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Hacking Guide

Overview

This guide contains instructions for those looking to hack on rkt. For more information on the rkt internals, see the devel documentation.

Building rkt

rkt should be able to be built on any modern Linux system. For the most part the codebase is self-contained (e.g. all dependencies are vendored), but assembly of the stage1 requires some other tools to be installed on the system.

Requirements

  • Linux 3.8+
    • make
    • gcc
    • glibc development and static pieces (on Fedora/RHEL/Centos: glibc-devel and glibc-static packages, on Debian/Ubuntu libc6-dev package)
    • cpio
    • squashfs-tools
    • realpath
    • gpg
  • Go 1.3+

Once the requirements have been met you can build rkt by running the following commands:

git clone https://github.com/coreos/rkt.git
cd rkt
./build

With Docker

Alternatively, you can build rkt in a Docker container with the following command. Replace $SRC with the absolute path to your rkt source code:

$ sudo docker run -v $SRC:/opt/rkt -i -t golang:1.3 /bin/bash -c "apt-get update && apt-get install -y coreutils cpio squashfs-tools realpath && cd /opt/rkt && go get github.com/appc/spec/... && ./build"

Managing Dependencies

rkt uses godep to manage third-party dependencies. The build script is crafted to make this transparent to most users (i.e. if you're just building rkt from source, or modifying any of the codebase without changing dependencies, you should have no need to interact with godep). But occasionally the need arises to either a) add a new dependency or b) update/remove an existing dependency. At this point, the ramblings below from an experienced Godep victim^Wenthusiast might prove of use...

Update godep

Step zero is generally to ensure you have the latest version of godep available in your PATH.

Having the right directory layout (i.e. GOPATH)

To work with godep, you'll need to have the repository (i.e. github.com/coreos/rkt) checked out in a valid GOPATH. If you use the standard Go workflow, with every package in its proper place in a workspace, this should be no problem. As an example, if one was obtaining the repository for the first time, one would do the following:

$ export GOPATH=/tmp/foo               # or any directory you please
$ go get -d github.com/coreos/rkt/...  # or 'git clone https://github.com/coreos/rkt $GOPATH/src/github.com/coreos/rkt'
$ cd $GOPATH/src/github.com/coreos/rkt

If, however, you instead prefer to manage your source code in directories like ~/src/rkt, there's a problem: godep doesn't like symbolic links (which is what the rkt build script uses to create a self-contained GOPATH). Hence, you'll need to work around this with bind mounts, with something like the following:

$ export GOPATH=/tmp/foo        # or any directory you please
$ mkdir -p $GOPATH/src/github.com/coreos/rkt
$ sudo mount --bind ~/src/rkt $GOPATH/src/github.com/coreos/rkt
$ cd $GOPATH/src/github.com/coreos/rkt

One benefit of this approach over the single-workspace workflow is that checking out different versions of dependencies in the GOPATH (as we are about to do) is guarnteed to not affect any other packages in the GOPATH. (Using gvm or other such tomfoolery to manage GOPATHs is an exercise left for the reader.)

Restoring the current state of dependencies

Now that we have a functional GOPATH, use godep to restore the full set of vendored dependencies to their correct versions. (What this command does is essentially just loop over the set of dependencies codified in Godeps/Godeps.json, using go get to retrieve and then git checkout (or equivalent) to set each to their correct revision.)

$ godep restore # might take a while if it's the first time...

At this stage, your path forks, depending on what exactly you want to do: add, update or remove a dependency. But in all three cases, the procedure finishes with the same save command.

Add a new dependency

In this case you'll first need to retrieve the dependency you're working with into GOPATH. As a simple example, assuming we're adding github.com/fizz/buzz:

$ go get -d github.com/fizz/buzz

Then add your new dependency into godep's purview by simply importing the standard package name in one of your sources:

$ vim $GOPATH/src/github.com/coreos/rkt/some/file.go
...
import "github.com/fizz/buzz"
...

Now, GOTO saving

Update an existing dependency

In this case, assuming we're updating github.com/foo/bar:

$ cd $GOPATH/src/github.com/foo/bar
$ git pull   # or 'go get -d -u github.com/foo/bar/...' 
$ git checkout $DESIRED_REVISION
$ cd $GOPATH/src/github.com/coreos/rkt
$ godep update github.com/foo/bar/...

Now, GOTO saving

Removing an existing dependency

This is the simplest case of all: simply remove all references to a dependency from the source files.

Now, GOTO saving

Saving the set of dependencies

Finally, here we are, the magic command, the holy grail, the ultimate conclusion of all godep operations. Provided you have followed the preceding instructions, regardless of whether you are adding/removing/modifying dependencies, this command will cast the necessary spells to solve all of your dependency worries:

$ godep save -r ./...

Finishing up

At this point, you should be good to PR. As well as a simple sanity check that the code actually builds and tests pass, here are some things to look out for:

  • git status Godeps/ should show only a minimal and relevant change (i.e. only the dependencies you actually intended to touch).
  • git diff Godeps/ should be free of any changes to import paths within the vendored dependencies
  • git diff should show all third-party import paths prefixed with Godeps/_workspace

If something looks awry, restart, pray to your preferred deity, and try again.