Fix cp bug: pre-write permission change (#2769)

src/uu/cp/src/cp.rs

@@ -63,7 +63,7 @@ quick_error! {
     #[derive(Debug)]
     pub enum Error {
         /// Simple io::Error wrapper
-        IoErr(err: io::Error) { from() cause(err) display("{}", err) }
+        IoErr(err: io::Error) { from() cause(err) display("{}", err)}
 
         /// Wrapper for io::Error with path context
         IoErrContext(err: io::Error, path: String) {
@@ -1292,7 +1292,13 @@ fn copy_file(source: &Path, dest: &Path, options: &Options) -> CopyResult<()> {
         .map(|meta| !meta.file_type().is_symlink())
         .unwrap_or(false)
     {
-        fs::set_permissions(&dest, dest_permissions).unwrap();
+        // Here, to match GNU semantics, we quietly ignore an error
+        // if a user does not have the correct ownership to modify
+        // the permissions of a file.
+        //
+        // FWIW, the OS will throw an error later, on the write op, if
+        // the user does not have permission to write to the file.
+        fs::set_permissions(&dest, dest_permissions).ok();
     }
     for attribute in &options.preserve_attributes {
         copy_attribute(&source, &dest, attribute)?;
@@ -1312,7 +1318,7 @@ fn copy_helper(source: &Path, dest: &Path, options: &Options, context: &str) ->
         /* workaround a limitation of fs::copy
          * https://github.com/rust-lang/rust/issues/79390
          */
-        File::create(dest)?;
+        File::create(dest).context(dest.display().to_string())?;
     } else if is_symlink {
         copy_link(source, dest)?;
     } else if options.reflink_mode != ReflinkMode::Never {

tests/by-util/test_cp.rs

@@ -725,6 +725,12 @@ fn test_cp_parents_dest_not_directory() {
         .stderr_contains("with --parents, the destination must be a directory");
 }
 
+#[test]
+#[cfg(unix)]
+fn test_cp_writable_special_file_permissions() {
+    new_ucmd!().arg("/dev/null").arg("/dev/zero").succeeds();
+}
+
 #[test]
 fn test_cp_preserve_no_args() {
     new_ucmd!()