eclass: add basic eclass for building Go binaries

This isn't particularly interesting right now but will be more important
once it grows support for cgo and cross-compiling, things which our
standard go build scripts don't handle.

sdk_container/src/third_party/coreos-overlay/eclass/coreos-go.eclass

@@ -0,0 +1,61 @@
+# Copyright 2014 CoreOS, Inc.
+# Distributed under the terms of the GNU General Public License v2
+# $Header: $
+
+# @ECLASS: coreos-go.eclass
+# @BLURB: utility functions for building Go binaries
+
+# @ECLASS-VARIABLE: COREOS_GO_PACKAGE
+# @DESCRIPTION:
+# Name of the Go package unpacked to ${S}, this is required.
+
+[[ ${EAPI} != "5" ]] && die "Only EAPI=5 is supported"
+
+inherit multiprocessing
+
+DEPEND="dev-lang/go"
+
+# @FUNCTION: go_build
+# @USAGE: <package-name> [<binary-name>]
+go_build() {
+	debug-print-function ${FUNCNAME} "$@"
+
+	[[ $# -eq 0 || $# -gt 2 ]] && die "go_install: incorrect # of arguments"
+	local package_name="$1"
+	local binary_name="${package_name##*/}"
+
+	# TODO: handle cgo, cross-compiling, etc etc...
+	CGO_ENABLED=0 go build -x -p "$(makeopts_jobs)" \
+		-o "${GOPATH}/bin/${binary_name}" "${package_name}" \
+		|| die "go build failed"
+}
+
+coreos-go_src_prepare() {
+	debug-print-function ${FUNCNAME} "$@"
+
+	export GOPATH="${WORKDIR}/gopath"
+	export GOBIN="${GOPATH}/bin"
+	mkdir -p "${GOBIN}" || die
+
+	if [[ -z "${COREOS_GO_PACKAGE}" ]]; then
+		die "COREOS_GO_PACKAGE must be defined by the ebuild"
+	fi
+
+	local package_path="${GOPATH}/src/${COREOS_GO_PACKAGE}"
+	mkdir -p "${package_path%/*}" || die
+	ln -sT "${S}" "${package_path}" || die
+}
+
+coreos-go_src_compile() {
+	debug-print-function ${FUNCNAME} "$@"
+
+	go_build "${COREOS_GO_PACKAGE}"
+}
+
+coreos-go_src_install() {
+	debug-print-function ${FUNCNAME} "$@"
+
+	dobin "${GOBIN}"/*
+}
+
+EXPORT_FUNCTIONS src_prepare src_compile src_install