Contributing to Bazel

How can I contribute to Bazel?

You should first read the Bazel governance plan. Then you can read this page that explains how to set-up and work on the Bazel code and how to submit a patch to Bazel.

Setting-up your coding environment

For now we only have partial support for the Eclipse IDE for Java. We don't have IDE support for others languages in Bazel right now.

Creating an Eclipse project

To work with Eclipse, run from the root of the source tree sh scripts/setup-eclipse.sh and it will create the .project and the .classpath files (if a .project file is present, only the .classpath will get overwritten). You can then import the project in Eclipse.

You might see some errors in Eclipse concerning Truth assertions

Compiling Bazel

To test out bazel, you need to compile it. There is currently two ways of compiling it:

• sh compile.sh build a Bazel binary without Bazel, it should only be used to bootstrap Bazel itself. The resulting binary can be found at output/bazel.
• bazel build //src:bazel builds the Bazel binary using Bazel and the resulting binary can be found at bazel-bin/src/bazel.

In addition to the Bazel binary, you might want to build the various tools Bazel uses:

• For Java support
  • JavaBuilder is the java compiler wrapper used by Bazel and its target can be found at //src/java_tools/buildjar:JavaBuilder_deploy.jar.
  • SingleJar is a tool to assemble a single jar composed of all classes from all jar dependencies (build the *_deploy.jar files), it can be found at //src/java_tools/singlejar:SingleJar_deploy.jar.
  • ijar is a tool to extracts the class interfaces of jars and is a third party software at //third_party/ijar.
• For Objective-C / iOS support
  • actoolzip is a utility that runs OS X's actool and zips up its output for further processing. It is currently compiled and placed into tools/objc/ by compile.sh.
  • ibtoolzip is a utility that runs OS X's ibtool and zips up its output for further processing. It is currently compiled and placed into tools/objc/ by compile.sh.
  • momczip is a utility that runs OS X's momc and zips up its output for further processing. It is currently compiled and placed into tools/objc/ by compile.sh.
  • bundlemerge is a tool that can construct iOS bundles (such as .ipa files or .bndl directories), including plist merging and zip creation. It is currently compiled and placed into tools/objc/ by compile.sh.
  • plmerge is a tool used for merging plists. It is currently compiled and placed into tools/objc/ by compile.sh.
  • xcodegen is a tool that assembles an Xcode project file matching Bazel build targets. It is currently compiled and placed into tools/objc/ by compile.sh.
  • iossim allows us to run iOS applications built by Bazel on Xcode's iOS simulator and is third party software located at //third_party/iossim

When modifying Bazel, you want to make sure that the following still works:

• Bootstrap test with sh bootstrap_test.sh all after having removed the output directory: it rebuilds Bazel with ./compile.sh, Bazel with the compile.sh Bazel and Bazel with the Bazel-built binary. It compares if the constructed Bazel builts are identical and then run all bazel tests with bazel test //src/....
• ijar's tests with bazel test //third_party/ijar/test/...
• Building testing the examples in the base_workspace directory. bazel test --nobuild_tests_only //... in that directory should build and test everything (you might need to set-up extra tools following the instructions in the README files for each example).

Debugging Bazel

Bazel has support for debugging its Java code:

• Make sure you compile it with debugging enabled by adding the misc = ["-g"] attributes to the toolchain rule of the tools/jdk/BUILD file or by using the --javacopt="-g" option on the Bazel command-line.
• Run Bazel with debugging option --host_jvm_debug before the command (e.g., bazel --batch --host_jvm_debug build //src:bazel).
• Attach a debugger to the port 5005. With jdb for instance, just run jdb -attach localhost:5005. From within Eclipse, use the remote Java application launch configuration.

If you want to debug the C++ client, ensure that you have the -g option activated for C++ build, rebuild Bazel and use your favorite C++ debugger.

Bazel's code description

Bazel is organized in several parts:

• A client code in C++ that talk to the server code in java and provide the command-line interface. Its code is in src/main/cpp.
• Protocol buffers in src/main/protobuf.
• The server code in Java (in src/main/java and src/test/java)
  • Core code which is mostly composed of SkyFrame and some utilities.
  • Skylark rules are defined in tools/build_rules. If you want to add rules, consider using Skylark first.
  • Builtin rules in com.google.devtools.build.lib.rules and in com.google.devtools.build.lib.bazel.rules.
• Java native interfaces in src/main/native.
• Various tooling for language support (see the list in the compiling Bazel section.

Patch Acceptance Process

1. Discuss your plan and design, and get agreement on our mailing list.
2. Prepare a patch that implements the feature. Don't forget to add tests.
3. Upload to Gerrit; Gerrit upload requires that you have signed a Contributor License Agreement.
4. Complete a code review with a core contributor.
5. An engineer at Google applies the patch to our internal version control system.
6. The patch is exported as a Git commit, at which point the Gerrit code review is closed.

We do not currently accept pull requests on GitHub.

We will make changes to this process as necessary, and we're hoping to move closer to a fully open development model in the future (also see Is Bazel developed fully in the open?).