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# Go with Bzlmod | ||
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This document describes how to use rules_go and Gazelle with Bazel's new external dependency subsystem [Bzlmod](https://bazel.build/external/overview#bzlmod), which is meant to replace `WORKSPACE` files eventually. | ||
Usages of rules_go and Gazelle in `BUILD` files are not affected by this, refer to the existing documentation on rules and configuration options for them. | ||
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## Setup | ||
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Add the following lines to your `MODULE.bazel` file: | ||
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```starlark | ||
bazel_dep(name = "rules_go", version = "0.39.1") | ||
bazel_dep(name = "gazelle", version = "0.29.0") | ||
``` | ||
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The latest versions are always listed on https://registry.bazel.build/. | ||
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If you have WORKSPACE dependencies that reference rules_go and/or Gazelle, you can still use the legacy repository names for the two repositories: | ||
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```starlark | ||
bazel_dep(name = "rules_go", version = "0.39.1", repo_name = "io_bazel_rules_go") | ||
bazel_dep(name = "gazelle", version = "0.29.0", repo_name = "bazel_gazelle") | ||
``` | ||
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## Registering Go SDKs | ||
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rules_go automatically downloads and registers a recent Go SDK, so unless a particular version is required, no manual steps are required. | ||
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To register a particular version of the Go SDK, use the `go_sdk` module extension: | ||
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```starlark | ||
go_sdk = use_extension("@rules_go//go:extensions.bzl", "go_sdk") | ||
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# Download an SDK for the host OS & architecture. | ||
go_sdk.download(version = "1.20.3") | ||
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# Download an SDK for a fixed OS/architecture, e.g. for remote execution. | ||
go_sdk.download( | ||
version = "1.20.3", | ||
goarch = "amd64", | ||
goos = "linux", | ||
) | ||
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# Register the Go SDK installed on the host. | ||
go_sdk.host() | ||
``` | ||
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Multiple Go SDKs can be registered and selected on a per-target basis using [`go_cross_binary`](rules.md#go_cross_binary). | ||
The usual rules of [toolchain resolution](https://bazel.build/extending/toolchains#toolchain-resolution) apply, with SDKs registered in the root module taking precedence over those registered in dependencies. | ||
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### Using a Go SDK | ||
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By default, Go SDK repositories are created with mangled names and are not expected to be referenced directly. | ||
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For build actions, toolchain resolution is used to select the appropriate SDK for a given target. | ||
[`go_cross_binary`](rules.md#go_cross_binary) can be used to influence the outcome of the resolution. | ||
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The `go` tool of the SDK registered for the host is available via the `@rules_go//go` target. | ||
Prefer running it via this target over running `go` directly to ensure that all developers use the same version. | ||
The `@rules_go//go` target can be used in scripts executed via `bazel run`, but cannot be used in build actions. | ||
Note that `go` command arguments starting with `-` require the use of the double dash separator with `bazel run`: | ||
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```sh | ||
bazel run @rules_go//go -- mod tidy -v | ||
``` | ||
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If you really do need direct access to a Go SDK, you can provide the `name` attribute on the `go_sdk.download` or `go_sdk.host` tag and then bring the repository with that name into scope via `use_repo`. | ||
Note that modules using this attribute cannot be added to registries such as the Bazel Central Registry (BCR). | ||
If you have a use case that would require this, please explain it in an issue. | ||
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### Not yet supported | ||
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* `go_local_sdk` | ||
* `go_wrap_sdk` | ||
* nogo | ||
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## Generating BUILD files | ||
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Add the following to your top-level BUILD file, including your Go module's path in a [Gazelle directive](https://github.com/bazelbuild/bazel-gazelle#directives): | ||
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```starlark | ||
load("@gazelle//:defs.bzl", "gazelle") | ||
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# gazelle:prefix github.com/example/project | ||
gazelle(name = "gazelle") | ||
``` | ||
