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Contributing to Packer

First: if you're unsure or afraid of anything, just ask or submit the issue or pull request anyways. You won't be yelled at for giving your best effort. The worst that can happen is that you'll be politely asked to change something. We appreciate any sort of contributions, and don't want a wall of rules to get in the way of that.

However, for those individuals who want a bit more guidance on the best way to contribute to the project, read on. This document will cover what we're looking for. By addressing all the points we're looking for, it raises the chances we can quickly merge or address your contributions.

Issues

Reporting an Issue

Make sure you test against the latest released version. It is possible we already fixed the bug you're experiencing.
Run the command with debug output with the environment variable PACKER_LOG. For example: PACKER_LOG=1 packer build template.json. Take the entire output and create a gist for linking to in your issue. Packer should strip sensitive keys from the output, but take a look through just in case.
Provide a reproducible test case. If a contributor can't reproduce an issue, then it dramatically lowers the chances it'll get fixed. And in some cases, the issue will eventually be closed.
Respond promptly to any questions made by the Packer team to your issue. Stale issues will be closed.

Issue Lifecycle

The issue is reported.
The issue is verified and categorized by a Packer collaborator. Categorization is done via tags. For example, bugs are marked as "bugs" and easy fixes are marked as "easy".
Unless it is critical, the issue is left for a period of time (sometimes many weeks), giving outside contributors a chance to address the issue.
The issue is addressed in a pull request or commit. The issue will be referenced in the commit message so that the code that fixes it is clearly linked.
The issue is closed.

Setting up Go

If you have never worked with Go before, you will have to install its runtime in order to build packer.

Install go

Setting up Packer for dev

If/when you have go installed you can already go get packer and make in order to compile and test Packer. These instructions target POSIX-like environments (Mac OS X, Linux, Cygwin, etc.) so you may need to adjust them for Windows or other shells. The instructions below are for go 1.7. or later.

Download the Packer source (and its dependencies) by running go get github.com/hashicorp/packer. This will download the Packer source to $GOPATH/src/github.com/hashicorp/packer.
When working on Packer, first cd $GOPATH/src/github.com/hashicorp/packer so you can run make and easily access other files. Run make help to get information about make targets.
Make your changes to the Packer source. You can run make in $GOPATH/src/github.com/hashicorp/packer to run tests and build the Packer binary. Any compilation errors will be shown when the binaries are rebuilding. If you don't have make you can simply run go build -o bin/packer . from the project root.
After running building Packer successfully, use $GOPATH/src/github.com/hashicorp/packer/bin/packer to build a machine and verify your changes work. For instance: $GOPATH/src/github.com/hashicorp/packer/bin/packer build template.json.
If everything works well and the tests pass, run go fmt on your code before submitting a pull-request.

Opening an Pull Request

Thank you for contributing! When you are ready to open a pull-request, you will need to fork Packer, push your changes to your fork, and then open a pull-request.

For example, my github username is cbednarski, so I would do the following:

git checkout -b f-my-feature
# Develop a patch.
git push https://github.com/cbednarski/Packer f-my-feature

From there, open your fork in your browser to open a new pull-request.

Note: Go infers package names from their file paths. This means go build will break if you git clone your fork instead of using go get on the main Packer project.

Pull Request Lifecycle

You are welcome to submit your pull request for commentary or review before it is fully completed. Please prefix the title of your pull request with "[WIP]" to indicate this. It's also a good idea to include specific questions or items you'd like feedback on.
Once you believe your pull request is ready to be merged, you can remove any "[WIP]" prefix from the title and a core team member will review.
One of Packer's core team members will look over your contribution and either provide comments letting you know if there is anything left to do. We do our best to provide feedback in a timely manner, but it may take some time for us to respond.
Once all outstanding comments and checklist items have been addressed, your contribution will be merged! Merged PRs will be included in the next Packer release. The core team takes care of updating the CHANGELOG as they merge.
In rare cases, we might decide that a PR should be closed. We'll make sure to provide clear reasoning when this happens.

Tips for Working on Packer

Working on forks

The easiest way to work on a fork is to set it as a remote of the Packer project. After following the steps in "Setting up Go to work on Packer":

Navigate to $GOPATH/src/github.com/hashicorp/packer
Add the remote by running git remote add <name of remote> <github url of fork>. For example: git remote add mwhooker https://github.com/mwhooker/packer.git.
Checkout a feature branch: git checkout -b new-feature
Make changes
(Optional) Push your changes to the fork: git push -u <name of remote> new-feature

This way you can push to your fork to create a PR, but the code on disk still lives in the spot where the go cli tools are expecting to find it.

Govendor

If you are submitting a change that requires new or updated dependencies, please include them in vendor/vendor.json and in the vendor/ folder. This helps everything get tested properly in CI.

Note that you will need to use govendor to do this. This step is recommended but not required; if you don't use govendor please indicate in your PR which dependencies have changed and to what versions.

Use govendor fetch <project> to add dependencies to the project. See govendor quick start for examples.

Please only apply the minimal vendor changes to get your PR to work. Packer does not attempt to track the latest version for each dependency.

Running Unit Tests

You can run tests for individual packages using commands like this:

make test TEST=./builder/amazon/...

Running Acceptance Tests

Packer has acceptance tests for various builders. These typically require an API key (AWS, GCE), or additional software to be installed on your computer (VirtualBox, VMware).

If you're working on a new builder or builder feature and want verify it is functioning (and also hasn't broken anything else), we recommend running the acceptance tests.

Warning: The acceptance tests create/destroy/modify real resources, which may incur costs for real money. In the presence of a bug, it is possible that resources may be left behind, which can cost money even though you were not using them. We recommend running tests in an account used only for that purpose so it is easy to see if there are any dangling resources, and so production resources are not accidentally destroyed or overwritten during testing.

To run the acceptance tests, invoke make testacc:

make testacc TEST=./builder/amazon/ebs
...

The TEST variable lets you narrow the scope of the acceptance tests to a specific package / folder. The TESTARGS variable is recommended to filter down to a specific resource to test, since testing all of them at once can sometimes take a very long time.

To run only a specific test, use the -run argument:

make testacc TEST=./builder/amazon/ebs TESTARGS="-run TestBuilderAcc_forceDeleteSnapshot"

Acceptance tests typically require other environment variables to be set for things such as API tokens and keys. Each test should error and tell you which credentials are missing, so those are not documented here.

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