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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 anyway. 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 simple fixes are marked as "good first issue".
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 (macOS, 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.

Note: See 'Working with forks' for a better way to use git push ....

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 merge, or 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. We may also have questions that we need answered about the code, either because something doesn't make sense to us or because we want to understand your thought process.
If we have requested changes, you can either make those changes or, if you disagree with the suggested changes, we can have a conversation about our reasoning and agree on a path forward. This may be a multi-step process. Our view is that pull requests are a chance to collaborate, and we welcome conversations about how to do things better. It is the contributor's responsibility to address any changes requested. While reviewers are happy to give guidance, it is unsustainable for us to perform the coding work necessary to get a PR into a mergeable state.
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.md as they merge.
In rare cases, we might decide that a PR should be closed without merging. We'll make sure to provide clear reasoning when this happens.

Tips for Working on Packer

Getting Your Pull Requests Merged Faster

It is much easier to review pull requests that are:

Well-documented: Try to explain in the pull request comments what your change does, why you have made the change, and provide instructions for how to produce the new behavior introduced in the pull request. If you can, provide screen captures or terminal output to show what the changes look like. This helps the reviewers understand and test the change.
Small: Try to only make one change per pull request. If you found two bugs and want to fix them both, that's awesome, but it's still best to submit the fixes as separate pull requests. This makes it much easier for reviewers to keep in their heads all of the implications of individual code changes, and that means the PR takes less effort and energy to merge. In general, the smaller the pull request, the sooner reviewers will be able to make time to review it.
Passing Tests: Based on how much time we have, we may not review pull requests which aren't passing our tests. (Look below for advice on how to run unit tests). If you need help figuring out why tests are failing, please feel free to ask, but while we're happy to give guidance it is generally your responsibility to make sure that tests are passing. If your pull request changes an interface or invalidates an assumption that causes a bunch of tests to fail, then you need to fix those tests before we can merge your PR.

If we request changes, try to make those changes in a timely manner. Otherwise, PRs can go stale and be a lot more work for all of us to merge in the future.

Even with everyone making their best effort to be responsive, it can be time-consuming to get a PR merged. It can be frustrating to deal with the back-and-forth as we make sure that we understand the changes fully. Please bear with us, and please know that we appreciate the time and energy you put into the project.

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 the code:

cd $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.

Go modules & go vendor

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

Note that you will need to use go mod to do this. This step is recommended but not required.

Use go get <project> to add dependencies to the project and go mod vendor to make vendored copy of dependencies. See go mod 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 to 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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