Add a "declarative program header" and introspection of options and verbs (#43152)

test/units/TEST-74-AUX-UTILS.introspect-cli.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -eux
+set -o pipefail
+
+# Just a smoke test for the introspection code
+for i in systemd-ac-power systemd-analyze systemd-ask-password systemd-dissect \
+         systemd-id128 systemd-notify homectl resolvectl timedatectl varlinkctl; do
+    command -v "$i" >/dev/null || continue
+
+    $i --introspect-cli | jq -e \
+        '.mediaType == "application/vnd.io.systemd.cli-introspection-0"'
+    $i --introspect-cli | jq -e --arg name "$i" \
+        'any(.commands[]; .names[0] == $name)'
+    $i --introspect-cli | jq -e \
+        'any(.commands[]; [.options[].names[-1]] | contains(["--help", "--version", "--introspect-cli"]))'
+    $i --intro | grep -e --help
+done
+
+# Those are not in $PATH and do not do regular option parsing, so the option must
+# be given verbatim
+for i in /usr/lib/systemd/systemd-backlight /usr/lib/systemd/systemd-veritysetup; do
+    [[ -x "$i" ]] || continue
+
+    $i --introspect-cli | jq -e \
+        '.mediaType == "application/vnd.io.systemd.cli-introspection-0"'
+done
+
+# systemd-id128 defines verbs, check that they are described
+systemd-id128 --introspect-cli | jq -e \
+    '.commands[0].verbs | map(.names[0]) | contains(["new", "machine-id", "show", "help"])'
+
+# resolvectl is a multicall binary, check that both commands are described
+resolvectl --introspect-cli | jq -e \
+    '[.commands[].names[0]] | sort == ["resolvconf", "resolvectl", "systemd-resolve"]'