// -*- mode:doc; -*-
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=== Infrastructure for Go packages

This infrastructure applies to Go packages that use the standard
build system and use bundled dependencies.

[[golang-package-tutorial]]

==== +golang-package+ tutorial

First, let's see how to write a +.mk+ file for a go package,
with an example :

----
01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06:
07: FOO_VERSION = 1.0
08: FOO_SITE = $(call github,bar,foo,$(FOO_VERSION))
09: FOO_LICENSE = BSD-3-Clause
10: FOO_LICENSE_FILES = LICENSE
11:
12: $(eval $(golang-package))
----

On line 7, we declare the version of the package.

On line 8, we declare the upstream location of the package, here
fetched from Github, since a large number of Go packages are hosted on
Github.

On line 9 and 10, we give licensing details about the package.

Finally, on line 12, we invoke the +golang-package+ macro that
generates all the Makefile rules that actually allow the package to be
built.

[[golang-package-reference]]

==== +golang-package+ reference

In their +Config.in+ file, packages using the +golang-package+
infrastructure should depend on +BR2_PACKAGE_HOST_GO_TARGET_ARCH_SUPPORTS+
because Buildroot will automatically add a dependency on +host-go+
to such packages.
If you need CGO support in your package, you must add a dependency on
+BR2_PACKAGE_HOST_GO_TARGET_CGO_LINKING_SUPPORTS+; for host packages,
add a dependency on +BR2_PACKAGE_HOST_GO_HOST_CGO_LINKING_SUPPORTS+.

The main macro of the Go package infrastructure is
+golang-package+. It is similar to the +generic-package+ macro. The
ability to build host packages is also available, with the
+host-golang-package+ macro.
Host packages built by +host-golang-package+ macro should depend on
+BR2_PACKAGE_HOST_GO_HOST_ARCH_SUPPORTS+.

Just like the generic infrastructure, the Go infrastructure works
by defining a number of variables before calling the +golang-package+
macro.

All the package metadata information variables that exist in the
xref:generic-package-reference[generic package infrastructure] also
exist in the Go infrastructure.

Note that it is not necessary to add +host-go+ in the
+FOO_DEPENDENCIES+ variable of a package, since this basic dependency
is automatically added as needed by the Go package infrastructure.

A few additional variables, specific to the Go infrastructure, can
optionally be defined, depending on the package's needs. Many of them
are only useful in very specific cases, typical packages will
therefore only use a few of them, or none.

* The package must specify its Go module name in the +FOO_GOMOD+
  variable. If not specified, it defaults to
  +URL-domain/1st-part-of-URL/2nd-part-of-URL+, e.g +FOO_GOMOD+ will
  take the value +github.com/bar/foo+ for a package that specifies
  +FOO_SITE = $(call github,bar,foo,$(FOO_VERSION))+. The Go package
  infrastructure will automatically generate a minimal +go.mod+ file
  in the package source tree if it doesn't exist.

* +FOO_LDFLAGS+, +FOO_EXTLDFLAGS+, and +FOO_TAGS+ can be used to pass
  respectively the go +LDFLAGS+ (via the `-ldflags` command line flag),
  the external linker flags +EXTLDFLAGS+ (via the `-extldflags`
  command line flag), or the +TAGS+ to the +go+ build command.

* +FOO_BUILD_TARGETS+ can be used to pass the list of targets that
  should be built. If +FOO_BUILD_TARGETS+ is not specified, it
  defaults to +.+. We then have two cases:

** +FOO_BUILD_TARGETS+ is +.+. In this case, we assume only one binary
   will be produced, and that by default we name it after the package
   name. If that is not appropriate, the name of the produced binary
   can be overridden using +FOO_BIN_NAME+.

** +FOO_BUILD_TARGETS+ is not +.+. In this case, we iterate over the
   values to build each target, and for each produced a binary that is
   the non-directory component of the target. For example if
   +FOO_BUILD_TARGETS = cmd/docker cmd/dockerd+ the binaries produced
   are +docker+ and +dockerd+.

* +FOO_INSTALL_BINS+ can be used to pass the list of binaries that
  should be installed in +/usr/bin+ on the target. If
  +FOO_INSTALL_BINS+ is not specified, it defaults to the lower-case
  name of package.

With the Go infrastructure, all the steps required to build and
install the packages are already defined, and they generally work well
for most Go-based packages. However, when required, it is still
possible to customize what is done in any particular step:

* By adding a post-operation hook (after extract, patch, configure,
  build or install). See xref:hooks[] for details.

* By overriding one of the steps. For example, even if the Go
  infrastructure is used, if the package +.mk+ file defines its own
  +FOO_BUILD_CMDS+ variable, it will be used instead of the default Go
  one. However, using this method should be restricted to very
  specific cases. Do not use it in the general case.

A Go package can depend on other Go modules, listed in its +go.mod+
file. Buildroot automatically takes care of downloading such
dependencies as part of the download step of packages that use the
+golang-package+ infrastructure. Such dependencies are then kept
together with the package source code in the tarball cached in
Buildroot's +DL_DIR+, and therefore the hash of the package's tarball
includes such dependencies.

This mechanism ensures that any change in the dependencies will be
detected, and allows the build to be performed completely offline.
