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Contributing to format_parser

Please take a moment to review this document in order to make the contribution process easy and effective for everyone involved.

Following these guidelines helps to communicate that you respect the time of the developers managing and developing this open source project. In return, they should reciprocate that respect in addressing your issue or assessing patches and features.

What do I need to know to help?

If you are already familiar with the Ruby Programming Language you can start contributing code right away, otherwise look for issues labeled with documentation or good first issue to get started.

If you are interested in contributing code and would like to learn more about the technologies that we use, check out the (non-exhaustive) list below. You can also get in touch with us via an issue or email to [email protected] and/or [email protected] to get additional information.

How do I make a contribution?

Using the issue tracker

The issue tracker is the preferred channel for bug reports, feature requests and submitting pull requests, but please respect the following restrictions:

  • Please do not derail or troll issues. Keep the discussion on topic and respect the opinions of others. Adhere to the principles set out in the Code of Conduct.

Bug reports

A bug is a demonstrable problem that is caused by code in the repository.

Good bug reports are extremely helpful-thank you!

Guidelines for bug reports:

  1. Use the GitHub issue search – check if the issue has already been reported.

  2. Check if the issue has been fixed – try to reproduce it using the latest master branch in the repository.

  3. Isolate the problem – create a reduced test case and a live example.

A good bug report shouldn't leave others needing to chase you up for more information. Please try to be as detailed as possible in your report. What is your environment? What steps will reproduce the issue? What tool(s) or OS will experience the problem? What would you expect to be the outcome? All these details will help people to fix any potential bugs.

Example:

Short and descriptive example bug report title

A summary of the issue and the OS environment in which it occurs. If suitable, include the steps required to reproduce the bug.

  1. This is the first step
  2. This is the second step
  3. Further steps, etc.

<url> - a link to the reduced test case, if possible. Feel free to use a Gist.

Any other information you want to share that is relevant to the issue being reported. This might include the lines of code that you have identified as causing the bug, and potential solutions (and your opinions on their merits).

Feature requests

Feature requests are welcome. But take a moment to find out whether your idea fits with the scope and aims of the project. It's up to you to make a strong case to convince the project's developers of the merits of this feature. Please provide as much detail and context as possible.

So, you want to contribute a new parser

That's awesome! Please do take care to add example files that fit your parser use case. Make sure that the file you are adding is licensed for use within an MIT-licensed piece of software. Ideally, this file is going to be something you have produced yourself and you are permitted to share under the MIT license provisions.

When writing a parser, please try to ensure it returns a usable result as soon as possible, or nil as soon as possible (once you know the file is not fit for your specific parser). Bear in mind that we enforce read budgets per-parser, so you will not be allowed to perform too many reads, or perform reads which are too large.

In order to create new parsers, make a well-named class with an instance method call, and to register a single instance of that class as the parser - so that only one object needs to be stored in memory when parsing multiple inputs. In that case your object must be thread-safe and stateless - this is really important since FormatParser is thread-safe and multiple parsing procedures may be in progress concurrently against the same parser object. You can also create a Proc if your parser is fairly trivial.

If it will be difficult to have your parser thread-safe you can register your class itself as the parser and define the self.call method to parse using a fresh instance every time, allowing object-level state:

class MyParser
  def self.call(io)
    new.call(io)
  end

  def call(io)
    @state = ...
  end

call accepts a single argument - an IO-ish object which is guaranteed to respond to the same methods as the ones defined in IOConstraint - that is, it is a strict subset of a standard Ruby IO object. All reads from this IO object are guaranteed to be returned in binary encoding. The IO will be at offset of 0 when your parsing proc receives it and there will be no concurrent calls to that object until your proc returns.

Your parsing procedure may read from this IO object, and should return either a Result-like object with the file metadata (if it could recover any) or nil if it couldn't. All files pass through all parsers by default, so if you are dealing with a file that is not "your" format - return nil from your method or break your Proc as early as possible. A blank return works fine too as it actually returns nil.

Your parser then needs to be registered using FormatParser.register_parser with the information on the formats and file natures it provides. This allows FormatParser to skip your parser if, say, the user only want to parse for :image nature files but your parser parses :audio.

