ZIP Unpacker extension used in ChromiumOS to support reading and unpacking of zip archives

Clone this repo:
  1. 643eff1 README: update URIs to https sites by Mike Frysinger · 5 years ago main master
  2. fd9ce6e normalize archive paths by Mike Frysinger · 6 years ago
  3. d3e5632 Merge "unpacker: drop stub close callback" by Mike Frysinger · 5 years ago
  4. a025514 Prevent mounting the volume after sign out and signing in again. by Tatsuhisa Yamaguchi · 6 years ago
  5. fd5ab5c Upgrade Polymer to 1.9.1. by Naoki Fukino · 6 years ago

ZIP Unpacker extension

This is the ZIP Unpacker extension used in Chrome OS to support reading and unpacking of zip archives.

Build steps


Since the code is built with NaCl, you'll need its toolchain.

$ cd third-party
$ make nacl_sdk

Webports (a.k.a. NaCl ports)

We'll use libraries from webports.

$ cd third-party
$ make depot_tools
$ make webports

npm Setup

First install npm using your normal packaging system. On Debian, you'll want something like:

$ sudo apt-get install npm

Your distro might have old versions of npm, so you'd have to install it yourself.

Then install the npm modules that we require. Do this in the root of the unpacker repo.

$ npm install bower vulcanize crisper

Unpacker Build

Once done, install the libarchive-fork/ from third-party/ of the unpacker project. Note that you cannot use libarchive nor libarchive-dev packages from webports at this moment, as not all patches in the fork are upstreamed.

$ cd third-party
$ make libarchive-fork

Polymer is used for UI. In order to fetch it, in the same directory type:

$ make polymer

Build the PNaCl module.

$ cd unpacker
$ make [debug]


The package can be found in the release or debug directory. You can run it directly from there using Chrome's “Load unpacked extension” feature, or you can zip it up for posting to the Chrome Web Store.

$ zip -r release/

Once it's loaded, you should be able to open ZIP archives in the Files app.

Source Layout

Paths that aren't linked below are dynamically created at build time.

  • node_modules/: All the locally installed npm modules used for building.
  • third-party/: The source for third-party NaCl & Polymer code.
  • unpacker/: The extension CSS/HTML/JS/NaCl source code.
    • cpp/: The NaCl module source.
    • css/: Any CSS needed for styling UI.
    • debug/: A debug build of the Chrome extension.
    • html/: Any HTML needed for UI.
    • icons/: Various extension images.
    • js/: The JavaScript code.
    • _locales/: Translations of strings shown to the user.
    • pnacl/: Compiled NaCl objects & module (debug & release).
    • release/: A release build of the Chrome extension.
  • unpacker-test/: Code for running NaCl & JavaScript unittests.

NaCl/JS Life Cycle

Some high level points to remember: the JS side reacts to user events and is the only part that has access to actual data on disk. It uses the NaCl module to do all the data parsing (e.g. gzip & tar), but it has to both send a request to the module (“parse this archive”), and respond to requests from the module when the module needs to read actual bytes on disk.

When the extension loads, background.js registers everything and goes idle.

When the Files app wants to mount an archive, callbacks in app.js are called to initialize the NaCl runtime. Creates an unpacker.Volume object for each mounted archive.

Requests on the archive (directory listing, metadata lookups, reading files) are routed through app.js and to volume.js unpacker.Volume. Then they are sent to the low level decompressor.js unpacker.Decompressor which talks to the NaCl module using the request.js unpacker.request protocol. Responses are passed back up.

When the NaCl module is loaded, NaclArchiveModule is instantiated. That instantiates NaclArchiveInstance for initial JS message entry points. It instantiates JavaScriptMessageSender for sending requests back to JS.

When JS requests come in, NaclArchiveInstance will create volume.h Volume objects on the fly, and pass requests down to them (using the protocol defined in request.h request::*).

volume.h Volume objects in turn use the volume_archive.h VolumeArchive abstract interface to handle requests from the JS side (using the protocol defined in request.h request:**). This way the lower levels don't have to deal with JS directly. VolumeArchiveLibarchive implements the VolumeArchive interface and uses libarchive as its backend to do all the decompression & archive format processing.

But NaCl code doesn't have access to any files or data itself. So the volume_reader.h VolumeReader abstract interface is passed to it to provide the low level data read functions. The VolumeReaderJavaScriptStream implements that by passing requests back up to the JS side via the javascript_requestor_interface.h JavaScriptRequestorInterface interface (which was passed down to it).

So requests (mount an archive, read a file, etc...) generally follow the path:

Then once VolumeArchive has processed the raw data stream, it can return results to the Volume object which takes care of posting JS status messages back to the Chrome side.

Source Layout

Here‘s the JavaScript code that matters. A few files have very specific purposes and can be ignored at a high level, so they’re in a separate section.

  • background.js
    • Main entry point.
    • Initializes the module/runtime.
    • Registers the extension with Chrome filesystem/runtime.
  • app.js
    • Main runtime for the extension.
    • Loads/unloads NaCl modules on demand (to save runtime memory).
    • Loads/unloads volumes as Chrome has requested.
    • Responds to Chrome filesystem callbacks.
    • Passes data back to Chrome from unpacker.Volume objects.
  • volume.js unpacker.Volume
    • Every mounted archive has a unpacker.Volume instance.
    • Provides high level interface to requests (like reading files & metadata).
  • decompressor.js unpacker.Decompressor
    • Provides low level interface for unpacker.Volume requests.
    • Talks to the NaCl module using the unpacker.request protocol.
  • request.js unpacker.request
    • Handle the JS<->NaCl protocol communication.
  • passphrase-manager.js unpacker.PassphraseManager
    • Interface for plumbing password requests between UI & JS & NaCl.

These are the boilerplate/simple JavaScript files you can generally ignore.

Here's the NaCl layout.


To see debug messages open chrome from a terminal and check the output. For output redirection see


Install Karma for tests runner, Mocha for asynchronous testings, Chai for assertions, and Sinon for spies and stubs.

$ npm install --save-dev \
  karma karma-chrome-launcher karma-cli \
  mocha karma-mocha karma-chai chai karma-sinon sinon

# Run tests:
$ cd unpacker-test
$ ./  # JavaScript tests.
$ ./  # C++ tests.

# Check JavaScript code using the Closure JS Compiler.
# See
$ cd unpacker
$ npm install google-closure-compiler
$ bash