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Contributing to Bazel

Contributing to Bazel

We welcome contributions! This page covers setting up your machine to develop Bazel and, when you've made a patch, how to submit it.

How can I contribute to Bazel?

In general, we prefer contributions that fix bugs or add features (as opposed to stylistic, refactoring, or "cleanup" changes). Please check with us on the dev list before investing a lot of time in a patch.

Patch Acceptance Process

  1. Read the Bazel governance plan.
  2. Discuss your plan and design, and get agreement on our mailing list.
  3. Prepare a git commit that implements the feature. Don't forget to add tests.
  4. Create a new code review on Gerrit by running: 
    $ git push https://bazel.googlesource.com/bazel HEAD:refs/for/master
    Gerrit upload requires that you:
    • Have signed a Contributor License Agreement.
    • Have an automatically generated "Change Id" line in your commit message. If you haven't used Gerrit before, it will print a bash command to create the git hook and then you will need to run `git commit --amend` to add the line.
    The HTTP password required by Gerrit can be obtained from your Gerrit settings page. See the Gerrit documentation for more information about uploading changes.
  5. Complete a code review with a core contributor. Amend your existing commit and re-push to make changes to your patch.
  6. An engineer at Google applies the patch to our internal version control system.
  7. The patch is exported as a Git commit, at which point the Gerrit code review is closed.

Setting up your coding environment

For now we have support for IntelliJ, and partial support for the Eclipse IDE for Java. We don't have IDE support for other languages in Bazel right now.

Creating an IntelliJ project

To work with IntelliJ, follow the instructions at ij.bazel.io.

Creating an Eclipse project

To work with Eclipse:

  • Install the e4b plugin.
  • Change the path to the Bazel binary in the plugin preferences.
  • Import the Bazel workspace as a Bazel project (File > New > Other > Import Bazel Workspace).
  • Select src > main > java and src > test > java as directories and add //src/main/java/... and //src/test/java/... as targets.

Compiling Bazel

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

  • sh compile.sh bootstraps Bazel from scratch, first compiling it without using Bazel, then rebuilding it again using the just built Bazel and optionally runs tests, too. The resulting binary can be found at output/bazel.
  • bazel build //src:bazel builds the Bazel binary using bazel from your PATH and the resulting binary can be found at bazel-bin/src/bazel. This is the recommended way of rebuilding Bazel once you have bootstrapped it.

In addition to the Bazel binary, you might want to build the various tools Bazel uses. They are located in //src/java_tools/..., //src/objc_tools/... and //src/tools/... and their directories contain README files describing their respective utility.

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

  • Bootstrap test with sh compile.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 runs all bazel tests with bazel test //src/... //third_party/ijar/.... This is what we use at Google to ensure that we don't break Bazel when pushing new commits, too.

Debugging Bazel

Start creating a debug configuration for both C++ and Java in your .bazelrc with the following:

build:debug -c dbg
build:debug --javacopt="-g"
build:debug --copt="-g"
build:debug --strip="never"

Then you can rebuild Bazel with bazel build --config debug //src:bazel and use your favorite debugger to start debugging.

For debugging the C++ client you can just run it from gdb or lldb as you normally would. But if you want to debug the Java code, you must attach to the server using the following:

  • 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, run jdb -attach localhost:5005. From within Eclipse, use the remote Java application launch configuration.
  • Our IntelliJ plugin has built-in debugging support

Bazel's code description

Bazel is organized in several parts:

  • Client code in src/main/cpp provides the command-line interface.
  • Protocol buffers in src/main/protobuf.
  • Server code in src/main/java and src/test/java.
    • Core code which is mostly composed of SkyFrame and some utilities.
    • Rules written in Bazel's extension language Skylark 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. You might want to read about the Challenges of Writing Rules first.
  • Java native interfaces in src/main/native.
  • Various tooling for language support (see the list in the compiling Bazel section).