Build open source projects

This document provides tips and best practices for building your open source project on CircleCI.

See the Using Credits page for up-to-date information on free credits available for open source projects

While open source can be a liberating practice, take care not to liberate sensitive information.

• If your repository is public, your CircleCI project and its build logs are also public. Pay attention to the information you choose to print.
• Environment variables set in the CircleCI application are hidden from the public, these variables will not be shared in forked PRs unless explicitly enabled.

The following features and settings are especially useful for open source projects.

Many projects require API tokens, SSH keys, or passwords. Private environment variables allow you to safely store secrets, even if your project is public.

For more information, see the Set an environment variable document.

By default, CircleCI builds every commit from every branch. This behavior may be too aggressive for open source projects, which often have significantly more commits than private projects.

To change this setting, go to the Project Settings>Advanced of your project and set the Only build pull requests option to On.

The ability to override the Only build pull requests setting is also supported. Specifically, CircleCI will run validation on all commits from additional, non-default branches that are specified via regular expression (for example, release.\*).

You can override the Only build pull requests setting by utilizing the API and following the steps outlined in this support article.

Enabling Only build pull requests may result in duplicate builds. You can find troubleshooting steps in this support article.

Many open source projects accept PRs from forked repositories. Building these PRs is an effective way to catch bugs before manually reviewing changes.

By default, CircleCI does not build PRs from forked repositories. To change this setting, go to the Project Settings > Advanced of your project and set the Build forked pull requests option to On.

If a user submits a pull request to your repository from a fork, but no pipeline is triggered, then the user most likely is following a project fork on their personal account rather than the project itself of CircleCI, causing the jobs to trigger under the user’s personal account and not the organization account. To resolve this issue, have the user unfollow their fork of the project on CircleCI and instead follow the source project. This will trigger their jobs to run under the organization when they submit pull requests.

Running an unrestricted build in a parent repository can be dangerous. Projects often contain sensitive information, and this information is freely available to anyone who can push code that triggers a build.

By default, CircleCI does not pass secrets to builds from forked PRs for open source projects and hides four types of configuration data:

• Environment variables set through the application.

• Deployment keys and user keys.

• Passphraseless private SSH keys you have added to CircleCI to access arbitrary hosts during a build.

• AWS permissions and configuration files.

Forked PR builds of open source projects that require secrets will not run successfully on CircleCI until you enable this setting.

If you wish to use OpenID Connect in your project, the OIDC tokens will only be generated if you have this setting turned on. This prevents your token data being shared with forked builds unless you require it. If you do choose to use OIDC with open source projects by enabling the Pass secrets to builds from forked pull requests option, you must check the oidc.circleci.com/vcs-origin claims in your policies to avoid forked builds having access to resources outside those that you require. For more information on OIDC, see the Using OpenID Connect tokens in jobs page.

If you are comfortable sharing secrets with anyone who forks your project and opens a PR, you can enable the Pass secrets to builds from forked pull requests option:

1. Navigate to the Project Settings > Advanced page in the CircleCI web app for your project.
2. Set the Pass secrets to builds from forked pull requests option to On.

Caches are isolated based on GitHub Repo for PRs. CircleCI uses the GitHub repository-id of the originator of the fork PR to identify the cache.

• PRs from the same fork repo will share a cache. For example, PRs from the main repo share a cache with the main repo branches (in particular the main branch).
• Two PRs in different fork repos will have different caches. That means that a PR from a fork will not share a cache with the main repo main branch.
• enabling the passing of secrets to build from forked pull requests will enable cache sharing between the original repo and all forked builds.

Currently there is no pre-population of caches because this optimization has not made it to the top of the priority list yet.

Following are a few examples of projects (big and small) that build on CircleCI:

• React - Facebook’s JavaScript based React is built with CircleCI (as well as other CI tools).
• React Native - Build native mobile apps using JavaScript and React.
• Flow - Adds static typing to JavaScript to improve developer productivity and code quality.
• Storybook - Interactive UI component dev & test: React, React Native, Vue, Angular, Ember.
• Electron - Build cross-platform desktop apps with JavaScript, HTML, and CSS.
• Angular - Framework for building browser and desktop web applications.
• Apollo - A community building flexible open source tools for GraphQL.
• PyTorch - Data manipulation and Machine Learning platform.
• Calypso - The next generation webapp powering WordPress.com.
• Fastlane - A build automatically tool for Android and iOS.

Refer to the Examples document for more public and open source project configuration links organized by CircleCI features and by programming language.