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Go toolchains

The Go toolchain is at the heart of the Go rules, and is the mechanism used to customize the behavior of the core Go rules.

Contents

Overview

The Go toolchain consists of three main layers: the SDK, the toolchain, and the context.

The SDK

The Go SDK (more commonly known as the Go distribution) is a directory tree containing sources for the Go toolchain and standard library and pre-compiled binaries for the same. You can download this from by visiting the Go website and downloading a binary distribution.

There are several Bazel rules for obtaining and configuring a Go SDK:

  • go_download_sdk: downloads a toolchain for a specific version of Go for a specific operating system and architecture.
  • go_host_sdk: uses the toolchain installed on the system where Bazel is run. The toolchain's location is specified with the GOROOT or by running go env GOROOT.
  • go_local_sdk: like go_host_sdk, but uses the toolchain in a specific directory on the host system.
  • go_wrap_sdk: configures a toolchain downloaded with another Bazel repository rule.

By default, if none of the above rules are used, the go_register_toolchains function creates a repository named @go_sdk using go_download_sdk, using a recent version of Go for the host operating system and architecture.

SDKs are specific to a host platform (e.g., linux_amd64) and a version of Go. They may target all platforms that Go supports. The Go SDK is naturally cross compiling.

By default, all go_binary, go_test, etc. rules will use the first declared Go SDK. If you would like to build a target using a specific Go SDK version, first ensure that you have declared a Go SDK of that version using one of the above rules (go_download_sdk, go_host_sdk, go_local_sdk, go_wrap_sdk). Then you can specify the sdk version to build with when running a bazel build by passing the flag --@io_bazel_rules_go//go/toolchain:sdk_version="version" where "version" is the SDK version you would like to build with, eg. "1.18.3". The SDK version can omit the patch, or include a prerelease part, eg. "1", "1.18", "1.18.0", and "1.19.0beta1" are all valid values for sdk_version. When go_host_sdk is used, "version" can be set to host to refer to the host Go SDK. It can also be set remote to match any non-host version.

The toolchain

The workspace rules above declare Bazel toolchains with go_toolchain implementations for each target platform that Go supports. Wrappers around the rules register these toolchains automatically. Bazel will select a registered toolchain automatically based on the execution and target platforms, specified with --host_platform and --platforms, respectively.

The workspace rules define the toolchains in a separate repository from the SDK. For example, if the SDK repository is @go_sdk, the toolchains will be defined in @go_sdk_toolchains. The @go_sdk_toolchains repository must be eagerly fetched in order to register the toolchain, but fetching the @go_sdk repository may be delayed until the toolchain is needed to build something. To activate lazily fetching the SDK, you must provide a version attribute to the workspace rule that defines the SDK (go_download_sdk, go_host_sdk, go_local_sdk, go_wrap_sdk, or go_register_toolchains). The value must match the actual version of the SDK; rules_go will validate this when the toolchain is used.

The toolchain itself should be considered opaque. You should only access its contents through the context.

The context

The context is the type you need if you are writing custom rules that need to be compatible with rules_go. It provides information about the SDK, the toolchain, and the standard library. It also provides a convenient way to declare mode-specific files, and to create actions for compiling, linking, and more.

Customizing

Normal usage

This is an example of normal usage for the other examples to be compared against. This will download and use a specific version of Go for the host platform.

# WORKSPACE

load("@io_bazel_rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_toolchains")

go_rules_dependencies()

go_register_toolchains(version = "1.15.5")

Using the installed Go SDK

You can use the Go SDK that's installed on the system where Bazel is running. This may result in faster builds, since there's no need to download an SDK, but builds won't be reproducible across systems with different SDKs installed.

# WORKSPACE

load("@io_bazel_rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_toolchains")

go_rules_dependencies()

go_register_toolchains(version = "host")

Registering a custom SDK

If you download the SDK through another repository rule, you can configure it with go_wrap_sdk. It must still be named go_sdk, but this is a temporary limitation that will be removed in the future.

# WORKSPACE

load("@io_bazel_rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_toolchains", "go_wrap_sdk")

unknown_download_sdk(
    name = "go",
    ...,
)

go_wrap_sdk(
    name = "go_sdk",
    root_file = "@go//:README.md",
)

go_rules_dependencies()

go_register_toolchains()

Writing new Go rules

If you are writing a new Bazel rule that uses the Go toolchain, you need to do several things to ensure you have full access to the toolchain and common dependencies.

