Go rules for Bazel

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Announcements

September 13, 2017: Release 0.5.5 is now available. This is a bug fix release on top of 0.5.4 that removes the sha256 from some of our dependencies, since it changed upstream.
August 28, 2017: Release 0.5.4 is now available! This will be the last stable tag before requiring Bazel 0.5.4 and toolchains support.
August 9, 2017: Release 0.5.3 is now available!

Quick links

Overview

The rules are in the alpha stage of development. They support:

libraries
binaries
tests
vendoring
cgo
cross compilation
auto generating BUILD files via gazelle
protocol buffers

They currently do not support (in order of importance):

bazel-style auto generating BUILD (where the library name is other than go_default_library)
C/C++ interoperation except cgo (swig etc.)
coverage
test sharding

Note: The latest version of these rules (0.5.5) require Bazel ≥ 0.5.2 to work.

The master branch is only guaranteed to work with the latest version of Bazel.

Setup

Create a file at the top of your repository named WORKSPACE, and add one of the following snippets, verbatim. This will let Bazel fetch necessary dependencies from this repository and a few others. If you're using the latest stable release you can use the following contents:

http_archive(
    name = "io_bazel_rules_go",
    url = "https://github.com/bazelbuild/rules_go/releases/download/0.5.5/rules_go-0.5.5.tar.gz",
    sha256 = "ca58b0b856dc95473b93f2228ab117913b82a6617fc0deabd107346e3981522a",
)
load("@io_bazel_rules_go//go:def.bzl", "go_repositories")

go_repositories()

If you're using rules_go at or near the HEAD of master, you can use the following contents (optionally replacing the commit with something newer):

git_repository(
    name = "io_bazel_rules_go",
    remote = "https://github.com/bazelbuild/rules_go.git",
    commit = "d8d73c918ed7b59a5584e0cab4f5274d2f91faab",
)
load("@io_bazel_rules_go//go:def.bzl", "go_rules_dependencies", "go_register_toolchains")

go_rules_dependencies()
go_register_toolchains()

You can add more external dependencies to this file later (see go_repository).

Add a file named BUILD.bazel in the root directory of your project. In general, you need one of these files in every directory with Go code, but you need one in the root directory even if your project doesn't have any Go code there.
If your project can be built with go build, you can generate your build files using Gazelle. If your project isn't compatible with go build or if you prefer not to use Gazelle, you can write build files by hand.

Generating build files

If your project can be built with go build, you can generate and update your build files automatically using gazelle, a tool included in this repository.

Add the code below to the BUILD.bazel file in your repository's root directory. Replace the prefix string with the prefix you chose for your project earlier.
```
load("@io_bazel_rules_go//go:def.bzl", "gazelle")

gazelle(
    name = "gazelle",
    prefix = "github.com/example/project",
)
```
If your project uses vendoring, add external = "vendored", below the prefix line.
After adding the gazelle rule, run the command below:
```
bazel run //:gazelle
```
This will generate a BUILD.bazel file for each Go package in your repository. You can run the same command in the future to update existing build files with new source files, dependencies, and options.

Writing build files by hand

If your project doesn't follow go build conventions or you prefer not to use gazelle, you can write build files by hand.

In each directory that contains Go code, create a file named BUILD.bazel

Add a load statement at the top of the file for the rules you use.

load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library", "go_test")

For each library, add a go_library rule like the one below. Source files are listed in srcs. Other packages you import are listed in deps using Bazel labels that refer to other go_library rules. The library's import path should be specified with importpath.

go_library(
    name = "go_default_library",
    srcs = [
        "foo.go",
        "bar.go",
    ],
    deps = [
        "//tools:go_default_library",
        "@org_golang_x_utils//stuff:go_default_library",
    ],
    importpath = "github.com/example/project/foo",
    visibility = ["//visibility:public"],
)

For each test, add a go_test rule like either of the ones below. You'll need separate go_test rules for internal and external tests.

# Internal test
go_test(
    name = "go_default_test",
    srcs = ["foo_test.go"],
    importpath = "github.com/example/project/foo",
    library = ":go_default_library",
)

# External test
go_test(
    name = "go_default_xtest",
    srcs = ["bar_test.go"],
    deps = [":go_default_library"],
    importpath = "github.com/example/project/foo",
)

For each binary, add a go_binary rule like the one below.

go_binary(
    name = "foo",
    srcs = ["main.go"],
    deps = [":go_default_library"],
    importpath = "github.com/example/project/foo",
)

For instructions on how to depend on external libraries, see _vendoring

FAQ

Can I still use the `go` tool?

Yes, this setup was deliberately chosen to be compatible with go build. Make sure your project appears in GOPATH, and it should work.

Note that go build won't be aware of dependencies listed in WORKSPACE, so these will be downloaded into GOPATH. You may also need to check in generated files.

What's up with the `go_default_library` name?

This was used to keep import paths consistent in libraries that can be built with go build before the importpath attribute was available.

In order to compile and link correctly, the Go rules need to be able to translate Bazel labels to Go import paths. Libraries that don't set the importpath attribute explicitly have an implicit dependency on //:go_prefix, a special rule that specifies an import path prefix. The import path is the prefix concatenated with the Bazel package and target name. For example, if your prefix was github.com/example/project, and your library was //foo/bar:bar, the Go rules would decide the import path was github.com/example/project/foo/bar/bar. The stutter at the end is incompatible with go build, so if the label name is go_default_library, the import path is just the prefix concatenated with the package name. So if your library is //foo/bar:go_default_library, the import path is github.com/example/project/foo/bar.

We are working on deprecating go_prefix and making importpath mandatory (see #721). When this work is complete, the go_default_library name won't be needed. We may decide to stop using this name in the future (see #265).

diegs/rules_go