Down below you can find the most basic parser implementation which parses an imaginary IMGA file format:

MyParser = ->(io) {
  # ... Read the magic bytes from the start of IO - the IO is
  # guaranteed to be fed to you at offset 0, start-of-file.
  magic_bytes = io.read(4)

  # breaking the block returns `nil` to the caller signaling "no match"
  break if magic_bytes != 'IMGA'

  # Our file format stores the width and height as 2 32-bit unsigned integers
  parsed_witdh, parsed_height = io.read(8).unpack('VV')

  # ...and return the FileInformation::Image object with the metadata.
  FormatParser::Image.new(
    format: :imga,
    width_px: parsed_width,
    height_px: parsed_height,
  )
}

# Register the parser with the module, so that it will be applied to any
# document given to `FormatParser.parse()`. The supported natures are currently
#      - :audio
#      - :document
#      - :image
#      - :video
#      - :archive
FormatParser.register_parser MyParser, natures: :image, formats: :imga

If you are using a class, this is the skeleton to use:

class MyParser
  def call(io)
    # ... do some parsing with `io`
    # The instance will be discarded after parsing, so using instance variables
    # is permitted - they are not shared between calls to `call`
    magic_bytes = io.read(4)
    break if magic_bytes != 'IMGA'
    parsed_witdh, parsed_height = io.read(8).unpack('VV')
    FormatParser::Image.new(
      format: :imga,
      width_px: parsed_width,
      height_px: parsed_height,
    )
  end

  # Note that we register an instance of the class, not the class. It is the
  # instance that responds to `call()` and we can do this because our object
  # is stateless.
  FormatParser.register_parser new, natures: :image, formats: :bmp
end

If your parser supports file types which have a known filename extension, you can add a method to it called likely_match?, add this method on the object you register itself. For example, for the ZIP parser we use:

  def likely_match?(filename)
    filename =~ /\.(zip|docx|keynote|numbers|pptx|xlsx)$/i
  end

If your parser matches the filename it is going to be applied earlier, saving time. Since most FormatParser users are likely to only want the first result of the parsing, the sooner your parser gets applied - the sooner you can return the result, avoiding unnecessary reads.

Calling convention for preparing parsers

A parser that gets registered using register_parser must be an object that can be call()-ed, with an argument that conforms to IOConstraint

FormatParser is made to be used in threaded environments, and if you use instance variables you need your parser to be isolated from it's siblings in other threads - create a copy for one-off use inside your call method.

Pull requests

Good pull requests-patches, improvements, new features-are a fantastic help. They should remain focused in scope and avoid containing unrelated commits.

Please ask first before embarking on any significant pull request (e.g. implementing features, refactoring code, porting to a different language), otherwise you risk spending a lot of time working on something that the project's developers might not want to merge into the project.

Please adhere to the coding conventions used throughout the project (indentation, accurate comments, etc.) and any other requirements (such as test coverage).

The test suite can be run with bundle exec rspec.

Follow this process if you'd like your work considered for inclusion in the project:

  1. Fork the project, clone your fork, and configure the remotes:

    # Clone your fork of the repo into the current directory
git clone [email protected]:WeTransfer/format_parser.git
# Navigate to the newly cloned directory
cd format_parser
# Assign the original repo to a remote called "upstream"
git remote add upstream [email protected]:WeTransfer/format_parser.git

  2. If you cloned a while ago, get the latest changes from upstream:

    git checkout <dev-branch>
git pull upstream <dev-branch>

  3. Create a new topic branch (off the main project development branch) to contain your feature, change, or fix:

    git checkout -b <topic-branch-name>

  4. Commit your changes in logical chunks and/or squash them for readability and conciseness. Check out this post or this other post for some tips re: writing good commit messages.

  5. Locally merge (or rebase) the upstream development branch into your topic branch:

    git pull [--rebase] upstream <dev-branch>

  6. Push your topic branch up to your fork:

    git push origin <topic-branch-name>

  7. Open a Pull Request with a clear title and description.

IMPORTANT: By submitting a patch, you agree to allow the project owner to license your work under the same license as that used by the project, which you can see by clicking here. This provision also applies to the test files you include with the changed code as fixtures.

Changelog

When creating a new release you must add an entry in the CHANGELOG.md.

Testing locally

It's possible to run exe/format_parser_inspect FILE_NAME or exe/format_parser_inspect FILE_URI to test the new code without the necessity of installing the gem.