ref-manual: initial documentation for go and go-mod classes

This addresses [YOCTO #14582]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/classes.rst

@@ -758,6 +758,29 @@ software from the GNOME stack. This class sets
 mirrors as well as extending :term:`FILES` with the typical
 GNOME installation paths.
 
+.. _ref-classes-go:
+
+``go``
+======
+
+The :ref:`ref-classes-go` class supports building Go programs. The behavior of
+this class is controlled by the mandatory :term:`GO_IMPORT` variable, and
+by the optional :term:`GO_INSTALL` and :term:`GO_INSTALL_FILTEROUT` ones.
+
+To build a Go program with the Yocto Project, you can use the
+:yocto_git:`go-helloworld_0.1.bb </poky/tree/meta/recipes-extended/go-examples/go-helloworld_0.1.bb>`
+recipe as an example.
+
+.. _ref-classes-go-mod:
+
+``go-mod``
+==========
+
+The :ref:`ref-classes-go-mod` class allows to use Go modules, and inherits the
+:ref:`ref-classes-go` class.
+
+See the associated :term:`GO_WORKDIR` variable.
+
 .. _ref-classes-gobject-introspection:
 
 ``gobject-introspection``

documentation/ref-manual/variables.rst

@@ -3012,6 +3012,73 @@ system and gives an overview of their function and contents.
 
          GLIBC_GENERATE_LOCALES = "en_GB.UTF-8 en_US.UTF-8"
 
+   :term:`GO_IMPORT`
+      When inheriting the :ref:`ref-classes-go` class, this mandatory variable
+      sets the import path for the Go package that will be created for the code
+      to build. If you have a ``go.mod`` file in the source directory, this
+      typically matches the path in the ``module`` line in this file.
+
+      Other Go programs importing this package will use this path.
+
+      Here is an example setting from the
+      :yocto_git:`go-helloworld_0.1.bb </poky/tree/meta/recipes-extended/go-examples/go-helloworld_0.1.bb>`
+      recipe::
+
+          GO_IMPORT = "golang.org/x/example"
+
+   :term:`GO_INSTALL`
+      When inheriting the :ref:`ref-classes-go` class, this optional variable
+      specifies which packages in the sources should be compiled and
+      installed in the Go build space by the
+      `go install <https://go.dev/ref/mod#go-install>`__ command.
+
+      Here is an example setting from the
+      :oe_git:`crucible </meta-openembedded/tree/meta-oe/recipes-support/crucible/>`
+      recipe::
+
+         GO_INSTALL = "\
+             ${GO_IMPORT}/cmd/crucible \
+             ${GO_IMPORT}/cmd/habtool \
+         "
+
+      By default, :term:`GO_INSTALL` is defined as::
+
+         GO_INSTALL ?= "${GO_IMPORT}/..."
+
+      The ``...`` wildcard means that it will catch all
+      packages found in the sources.
+
+      See the :term:`GO_INSTALL_FILTEROUT` variable for
+      filtering out unwanted packages from the ones
+      found from the :term:`GO_INSTALL` value.
+
+   :term:`GO_INSTALL_FILTEROUT`
+      When using the Go "vendor" mechanism to bring in dependencies for a Go
+      package, the default :term:`GO_INSTALL` setting, which uses the ``...``
+      wildcard, will include the vendored packages in the build, which produces
+      incorrect results.
+
+      There are also some Go packages that are structured poorly, so that the
+      ``...`` wildcard results in building example or test code that should not
+      be included in the build, or could fail to build.
+
+      This optional variable allows for filtering out a subset of the sources.
+      It defaults to excluding everything under the ``vendor`` subdirectory
+      under package's main directory. This is the normal location for vendored
+      packages, but it can be overridden by a recipe to filter out other
+      subdirectories if needed.
+
+   :term:`GO_WORKDIR`
+      When using Go Modules, the current working directory must be the directory
+      containing the ``go.mod`` file, or one of its subdirectories. When the
+      ``go`` tool is used, it will automatically look for the ``go.mod`` file
+      in the Go working directory or in any parent directory, but not in
+      subdirectories.
+
+      When using the :ref:`ref-classes-go-mod` class to use Go modules,
+      the optional :term:`GO_WORKDIR` variable, defaulting to the value
+      of :term:`GO_IMPORT`, allows to specify a different Go working directory.
+
    :term:`GROUPADD_PARAM`
       When inheriting the :ref:`ref-classes-useradd` class,
       this variable specifies for a package what parameters should be