.mode()
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m2755", source, dest])
+        .succeeds()
+        .stdout_contains(format!("'{source}' -> '{dest}'"));
+
+    // Second install with same setgid mode should ALSO copy (not skip)
+    // because -C option should be ignored when special mode bits are present
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m2755", source, dest])
+        .succeeds()
+        .stdout_contains("removed")
+        .stdout_contains(format!("'{source}' -> '{dest}'"));
+
+    // Special mode bits - setuid
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m4755", source, dest])
+        .succeeds()
+        .stdout_contains("removed")
+        .stdout_contains(format!("'{source}' -> '{dest}'"));
+
+    // Second install with setuid should also copy
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m4755", source, dest])
+        .succeeds()
+        .stdout_contains("removed")
+        .stdout_contains(format!("'{source}' -> '{dest}'"));
+
+    // Special mode bits - sticky bit
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m1755", source, dest])
+        .succeeds()
+        .stdout_contains("removed")
+        .stdout_contains(format!("'{source}' -> '{dest}'"));
+
+    // Second install with sticky bit should also copy
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m1755", source, dest])
+        .succeeds()
+        .stdout_contains("removed")
+        .stdout_contains(format!("'{source}' -> '{dest}'"));
+
+    // Back to normal mode - compare should work again
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m644", source, dest])
+        .succeeds()
+        .stdout_contains("removed")
+        .stdout_contains(format!("'{source}' -> '{dest}'"));
+
+    // Second install with normal mode should be no-op
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m644", source, dest])
+        .succeeds()
+        .no_stdout();
+}
+
+#[test]
+#[cfg(not(target_os = "openbsd"))]
+fn test_install_compare_group_ownership() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    let source = "source_file";
+    let dest = "dest_file";
+
+    at.write(source, "test content");
+
+    let user_group = std::process::Command::new("id")
+        .arg("-nrg")
+        .output()
+        .map_or_else(
+            |_| "users".to_string(),
+            |output| String::from_utf8_lossy(&output.stdout).trim().to_string(),
+        ); // fallback group name
+
+    // Install with explicit group
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m664", "-g", &user_group, source, dest])
+        .succeeds()
+        .stdout_contains(format!("'{source}' -> '{dest}'"));
+
+    // Install without group - this should detect that no copy is needed
+    // because the file already has the correct group (user's group)
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m664", source, dest])
+        .succeeds()
+        .no_stdout(); // Should be no-op if group ownership logic is correct
+}
+
+#[test]
+#[cfg(not(target_os = "openbsd"))]
+fn test_install_compare_symlink_handling() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    let source = "source_file";
+    let symlink_dest = "symlink_dest";
+    let target_file = "target_file";
+
+    at.write(source, "test content");
+    at.write(target_file, "test content"); // Same content to test that symlinks are always replaced
+    at.symlink_file(target_file, symlink_dest);
+
+    // Create a symlink as destination pointing to a different file - should always be replaced
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m644", source, symlink_dest])
+        .succeeds()
+        .stdout_contains("removed")
+        .stdout_contains(format!("'{source}' -> '{symlink_dest}'"));
+
+    // Even if content would be the same, symlink destination should be replaced
+    // Now symlink_dest is a regular file, so compare should work normally
+    scene
+        .ucmd()
+        .args(&["-Cv", "-m644", source, symlink_dest])
+        .succeeds()
+        .no_stdout(); // Now it's a regular file, so compare should work
+}
+
 #[test]
 // Matches part of tests/install/basic-1
 fn test_t_exist_dir() {