build: Consolidate the binary used to generate the shell completions

The Toolbx source code was being repeatedly re-built with 'go run ...'
to generate each of the shell completions.  A following commit will use
CGO to link to libsubid.so, which will sufficiently complicate the build
that a simple 'go run ...' won't be enough.

Hence, it's better to use the same mechanisms in src/go-build-wrapper
that are used to generate the Toolbx binary.

However, the main Toolbx binary only works if /run/host exists [1],
which is only created after 'meson install' and may not exist during
'meson compile'.  This is solved by using a throwaway binary that lacks
the ABI protections necessary to run as a container's entry, but is
enough to generate the shell completions.

Fallout from bafbbe8

[1] Commit 6063eb2
    containers#821

containers#1074

src/go-build-wrapper

@@ -77,4 +77,11 @@ go build \
         -ldflags "-extldflags '-Wl,-dynamic-linker,$dynamic_linker -Wl,-rpath,/run/host$libc_dir_canonical_dirname' -linkmode external -X github.com/containers/toolbox/pkg/version.currentVersion=$4" \
         -o "$2/$3"
 
+# shellcheck disable=SC2086
+go build \
+        $tags \
+        -trimpath \
+        -ldflags "-X github.com/containers/toolbox/pkg/version.currentVersion=$4" \
+        -o "$2/$3-throwaway"
+
 exit "$?"

src/meson.build

@@ -67,13 +67,15 @@ toolbox_go = custom_target(
   output: 'toolbox',
 )
 
+toolbox_go_throwaway = toolbox_go.full_path() + '-throwaway'
+
 if bashcompletionsdir != ''
   custom_target(
     'bash-completion',
     capture: true,
     command: [
       meson_generate_completions_program,
-      meson.current_source_dir(),
+      toolbox_go_throwaway,
       'bash',
     ],
     depends: [toolbox_go],
@@ -89,7 +91,7 @@ if fishcompletionsdir != ''
     capture: true,
     command: [
       meson_generate_completions_program,
-      meson.current_source_dir(),
+      toolbox_go_throwaway,
       'fish',
     ],
     depends: [toolbox_go],
@@ -104,7 +106,7 @@ custom_target(
   capture: true,
   command: [
     meson_generate_completions_program,
-    meson.current_source_dir(),
+    toolbox_go_throwaway,
     'zsh',
   ],
   depends: [toolbox_go],

src/meson_generate_completions.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python3
 #
 # Copyright © 2022 Ondřej Míchal
-# Copyright © 2022 Red Hat Inc.
+# Copyright © 2022 – 2023 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.
@@ -16,19 +16,17 @@
 # limitations under the License.
 #
 
-import os
 import subprocess
 import sys
 
 if len(sys.argv) != 3:
     print('{}: wrong arguments'.format(sys.argv[0]), file=sys.stderr)
-    print('Usage: {} [SOURCE DIR] [COMPLETION TYPE]'.format(sys.argv[0]), file=sys.stderr)
+    print('Usage: {} [toolbox] [COMPLETION TYPE]'.format(sys.argv[0]), file=sys.stderr)
     sys.exit(1)
 
-source_dir = sys.argv[1]
+toolbox = sys.argv[1]
 completion_type = sys.argv[2]
 
-os.chdir(source_dir)
-output = subprocess.run(['go', 'run', '.', 'completion', completion_type], check=True)
+output = subprocess.run([toolbox, 'completion', completion_type], check=True)
 
 sys.exit(0)