Gazelle build file generator

Gazelle is a build file generator for Go projects. It can create new BUILD.bazel files for a project that follows "go build" conventions, and it can update existing build files to include new files and options. Gazelle can be invoked directly in a project workspace, or it can be run on an external repository during the build as part of the go_repository rule.

Gazelle is under active development. Its interface and the rules it generates may change. Gazelle is not an official Google product.

Contents

Setup
- Running Gazelle with Bazel
- Running Gazelle with Go
Compatibility
Usage
Fix command transformations

Setup

Running Gazelle with Bazel

To use Gazelle in a new project, add the bazel_gazelle repository and its dependencies to your WORKSPACE file before go_rules_dependencies is called. It should look like this:

http_archive(
    name = "io_bazel_rules_go",
    url = "https://github.com/bazelbuild/rules_go/releases/download/0.10.1/rules_go-0.10.1.tar.gz",
    sha256 = "4b14d8dd31c6dbaf3ff871adcd03f28c3274e42abc855cb8fb4d01233c0154dc",
)
http_archive(
    name = "bazel_gazelle",
    url = "https://github.com/bazelbuild/bazel-gazelle/releases/download/0.10.0/bazel-gazelle-0.10.0.tar.gz",
    sha256 = "6228d9618ab9536892aa69082c063207c91e777e51bd3c5544c9c060cafe1bd8",
)
load("@io_bazel_rules_go//go:def.bzl", "go_rules_dependencies", "go_register_toolchains")
go_rules_dependencies()
go_register_toolchains()
load("@bazel_gazelle//:deps.bzl", "gazelle_dependencies")
gazelle_dependencies()

Add the code below to the BUILD or BUILD.bazel file in the root directory of your repository. Replace the string in prefix with the portion of your import path that corresponds to your repository.

load("@bazel_gazelle//:def.bzl", "gazelle")

gazelle(
    name = "gazelle",
    prefix = "github.com/example/project",
)

After adding this code, you can run Gazelle with Bazel.

bazel run //:gazelle

This will generate new BUILD.bazel files for your project. You can run the same command in the future to update existing BUILD.bazel files to include new source files or options.

Running Gazelle with Go

If you have a Go SDK installed, you can install Gazelle in your GOPATH with the command below:

go get -u github.com/bazelbuild/bazel-gazelle/cmd/gazelle

Make sure to re-run this command to upgrade Gazelle whenever you upgrade rules_go in your repository.

To generate BUILD.bazel files in a new project, run the command below, replacing the prefix with the portion of your import path that corresponds to your repository.

gazelle -go_prefix github.com/my/project

The prefix only needs to be specified the first time you run Gazelle. To update existing BUILD.bazel files, you can just run gazelle without arguments.

Compatibility

Gazelle generates build files that require a minimum version of rules_go to build. Check the table below to ensure that you're using compatible versions.

Gazelle version	Minimum rules_go version	Maximum rules_go version
0.8	0.8.0	n/a
0.9	0.9.0	n/a
0.10.0	0.9.0	n/a

Usage

Command line

gazelle <command> [flags...] [package-dirs...]

The first argument to Gazelle may be one of the commands below. If no command is specified, update is assumed. The remaining arguments are specific to each command and are documented below.

update: Scans sources files, then generates and updates build files.
fix: Same as the update command, but it also fixes deprecated usage of rules.
update-repos: Updates repository rules in the WORKSPACE file.

Bazel rule

Gazelle may be run via a rule. See Running Gazelle with Bazel for setup instructions. This rule builds Gazelle and generates a wrapper script that executes Gazelle with baked-in set of arguments. You can run this script with bazel run, or you can copy it into your workspace and run it directly.

The following attributes are available on the gazelle rule.

Name	Type	Default value
`gazelle`	label	`@bazel_gazelle//cmd/gazelle`
The `go_binary` rule that builds Gazelle. You can substitute a modified version of Gazelle with this.
`external`	string	`external`
The method for resolving unknown imports to Bazel dependencies. May be `external` or `vendored`.
`build_tags`	string_list	`[]`
The last of Go build tags that Gazelle should consider to always be true.
`prefix`	string	mandatory value
The import path that corresponds to the repository root directory. TODO(#26): this should be optional.
`extra_args`	string_list	`[]`
A list of extra command line arguments passed to Gazelle.
`command`	string	`update`
The Gazelle command to use. May be `fix` or `update`. To run a different command, e.g., `update-repos`, you'll need to copy the invoke the generated wrapper script directly with explicit arguments.