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Then, use `bazel run //:gazelle` to (re-)generate BUILD files. | ||
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## External dependencies | ||
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External Go dependencies are managed by the `go_deps` module extension provided by Gazelle. | ||
`go_deps` performs [Minimal Version Selection](https://go.dev/ref/mod#minimal-version-selection) on all transitive Go dependencies of all Bazel modules, so compared to the old WORKSPACE setup, every Bazel module only needs to declare its own Go dependencies. | ||
For every major version of a Go module, there will only ever be a single version in the entire build, just as in regular Go module builds. | ||
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### Specifying external dependencies | ||
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Even though this is not a strict requirement, for interoperability with Go tooling that isn't Bazel-aware, it is recommended to manage Go dependencies via `go.mod`. | ||
The `go_deps` extension can directly parse this file, so external tooling such as `gazelle update-repos` is no longer needed. | ||
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Register the `go.mod` file with the `go_deps` extension as follows: | ||
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```starlark | ||
go_deps = use_extension("@gazelle//:extensions.bzl", "go_deps") | ||
go_deps.from_file(go_mod = "//:go.mod") | ||
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# All *direct* Go dependencies of the module have to be listed explicitly. | ||
use_repo( | ||
go_deps, | ||
"com_github_gogo_protobuf", | ||
"com_github_golang_mock", | ||
"com_github_golang_protobuf", | ||
"org_golang_x_net", | ||
) | ||
``` | ||
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There is [ongoing work](https://github.com/bazelbuild/bazel/issues/17908) targeted for Bazel 6.2.0 to automate the generation of the `use_repo` statement. | ||
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Alternatively, you can specify a module extension tag to add an individual dependency. | ||
This can for example be useful for dependencies of generated code that `go mod tidy` would remove (there is [ongoing work](https://github.com/bazelbuild/bazel-gazelle/pull/1495) to provide a Bazel-aware version of `tidy`). | ||
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```starlark | ||
go_deps.module( | ||
path = "google.golang.org/grpc", | ||
sum = "h1:fPVVDxY9w++VjTZsYvXWqEf9Rqar/e+9zYfxKK+W+YU=", | ||
version = "v1.50.0", | ||
) | ||
``` | ||
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### Managing `go.mod` | ||
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An initial `go.mod` file can be created via | ||
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```sh | ||
bazel run @rules_go//go mod init github.com/example/project | ||
``` | ||
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A dependency can be added via | ||
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```sh | ||
bazel run @rules_go//go get golang.org/x/[email protected] | ||
``` | ||
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### Overrides | ||
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The root module can override certain aspects of the dependency resolution performed by the `go_deps` extension. | ||
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#### `replace` | ||
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[`replace` directives](https://go.dev/ref/mod#go-mod-file-replace) in `go.mod` can be used to replace particular versions of dependencies with other versions or entirely different modules. | ||
At the moment the only supported form is: | ||
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``` | ||
replace( | ||
golang.org/x/net v1.2.3 => example.com/fork/net v1.4.5 | ||
) | ||
``` | ||
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#### Gazelle directives | ||
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Some external Go modules may require tweaking how Gazelle generates BUILD files for them via [Gazelle directives](https://github.com/bazelbuild/bazel-gazelle#directives). | ||
The `go_deps` extension provides a dedicated `go_deps.gazelle_override` tag for this purpose: | ||
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```starlark | ||
go_deps.gazelle_override( | ||
directives = [ | ||
"gazelle:go_naming_convention go_default_library", | ||
], | ||
path = "github.com/stretchr/testify", | ||
) | ||
``` | ||
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If you need to use a `gazelle_override` to get a public Go module to build with Bazel, consider contributing the directives to the [public registry for Gazelle directives](internal/bzlmod/directives.bzl) via a PR. | ||
This will allow you to drop the `gazelle_override` tag and also makes the Go module usable in non-root Bazel modules. | ||
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### Not yet supported | ||
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* Fetching dependencies from Git repositories or via HTTP | ||
* `go.mod` `replace` directives matching all versions of a module | ||
* `go.mod` `replace` directives referencing local files | ||
* `go.mod` `exclude` directices |