Partially implement --preserve

Needs better implementation once stable Rust improves,
or perhaps see if there's a crate for this

Reference #4 but keep open

CHANGELOG.md

@@ -1,6 +1,7 @@
-**Version 0.5.1 (unreleased)**
+**Version 0.6.0 (unreleased)**
  - Fix issue where output directory would not be created if it did not exist
  - Use miniz for compression strategies where it outperforms zlib
+ - [SEMVER_MINOR] Partially implement -p / --preserve, as far as stable Rust will allow for now
 
 **Version 0.5.0**
  - [SEMVER_MINOR] Palette entries can now reduced, on by default ([#11](https://github.com/shssoichiro/oxipng/issues/11))

src/lib.rs

@@ -275,6 +275,51 @@ pub fn optimize(filepath: &Path, opts: &Options) -> Result<(), String> {
                     return Err(format!("Unable to write to file {}", opts.out_file.display()))
                 }
             };
+
+            if opts.preserve_attrs {
+                match File::open(filepath) {
+                    Ok(f) => {
+                        match f.metadata() {
+                            Ok(metadata) => {
+                                // TODO: Implement full permission changing on Unix
+                                // Not available in stable, requires block cfg statements
+                                // See https://github.com/rust-lang/rust/issues/15701
+                                {
+                                    match out_file.metadata() {
+                                        Ok(out_meta) => {
+                                            let readonly = metadata.permissions()
+                                                                   .readonly();
+                                            out_meta.permissions()
+                                                    .set_readonly(readonly);
+                                        }
+                                        Err(_) => {
+                                            if opts.verbosity.is_some() {
+                                                writeln!(&mut stderr(),
+                                                         "Failed to set permissions on output file")
+                                                    .ok();
+                                            }
+                                        }
+                                    }
+                                }
+                            }
+                            Err(_) => {
+                                if opts.verbosity.is_some() {
+                                    writeln!(&mut stderr(),
+                                             "Failed to read permissions on input file")
+                                        .ok();
+                                }
+                            }
+                        }
+                    }
+                    Err(_) => {
+                        if opts.verbosity.is_some() {
+                            writeln!(&mut stderr(), "Failed to read permissions on input file")
+                                .ok();
+                        }
+                    }
+                };
+            }
+
             let mut buffer = BufWriter::new(out_file);
             match buffer.write_all(&output_data) {
                 Ok(_) => {

tests/flags.rs

@@ -463,3 +463,21 @@ fn interlaced_0_to_1_other_filter_mode() {
 
     remove_file(output).ok();
 }
+
+#[test]
+fn preserve_attrs() {
+    let input = PathBuf::from("tests/files/preserve_attrs.png");
+    let mut opts = get_opts(&input);
+    opts.preserve_attrs = true;
+    let output = opts.out_file.clone();
+
+    test_it_converts(&input,
+                     &output,
+                     &opts,
+                     png::ColorType::RGB,
+                     png::BitDepth::Eight,
+                     png::ColorType::RGB,
+                     png::BitDepth::Eight);
+
+    // TODO: Actually check permissions
+}