udevadm: introduce cat command

This introduces 'udevadm cat' command, that shows udev rules files or
udev.conf, which may be useful for debugging.

Closes #35818.

src/udev/meson.build

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
 udevadm_sources = files(
+        'udevadm-cat.c',
         'udevadm-control.c',
         'udevadm-hwdb.c',
         'udevadm-info.c',

src/udev/udevadm-cat.c

@@ -0,0 +1,110 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include <getopt.h>
+
+#include "log.h"
+#include "parse-argument.h"
+#include "pretty-print.h"
+#include "static-destruct.h"
+#include "strv.h"
+#include "udevadm.h"
+#include "udevadm-util.h"
+
+static char *arg_root = NULL;
+static CatFlags arg_cat_flags = 0;
+static bool arg_config = false;
+
+STATIC_DESTRUCTOR_REGISTER(arg_root, freep);
+
+static int help(void) {
+        _cleanup_free_ char *link = NULL;
+        int r;
+
+        r = terminal_urlify_man("udevadm", "8", &link);
+        if (r < 0)
+                return log_oom();
+
+        printf("%s cat [OPTIONS] [FILE...]\n"
+               "\n%sShow udev rules files.%s\n\n"
+               "  -h --help            Show this help\n"
+               "  -V --version         Show package version\n"
+               "     --root=PATH       Operate on an alternate filesystem root\n"
+               "     --tldr            Skip comments and empty lines\n"
+               "     --config          Show udev.conf rather than udev rules files\n"
+               "\nSee the %s for details.\n",
+               program_invocation_short_name,
+               ansi_highlight(),
+               ansi_normal(),
+               link);
+
+        return 0;
+}
+
+static int parse_argv(int argc, char *argv[]) {
+        enum {
+                ARG_ROOT = 0x100,
+                ARG_TLDR,
+                ARG_CONFIG,
+        };
+        static const struct option options[] = {
+                { "help",          no_argument,       NULL, 'h'             },
+                { "version",       no_argument,       NULL, 'V'             },
+                { "root",          required_argument, NULL, ARG_ROOT        },
+                { "tldr",          no_argument,       NULL, ARG_TLDR        },
+                { "config",        no_argument,       NULL, ARG_CONFIG      },
+                {}
+        };
+
+        int r, c;
+
+        assert(argc >= 0);
+        assert(argv);
+
+        while ((c = getopt_long(argc, argv, "hVN:", options, NULL)) >= 0)
+                switch (c) {
+                case 'h':
+                        return help();
+                case 'V':
+                        return print_version();
+                case ARG_ROOT:
+                        r = parse_path_argument(optarg, /* suppress_root= */ true, &arg_root);
+                        if (r < 0)
+                                return r;
+                        break;
+                case ARG_TLDR:
+                        arg_cat_flags = CAT_TLDR;
+                        break;
+                case ARG_CONFIG:
+                        arg_config = true;
+                        break;
+                case '?':
+                        return -EINVAL;
+                default:
+                        assert_not_reached();
+                }
+
+        if (arg_config && optind < argc)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "Combination of --config and FILEs is not supported.");
+
+        return 1;
+}
+
+int cat_main(int argc, char *argv[], void *userdata) {
+        int r;
+
+        r = parse_argv(argc, argv);
+        if (r <= 0)
+                return r;
+
+        if (arg_config)
+                return conf_files_cat(arg_root, "udev/udev.conf", arg_cat_flags);
+
+        _cleanup_strv_free_ char **files = NULL;
+        r = search_rules_files(strv_skip(argv, optind), arg_root, &files);
+        if (r < 0)
+                return r;
+
+        /* udev rules file does not support dropin configs. So, we can safely pass multiple files as dropins. */
+        return cat_files(/* file = */ NULL, /* dropins = */ files, arg_cat_flags);
+}

src/udev/udevadm.c

@@ -26,6 +26,7 @@ static int help(void) {
                 { "test",         "Test an event run"                 },
                 { "test-builtin", "Test a built-in command"           },
                 { "verify",       "Verify udev rules files"           },
+                { "cat",          "Show udev rules files"             },
                 { "wait",         "Wait for device or device symlink" },
                 { "lock",         "Lock a block device"               },
         };
@@ -97,6 +98,7 @@ static int help_main(int argc, char *argv[], void *userdata) {
 
 static int udevadm_main(int argc, char *argv[]) {
         static const Verb verbs[] = {
+                { "cat",          VERB_ANY, VERB_ANY, 0, cat_main     },
                 { "info",         VERB_ANY, VERB_ANY, 0, info_main    },
                 { "trigger",      VERB_ANY, VERB_ANY, 0, trigger_main },
                 { "settle",       VERB_ANY, VERB_ANY, 0, settle_main  },

src/udev/udevadm.h

@@ -5,6 +5,7 @@
 
 #include "macro.h"
 
+int cat_main(int argc, char *argv[], void *userdata);
 int info_main(int argc, char *argv[], void *userdata);
 int trigger_main(int argc, char *argv[], void *userdata);
 int settle_main(int argc, char *argv[], void *userdata);