  • Declare a dependency on a toolchain of type @io_bazel_rules_go//go:toolchain. Bazel will select an appropriate, registered toolchain automatically.
  • Declare an implicit attribute named _go_context_data that defaults to @io_bazel_rules_go//:go_context_data. This target gathers configuration information and several common dependencies.
  • Use the go_context function to gain access to the context. This is your main interface to the Go toolchain.
load("@io_bazel_rules_go//go:def.bzl", "go_context")

def _my_rule_impl(ctx):
    go = go_context(ctx)
    ...

my_rule = rule(
    implementation = _my_rule_impl,
    attrs = {
        ...
        "_go_context_data": attr.label(
            default = "@io_bazel_rules_go//:go_context_data",
        ),
    },
    toolchains = ["@io_bazel_rules_go//go:toolchain"],
)

Rules and functions

go_register_toolchains

Installs the Go toolchains. If version is specified, it sets the SDK version to use (for example, "1.15.5").

Name Type Default value
version string mandatory value

Specifies the version of Go to download if one has not been declared.

If a toolchain was already declared with go_download_sdk or a similar rule, this parameter may not be set.

Normally this is set to a Go version like "1.15.5". It may also be set to "host", which will cause rules_go to use the Go toolchain installed on the host system (found using GOROOT or PATH).

If version is specified and is not set to "host", the SDK will be fetched only when the build uses a Go toolchain and toolchain resolution results in this SDK being chosen. Otherwise it will be fetched unconditionally.
nogo label None
The nogo attribute refers to a nogo rule that builds a binary used for static analysis. The nogo binary will be used alongside the Go compiler when building packages.
experiments string_list []
Go experiments to enable via GOEXPERIMENT.

go_download_sdk

This downloads a Go SDK for use in toolchains.

Name Type Default value
name string mandatory value
A unique name for this SDK. This should almost always be go_sdk if you want the SDK to be used by toolchains.
goos string None
The operating system the binaries in the SDK are intended to run on. By default, this is detected automatically, but if you're building on an unusual platform, or if you're using remote execution and the execution platform is different than the host, you may need to specify this explictly.
goarch string None
The architecture the binaries in the SDK are intended to run on. By default, this is detected automatically, but if you're building on an unusual platform, or if you're using remote execution and the execution platform is different than the host, you may need to specify this explictly.
version string latest Go version
The version of Go to download, for example 1.12.5. If unspecified, go_download_sdk will list available versions of Go from golang.org, then pick the highest version. If version is specified but sdks is unspecified, go_download_sdk will list available versions on golang.org to determine the correct file name and SHA-256 sum. If version is specified, the SDK will be fetched only when the build uses a Go toolchain and toolchain resolution results in this SDK being chosen. Otherwise it will be fetched unconditionally.
urls string_list [https://dl.google.com/go/{}]

A list of mirror urls to the binary distribution of a Go SDK. These must contain the {} used to substitute the sdk filename being fetched (using .format. It defaults to the official repository "https://dl.google.com/go/{}".

This attribute is seldom used. It is only needed for downloading Go from an alternative location (for example, an internal mirror).
strip_prefix string "go"
A directory prefix to strip from the extracted files. Used with urls.
sdks string_list_dict see description

This consists of a set of mappings from the host platform tuple to a list of filename and sha256 for that file. The filename is combined the urls to produce the final download urls to use.

This option is seldom used. It is only needed for downloading a modified Go distribution (with a different SHA-256 sum) or a version of Go not supported by rules_go (for example, a beta or release candidate).

Example:

load(
    "@io_bazel_rules_go//go:deps.bzl",
    "go_download_sdk",
    "go_register_toolchains",
    "go_rules_dependencies",
)

go_download_sdk(
    name = "go_sdk",
    goos = "linux",
    goarch = "amd64",
    version = "1.18.1",
    sdks = {
        # NOTE: In most cases the whole sdks attribute is not needed.
        # There are 2 "common" reasons you might want it:
        #
        # 1. You need to use an modified GO SDK, or an unsupported version
        #    (for example, a beta or release candidate)
        #
        # 2. You want to avoid the dependency on the index file for the
        #    SHA-256 checksums. In this case, You can get the expected
        #    filenames and checksums from https://go.dev/dl/
        "linux_amd64": ("go1.18.1.linux-amd64.tar.gz", "b3b815f47ababac13810fc6021eb73d65478e0b2db4b09d348eefad9581a2334"),
        "darwin_amd64": ("go1.18.1.darwin-amd64.tar.gz", "3703e9a0db1000f18c0c7b524f3d378aac71219b4715a6a4c5683eb639f41a4d"),
    },
)

go_rules_dependencies()

go_register_toolchains()

go_host_sdk

This detects and configures the host Go SDK for use in toolchains.