`fix` and `update`

The update command is the most common way of running Gazelle. Gazelle will scan sources in directories throughout the repository, then create and update build files.

The fix command does everything update does, but it also fixes deprecated usage of rules, analogous to go fix. For example, cgo_library will be consolidated with go_library. This command may delete or rename rules, so it's not on by default. See Fix command transformations for details.

Both commands accept a list of directories to process as positional arguments. If no directories are specified, Gazelle will process the current directory. Subdirectories will be processed recursively.

The following flags are accepted:

Name	Default value
`-build_file_name file1,file2,...`	`BUILD.bazel,BUILD`
Comma-separated list of file names. Gazelle recognizes these files as Bazel build files. New files will use the first name in this list. Use this if your project contains non-Bazel files named `BUILD` (or `build` on case-insensitive file systems).
`-build_tags tag1,tag2`
List of Go build tags Gazelle will consider to be true. Gazelle applies constraints when generating Go rules. It assumes certain tags are true on certain platforms (for example, `amd64,linux`). It assumes all Go release tags are true (for example, `go1.8`). It considers other tags to be false (for example, `ignore`). This flag overrides that behavior.
`-external external\|vendored`	`external`
Determines how Gazelle resolves import paths. May be `external` or `vendored`. Gazelle translates Go import paths to Bazel labels when resolving library dependencies. Import paths that start with the `go_prefix` are resolved to local labels, but other imports are resolved based on this mode. In `external` mode, paths are resolved using an external dependency in the WORKSPACE file (Gazelle does not create or maintain these dependencies yet). In `vendored` mode, paths are resolved to a library in the vendor directory.
`-go_prefix example.com/repo`
A prefix of import paths for libraries in the repository that corresponds to the repository root. Gazelle infers this from the `go_prefix` rule in the root BUILD.bazel file, if it exists. If not, this option is mandatory. This prefix is used to determine whether an import path refers to a library in the current repository or an external dependency.
`-known_import example.com`
Skips import path resolution for a known domain. May be repeated. When Gazelle resolves an import path to an external dependency, it attempts to discover the remote repository root over HTTP. Gazelle skips this discovery step for a few well-known domains with predictable structure, like golang.org and github.com. This flag specifies additional domains to skip, which is useful in situations where the lookup would fail for some reason.
`-mode fix\|print\|diff`	`fix`
Method for emitting merged build files. In `fix` mode, Gazelle writes generated and merged files to disk. In `print` mode, it prints them to stdout. In `diff` mode, it prints a unified diff.
`-proto default\|legacy\|disable`	`default`
Determines how Gazelle should generate rules for .proto files. See details in Directives below.
`-repo_root dir`
The root directory of the repository. Gazelle normally infers this to be the directory containing the WORKSPACE file. Gazelle will not process packages outside this directory.

`update-repos`

The update-repos command updates repository rules in the WORKSPACE file. Currently, this can only be used to import repositories from a vendoring tool's lock file. More functionality will be added in the future.

The following flags are accepted:

Name	Default value
`-from_file lock-file`
Import repositories from a vendoring tool's lock file as go_repository rules. These rules will be added to the bottom of WORKSPACE or merged with existing rules. The lock file format is inferred from the file's base name. Currently, only Gopkg.lock is supported.
`-repo_root dir`
The root directory of the repository. Gazelle normally infers this to be the directory containing the WORKSPACE file. Gazelle will not process packages outside this directory.

Bazel rule

When Gazelle is run by Bazel, most of the flags above can be encoded in the gazelle rule. For example:

load("@bazel_gazelle//:def.bzl", "gazelle")

gazelle(
    name = "gazelle",
    command = "fix",
    prefix = "github.com/example/project",
    external = "vendored",
    build_tags = [
        "integration",
        "debug",
    ],
    extra_args = [
        "-build_file_name",
        "BUILD,BUILD.bazel",
    ],
)

Directives

Gazelle supports several directives, written as top-level comments in build files. These are of the form # gazelle:key value. Most directories apply in both the current directory and in subdirectories.

