install: allow -C with --preserve-timestamps

src/uu/install/locales/en-US.ftl

@@ -46,7 +46,6 @@ install-error-extra-operand = extra operand { $operand }
 install-error-not-permitted = cannot remove { $path }: Operation not permitted
 install-error-invalid-mode = Invalid mode string: { $error }
 install-error-mutually-exclusive-target = Options --target-directory and --no-target-directory are mutually exclusive
-install-error-mutually-exclusive-compare-preserve = Options --compare and --preserve-timestamps are mutually exclusive
 install-error-mutually-exclusive-compare-strip = Options --compare and --strip are mutually exclusive
 install-error-missing-file-operand = missing file operand
 install-error-missing-destination-operand = missing destination file operand after { $path }

src/uu/install/locales/fr-FR.ftl

@@ -45,7 +45,6 @@ install-error-extra-operand = opérande supplémentaire { $operand }
   { $usage }
 install-error-invalid-mode = Chaîne de mode invalide : { $error }
 install-error-mutually-exclusive-target = Les options --target-directory et --no-target-directory sont mutuellement exclusives
-install-error-mutually-exclusive-compare-preserve = Les options --compare et --preserve-timestamps sont mutuellement exclusives
 install-error-mutually-exclusive-compare-strip = Les options --compare et --strip sont mutuellement exclusives
 install-error-missing-file-operand = opérande de fichier manquant
 install-error-missing-destination-operand = opérande de fichier de destination manquant après { $path }

src/uu/install/src/install.rs

@@ -396,13 +396,6 @@ fn behavior(matches: &ArgMatches) -> UResult<Behavior> {
     }
     .to_string();
 
-    if preserve_timestamps && compare {
-        show_error!(
-            "{}",
-            translate!("install-error-mutually-exclusive-compare-preserve")
-        );
-        return Err(1.into());
-    }
     if compare && strip {
         show_error!(
             "{}",
@@ -1313,6 +1306,12 @@ fn need_copy(from: &Path, to: &Path, b: &Behavior) -> bool {
         return true;
     }
 
+    // When preserving timestamps, a difference in modification time also
+    // requires a copy so the destination ends up with the source's timestamp.
+    if b.preserve_timestamps && from_meta.modified().ok() != to_meta.modified().ok() {
+        return true;
+    }
+
     if b.privileged {
         #[cfg(all(feature = "selinux", any(target_os = "linux", target_os = "android")))]
         if b.preserve_context && contexts_differ(from, to) {

tests/by-util/test_install.rs

@@ -491,6 +491,33 @@ fn test_install_preserve_timestamps() {
     );
 }
 
+#[test]
+#[cfg(not(target_os = "openbsd"))]
+fn test_install_compare_preserve_timestamps() {
+    use std::time::Duration;
+
+    let (at, mut ucmd) = at_and_ucmd!();
+    let source = "source_file";
+    let dest = "dest_file";
+
+    // Same contents, but destination has a newer timestamp than the source.
+    at.write(source, "data");
+    at.write(dest, "data");
+    let old = std::time::SystemTime::UNIX_EPOCH + Duration::from_secs(1_000_000);
+    filetime::set_file_mtime(at.plus(source), FileTime::from_system_time(old)).unwrap();
+
+    // With --preserve-timestamps, the timestamp difference forces the copy so
+    // the destination ends up with the source's modification time.
+    ucmd.args(&["-C", "--preserve-timestamps", source, dest])
+        .succeeds()
+        .no_output();
+
+    assert_eq!(
+        at.metadata(source).modified().ok(),
+        at.metadata(dest).modified().ok()
+    );
+}
+
 // These two tests are failing but should work
 #[test]
 fn test_install_copy_file() {
@@ -1856,11 +1883,12 @@ fn test_install_compare_option() {
         .args(&["-Cv", first, second])
         .succeeds()
         .stdout_contains(format!("removed '{second}'\n'{first}' -> '{second}'"));
+    // -C and --preserve-timestamps are no longer mutually exclusive
     scene
         .ucmd()
         .args(&["-C", "--preserve-timestamps", first, second])
-        .fails_with_code(1)
-        .stderr_contains("Options --compare and --preserve-timestamps are mutually exclusive");
+        .succeeds()
+        .no_output();
     scene
         .ucmd()
         .args(&["-C", "--strip", "--strip-program=echo", first, second])