If the GOROOT environment variable is set, the SDK in that directory is used. Otherwise, go env GOROOT is used.

Name Type Default value
name string mandatory value
A unique name for this SDK. This should almost always be go_sdk if you want the SDK to be used by toolchains.
version string None
The version of Go installed on the host. If specified, go_host_sdk will create its repository only when the build uses a Go toolchain and toolchain resolution results in this SDK being chosen. Otherwise it will be created unconditionally.
experiments string_list []
Go experiments to enable via GOEXPERIMENT.

go_local_sdk

This prepares a local path to use as the Go SDK in toolchains.

Name Type Default value
name string mandatory value
A unique name for this SDK. This should almost always be go_sdk if you want the SDK to be used by toolchains.
path string ""
The local path to a pre-installed Go SDK. The path must contain the go binary, the tools it invokes and the standard library sources.
version string None
The version of the Go SDK. If specified, go_local_sdk will create its repository only when the build uses a Go toolchain and toolchain resolution results in this SDK being chosen. Otherwise it will be created unconditionally.
experiments string_list []
Go experiments to enable via GOEXPERIMENT.

go_wrap_sdk

This configures an SDK that was downloaded or located with another repository rule.

Name Type Default value
name string mandatory value
A unique name for this SDK. This should almost always be go_sdk if you want the SDK to be used by toolchains.
root_file label None
A Bazel label referencing a file in the root directory of the SDK. Used to determine the GOROOT for the SDK. This attribute and root_files cannot be both provided.
root_files string_dict None
A set of mappings from the host platform to a Bazel label referencing a file in the SDK's root directory. This attribute and root_file cannot be both provided.
version string None
The version of the Go SDK. If specified, go_wrap_sdk will create its repository only when the build uses a Go toolchain and toolchain resolution results in this SDK being chosen. Otherwise it will be created unconditionally.
experiments string_list []
Go experiments to enable via GOEXPERIMENT.

Example:

load(
    "@io_bazel_rules_go//go:deps.bzl",
    "go_register_toolchains",
    "go_rules_dependencies",
    "go_wrap_sdk",
)

go_wrap_sdk(
    name = "go_sdk",
    root_file = "@other_repo//go:README.md",
)

go_rules_dependencies()

go_register_toolchains()

go_toolchain

This declares a toolchain that may be used with toolchain type "@io_bazel_rules_go//go:toolchain".

Normally, go_toolchain rules are declared and registered in repositories configured with go_download_sdk, go_host_sdk, go_local_sdk, or go_wrap_sdk. You usually won't need to declare these explicitly.

Name Type Default value
name string mandatory value
A unique name for the toolchain.
goos string mandatory value
The target operating system. Must be a standard GOOS value.
goarch string mandatory value
The target architecture. Must be a standard GOARCH value.
sdk label mandatory value
The SDK this toolchain is based on. The target must provide GoSDK. This is usually a `go_sdk`_ rule.
link_flags string_list []
Flags passed to the Go external linker.
cgo_link_flags string_list []
Flags passed to the external linker (if it is used).

go_context

This collects the information needed to form and return a GoContext from a rule ctx. It uses the attributes and the toolchains.

def _my_rule_impl(ctx):
    go = go_context(ctx)
    ...
Name Type Default value
ctx ctx mandatory value
The Bazel ctx object for the current rule.

The context object

GoContext is never returned by a rule, instead you build one using go_context(ctx) in the top of any custom starlark rule that wants to interact with the go rules. It provides all the information needed to create go actions, and create or interact with the other go providers.

When you get a GoContext from a context it exposes a number of fields and methods.

All methods take the GoContext as the only positional argument. All other arguments must be passed as keyword arguments. This allows us to re-order and deprecate individual parameters over time.

Fields

Name Type
toolchain ToolchainInfo
The underlying toolchain. This should be considered an opaque type subject to change.
sdk GoSDK
The SDK in use. This may be used to access sources, packages, and tools.
mode Mode
Controls the compilation setup affecting things like enabling profilers and sanitizers. See compilation modes for more information about the allowed values.
root string
Path of the effective GOROOT. If stdlib is set, this is the same as go.stdlib.root_file.dirname. Otherwise, this is the same as go.sdk.root_file.dirname.
go File
The main "go" binary used to run go sdk tools.
stdlib GoStdLib
The standard library and tools to use in this build mode. This may be the pre-compiled standard library that comes with the SDK, or it may be compiled in a different directory for this mode.
actions ctx.actions
The actions structure from the Bazel context, which has all the methods for building new bazel actions.
exe_extension string
The suffix to use for all executables in this build mode. Mostly used when generating the output filenames of binary rules.
shared_extension string
The suffix to use for shared libraries in this build mode. Mostly used when generating output filenames of binary rules.
crosstool list of File
The files you need to add to the inputs of an action in order to use the cc toolchain.
package_list File
A file that contains the package list of the standard library.
env dict of string to string
Environment variables to pass to actions. Includes GOARCH, GOOS, GOROOT, GOROOT_FINAL, CGO_ENABLED, and PATH.
tags list of string
List of build tags used to filter source files.