# gazelle:build_file_name BUILD.bazel,BUILD: a comma-separated list of file names that Gazelle will recognize as Bazel build files. When Gazelle creates a new file, it will use the first name in this list.
# gazelle:build_tags foo,bar,baz: a comma-separated list of build tags that Gazelle will consider as always true.
# gazelle:exclude path: prevents Gazelle from processing a file or directory. If the path refers to a source file, Gazelle won't include it in any rules. If the path refers to a directory, Gazelle won't recurse into it. The path may refer to something in a subdirectory, for example, a testdata directory somewhere in a vendor tree. This directive may be repeated to exclude multiple paths, one per line.
# gazelle:ignore: prevents Gazelle from updating the build file. Gazelle will still read rules in the build file and may modify build files in subdirectories. Use # gazelle:exclude to ignore a subdirectory.
# gazelle:importmap_prefix path: a prefix for importmap attributes on library rules. The full importmap for a package is determined by concatenating this prefix with the relative path from the directory where the prefix is set to the package directory. This is set automatically in vendor directories.
# gazelle:prefix example.com/repo: a prefix for importpath attributes on library rules. The full importpath for a package is determined by concatenating this prefix with the relative path from the directory where the prefix is set to the package directory. This may also be set with the -go_prefix parameter.
# gazelle:proto <mode>: Tells Gazelle how to generate rules for .proto files. Applies to the current directory and subdirectories. Valid values for mode are:
- default: proto_library, go_proto_library, go_grpc_library, and go_library rules are generated using @io_bazel_rules_go//proto:def.bzl. This is the default mode.
- legacy: filegroup rules are generated for use by @io_bazel_rules_go//proto:go_proto_library.bzl. go_proto_library rules must be written by hand. Gazelle will run in this mode automatically if go_proto_library.bzl is loaded to avoid disrupting existing projects, but this can be overridden with a directive.
- disable: .proto files are ignored. Gazelle will run in this mode automatically if go_proto_library is loaded from any other source, but this can be overridden with a directive.

In addition to directives, Gazelle supports # keep comments that protect parts of build files from being modified. # keep may be written before a rule, before an attribute, or after a string within a list.

Example

Suppose you have a library that includes a generated .go file. Gazelle won't know what imports to resolve, so you may need to add dependencies manually with # keep comments.

load("@io_bazel_rules_go//go:def.bzl", "go_library")
load("@com_github_example_gen//:gen.bzl", "gen_go_file")

gen_go_file(
    name = "magic",
    srcs = ["magic.go.in"],
    outs = ["magic.go"],
)

go_library(
    name = "go_default_library",
    srcs = ["magic.go"],
    visibility = ["//visibility:public"],
    deps = [
        "@com_github_example_gen//:go_default_library",  # keep
    ],
)

Fix command transformations

When Gazelle is invoked with the fix command, in addition to updating source files and dependencies of existing rules, Gazelle will remove deprecated usage of the Go rules, analogous to go fix. The following transformations are performed.

Squash cgo libraries: Gazelle will remove cgo_library rules named cgo_default_library and merge their attributes with a go_library rule in the same package named go_default_library. If no such go_library rule exists, a new one will be created. Other cgo_library rules will not be removed.

Remove legacy protos: Gazelle will remove usage of go_proto_library rules loaded from @io_bazel_rules_go//proto:go_proto_library.bzl and filegroup rules named go_default_library_protos. Newly generated proto rules will take their place. Since filegroup isn't needed anymore and go_proto_library has different attributes and was always written by hand, Gazelle will not attempt to merge anything from these rules with the newly generated rules.

This transformation is only applied in the default proto mode. Since Gazelle will run in legacy proto mode if go_proto_library.bzl is loaded, this transformation is not usually applied. You can set the proto mode explicitly using the directive # gazelle:proto default.

ando-masaki/bazel-gazelle

Gazelle build file generator

Example