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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

Design

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

The SDK

At the bottom is the Go SDK. This is the same thing you would get if you go to the main Go website and download a binary distribution.

In the past, this has always been a repository rule bound to @go_sdk, but we are moving away from this as part of an effort to enable the use of multiple SDKs in remote execution. It's best to access this through the toolchain.

The go_download_sdk, go_host_sdk, go_local_sdk, and go_wrap_sdk rules are responsible for downloading / locating and configuring an SDK by adding just enough of a build file to expose the contents to Bazel.

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.

SDKs are declared to Bazel with the go_sdk rule, which gathers information about the SDK into the GoSDK provider, which may be consumed by other rules through the context.

If you haven't declared an SDK before calling go_register_toolchains(), a go_download_sdk rule will be declared automatically. This is good enough in more situations, but if you need a forked version of Go, want to control the version, or use the installed SDK, declare a Go SDK repository before calling go_register_toolchains.

The toolchain

This a wrapper over the SDK that provides extra information about the target platform.

Toolchains are declared with the go_toolchain rule inside an SDK repository. They are registered within the macro that declares the SDK repository. Bazel will select a toolchain based on the target platform (specified with --platforms on the command line).

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 the latest Go SDK that was available when the version of rules_go you're using was released.

Forcing the Go version

You can select the version of the Go SDK to use by specifying it when you call go_register_toolchains but you must use a value that matches a known toolchain.

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.

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.

Writing new Go rules

If you are writing a new rule that wants to use the Go toolchain, you need to do a couple of things. First, you have to declare that you want to consume the toolchain on the rule declaration. The easiest way to do this is to use the go_rule wrapper, which adds in the toolchain and some hidden attributes that it consumes.

load("@io_bazel_rules_go//go:def.bzl", "go_context", "go_rule")

my_rule = go_rule(
    _my_rule_impl,
    attrs = {
        ...
    },
)

And then in the rule body, call go_context to get the context object. This will give you access to the toolchain and the SDK.

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

API

go_register_toolchains

Installs the Go toolchains. If go_version is specified, it sets the SDK version to use (for example, "1.10.3"). By default, the latest SDK will be used.

Name Type Default value
go_version string latest release
This specifies the version of the Go SDK to download. This is only used if no SDK has been declared with the name go_sdk before the call to go_register_toolchains. The default version is the latest version of Go that was released at the time the rules_go release you're using was tagged.
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.

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 download the latest version of Go that rules_go supports. Go versions that rules_go doesn't support may not be specified, since the download SHA-256 sums are not known.
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 = "1.12.5",
)

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.

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.

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 mandatory value
A Bazel label referencing a file in the root directory of the SDK. Used to determine the GOROOT for the SDK.

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_sdk

go_sdk collects information about a Go SDK. The SDK must have a normal GOROOT directory structure.

Normally, go_sdk rules are declared in repositories configured with go_download_sdk, go_host_sdk, go_local_sdk, or go_wrap_sdk. You usually won't need to declare this rule explicitly.

Name Type Default value
name string mandatory value
A unique name for the SDK rule. Typically go_sdk.
goos string mandatory value
The host operating system the SDK was built for. See go/platform/list.bzl for valid values.
goarch string mandatory value
The host architecture the SDK was built for. See go/platform/list.bzl for valid values.
root_file label mandatory value
A file in the SDK root directory. Used to determine GOROOT.
package_list label None
A text file containing a list of packages in the standard library that may be imported. If unspecified, this will be generated from srcs.
libs label_list []
Pre-compiled .a files for the standard library, built for the execution platform.
headers label_list []
.h files from pkg/include that may be included in assembly sources.
srcs label_list []
Source files for packages in the standard library.
tools label_list []
Executable files from pkg/tool, built for the execution platform.
go label mandatory value
The go binary.

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. It can only be used in the implementation of a rule that has the go toolchain attached and the go context data as an attribute. To do this declare the rule using the go_rule wrapper.

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

my_rule = go_rule(
    _my_rule_impl,
    attrs = {
        ...
    },
)
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 skylark 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.
asm

The asm function adds an action that runs go tool asm on a source file to produce an object, and returns the File of that object.

Name Type Default value
go GoContext mandatory value
This must be the same GoContext object you got this function from.
source File mandatory value
A source code artifact to assemble. This must be a .s file that contains code in the platform neutral go assembly language.
hdrs File iterable []
The list of .h files that may be included by the source.
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.
compile

The compile function adds an action that compiles a list of source files into a package archive (.a file).

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.
sources File iterable mandatory value
An iterable of source code artifacts. These must be pure .go files, no assembly or cgo is allowed.
importpath string ""
The import path this package represents. This is passed to the -p flag. When the actual import path is different than the source import path (i.e., when importmap is set in a go_library rule), this should be the actual import path.
archives GoArchive iterable []
An iterable of all directly imported libraries. The action will verify that all directly imported libraries were supplied, not allowing transitive dependencies to satisfy imports. It will not check that all supplied libraries were used though.
out_lib File mandatory value
The archive file that should be produced.
out_export File None
File where extra information about the package may be stored. This is used by nogo to store serialized facts about definitions. In the future, it may be used to store export data (instead of the .a file).
gc_goopts string_list []
Additional flags to pass to the compiler.
testfilter string "off"

Controls how files with a _test suffix are filtered.

  • "off" - files with and without a _test suffix are compiled.
  • "only" - only files with a _test suffix are compiled.
  • "exclude" - only files without a _test suffix are compiled.
asmhdr File None
If provided, the compiler will write an assembly header to this file.
cover

The cover function adds an action that runs go tool cover on a set of source files to produce copies with cover instrumentation.

Returns a covered GoSource with the required source files process for coverage.

Note that this removes most comments, including cgo comments.

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 source object to process. Any source files in the object that have been marked as needing coverage will be processed and substiuted in the returned GoSource.
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.
pack

The pack function adds an action that produces an archive from a base archive and a collection of additional object files.

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.
in_lib File mandatory value
The archive that should be copied and appended to. This must always be an archive in the common ar form (like that produced by the go compiler).
out_lib File mandatory value
The archive that should be produced. This will always be an archive in the common ar form (like that produced by the go compiler).
objects File iterable ()
An iterable of object files to be added to the output archive file.
archives list of File []
Additional archives whose objects will be appended to the output. These can be ar files in either common form or either the bsd or sysv variations.
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 rule being processed. In a normal rule implementation this would be ctx.attr.
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 _testmain_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.

go_rule

go_rule is a wrapper for the rule function. It's used to declare rules that have access to the Go toolchain and SDK.

Example:

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

my_rule = go_rule(
    _my_rule_impl,
    attrs = {
        ...
    },
)
Name Type Default value
implementation function mandatory value
The implementation function for the rule.
attrs dict {}
Attributes the rule accepts. Additional implicit attributes may be added to this list.
toolchains list See below
List of toolchains types that Bazel should select when invoking this rule. "@io_bazel_rules_go//go:toolchain" is automatically added to this list, so that does not need to be specified explicitly.