build: Hook up the Go code with Meson

Meson doesn't support Go [1], so this is implemented by a custom target
that invokes 'go build' to generate the binary.

Unfortunately, when using Go modules, 'go build' insists on being
invoked in the same source directory where the go.mod file lives,
while Meson insists on using a build directory separate from the
corresponding source directory. This is addressed by using a build
script that goes into the source directory and then invokes 'go build'.

Currently, the Go code is only built when a Go implementation is found,
and even then, it's not installed. Non-technical end-users are supposed
to continue using the POSIX shell implementation until the Go version
is blessed as stable.

[1] https://github.com/mesonbuild/meson/issues/123

https://github.com/containers/toolbox/pull/318

meson.build

@@ -5,6 +5,7 @@ project(
   meson_version: '>= 0.40.0',
 )
 
+go = find_program('go', required: false)
 go_md2man = find_program('go-md2man')
 shellcheck = find_program('shellcheck', required: false)
 
@@ -35,3 +36,4 @@ endif
 subdir('data')
 subdir('doc')
 subdir('profile.d')
+subdir('src')

src/go-build-wrapper

@@ -0,0 +1,31 @@
+#!/bin/sh
+#
+# Copyright © 2020 Red Hat, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+
+if [ "$#" -ne 2 ]; then
+    echo "go-build-wrapper: wrong arguments" >&2
+    echo "Usage: go-build-wrapper [SOURCE DIR] [OUTPUT DIR]" >&2
+    exit 1
+fi
+
+if ! cd "$1"; then
+    echo "go-build-wrapper: failed to enter source directory $1"
+    exit 1
+fi
+
+go build -o "$2"
+exit "$?"

src/meson.build

@@ -0,0 +1,21 @@
+go_build_wrapper_file = files('go-build-wrapper')
+go_build_wrapper_program = find_program('go-build-wrapper')
+
+sources = files(
+  'toolbox.go',
+  'cmd/root.go',
+)
+
+if go.found()
+  custom_target(
+    'toolbox-go',
+    build_by_default: true,
+    command: [go_build_wrapper_program, meson.current_source_dir(), meson.current_build_dir()],
+    input: sources,
+    output: 'toolbox',
+  )
+endif
+
+if shellcheck.found()
+  test('shellcheck go-build-wrapper', shellcheck, args: [go_build_wrapper_file])
+endif