Methods

archive

This emits actions to compile Go code into an archive. It supports embedding, cgo dependencies, coverage, and assembling and packing .s files.

It returns a GoArchive.

Name Type Default value
go GoContext mandatory value
This must be the same GoContext object you got this function from.
source GoSource mandatory value
The GoSource that should be compiled into an archive.
binary

This emits actions to compile and link Go code into a binary. It supports embedding, cgo dependencies, coverage, and assembling and packing .s files.

It returns a tuple containing GoArchive, the output executable file, and a runfiles object.

Name Type Default value
go GoContext mandatory value
This must be the same GoContext object you got this function from.
name string ""
The base name of the generated binaries. Required if executable is not given.
source GoSource mandatory value
The GoSource that should be compiled and linked.
test_archives list GoArchiveData []
List of archives for libraries under test. See link.
gc_linkopts string_list []
Go link options.
version_file File None
Version file used for link stamping. See link.
info_file File None
Info file used for link stamping. See link.
executable File None
Optional output file to write. If not set, binary will generate an output file name based on name, the target platform, and the link mode.
link

The link function adds an action that runs go tool link on a library.

It does not return anything.

Name Type Default value
go GoContext mandatory value
This must be the same GoContext object you got this function from.
archive GoArchive mandatory value
The library to link.
test_archives GoArchiveData list []
List of archives for libraries under test. These are excluded from linking if transitive dependencies of archive have the same package paths. This is useful for linking external test archives that depend internal test archives.
executable File mandatory value
The binary to produce.
gc_linkopts string_list []
Basic link options, these may be adjusted by the mode.
version_file File None
Version file used for link stamping.
info_file File None
Info file used for link stamping.
args

This creates a new Args object, using the ctx.actions.args method. The object is pre-populated with standard arguments used by all the go toolchain builders.

Name Type Default value
go GoContext mandatory value
This must be the same GoContext object you got this function from.
declare_file

This is the equivalent of ctx.actions.declare_file. It uses the current build mode to make the filename unique between configurations.

Name Type Default value
go GoContext mandatory value
This must be the same GoContext object you got this function from.
path string ""
A path for this file, including the basename of the file.
ext string ""
The extension to use for the file.
name string ""
A name to use for this file. If path is not present, this becomes a prefix to the path. If this is not set, the current rule name is used in it's place.
library_to_source

This is used to build a GoSource object for a given GoLibrary in the current build mode.

Name Type Default value
go GoContext mandatory value
This must be the same GoContext object you got this function from.
attr ctx.attr mandatory value

The attributes of the target being analyzed. For most rules, this should be ctx.attr. Rules can also pass in a struct with the same fields.

library_to_source looks for fields corresponding to the attributes of go_library and go_binary. This includes srcs, deps, embed, and so on. All fields are optional (and may not be defined in the struct argument at all), but if they are present, they must have the same types and allowed values as in go_library and go_binary. For example, srcs must be a list of Targets; gc_goopts must be a list of strings.

As an exception, deps, if present, must be a list containing either Targets or GoArchives.
library GoLibrary mandatory value
The GoLibrary that you want to build a GoSource object for in the current build mode.
coverage_instrumented bool mandatory value
This controls whether cover is enabled for this specific library in this mode. This should generally be the value of ctx.coverage_instrumented()
new_library

This creates a new GoLibrary. You can add extra fields to the go library by providing extra named parameters to this function, they will be visible to the resolver when it is invoked.

Name Type Default value
go GoContext mandatory value
This must be the same GoContext object you got this function from.
resolver function None

This is the function that gets invoked when converting from a GoLibrary to a GoSource. The function's signature must be

def _stdlib_library_to_source(go, attr, source, merge)

attr is the attributes of the rule being processed source is the dictionary of GoSource fields being generated merge is a helper you can call to merge
importable bool mandatory value
This controls whether the GoLibrary is supposed to be importable. This is generally only false for the "main" libraries that are built just before linking.