btrfs-progs: add support for dry-run options

Some commands could be run in a dry-run mode, i.e. not doing any
write/change actions, only printing the steps and ignoring errors.

There are two possibilities where to put the option:

- as a global one: btrfs --dry-run subvolume delete /path
- local option:    btrfs subvolume delete --dry-run /path

As we have several global options already, let's put it there, dry-run
should not be very common so the slight inconvenience of writing the
option out of order of command arguments should be acceptable.

Issue: #629
Signed-off-by: David Sterba <dsterba@suse.com>

btrfs.c

@@ -44,6 +44,7 @@ static const char * const btrfs_cmd_group_usage[] = {
 	"  -v|--verbose      increase verbosity of the subcommand\n"
 	"  -q|--quiet        print only errors\n"
 	"  --log <level>     set log level (default, info, verbose, debug, quiet)\n"
+	"  --dry-run         if supported, do not do any active/changing actions\n"
 	"\n"
 	"Options for the main command only:\n"
 	"  --help            print condensed help for all subcommands\n"
@@ -119,7 +120,7 @@ parse_command_token(const char *arg, const struct cmd_group *grp)
 	return cmd;
 }
 
-static void check_output_format(const struct cmd_struct *cmd)
+static void check_command_flags(const struct cmd_struct *cmd)
 {
 	if (cmd->next)
 		return;
@@ -129,6 +130,11 @@ static void check_output_format(const struct cmd_struct *cmd)
 			output_format_name(bconf.output_format));
 		exit(1);
 	}
+
+	if (bconf.dry_run && !(cmd->flags & CMD_DRY_RUN)) {
+		error("--dry-run option not supported for %s\n", cmd->token);
+		exit(1);
+	}
 }
 
 static void handle_help_options_next_level(const struct cmd_struct *cmd,
@@ -165,7 +171,7 @@ int handle_command_group(const struct cmd_struct *cmd, int argc,
 	subcmd = parse_command_token(argv[0], cmd->next);
 
 	handle_help_options_next_level(subcmd, argc, argv);
-	check_output_format(subcmd);
+	check_command_flags(subcmd);
 
 	fixup_argv0(argv, subcmd->token);
 	return cmd_execute(subcmd, argc, argv);
@@ -288,6 +294,7 @@ static int handle_global_options(int argc, char **argv)
 		{ "quiet", no_argument, NULL, 'q' },
 		{ "log", required_argument, NULL, OPT_LOG },
 		{ "param", required_argument, NULL, GETOPT_VAL_PARAM },
+		{ "dry-run", no_argument, NULL, GETOPT_VAL_DRY_RUN },
 		{ NULL, 0, NULL, 0}
 	};
 	int shift;
@@ -316,6 +323,9 @@ static int handle_global_options(int argc, char **argv)
 		case GETOPT_VAL_PARAM:
 			bconf_save_param(optarg);
 			break;
+		case GETOPT_VAL_DRY_RUN:
+			bconf_set_dry_run();
+			break;
 		case 'v':
 			bconf_be_verbose();
 			break;

cmds/commands.h

@@ -22,6 +22,7 @@ enum {
 	CMD_ALIAS = (1 << 1),	/* alias of next command in cmd_group */
 	CMD_FORMAT_TEXT = (1 << 2),	/* output as plain text */
 	CMD_FORMAT_JSON = (1 << 3),	/* output in json */
+	CMD_DRY_RUN	= (1 << 4),	/* Accepts global --dry-run option. */
 };
 
 #define CMD_FORMAT_MASK		(CMD_FORMAT_TEXT | CMD_FORMAT_JSON)

common/help.h

@@ -37,6 +37,7 @@ struct cmd_group;
 
 #define GETOPT_VAL_HELP				520
 #define GETOPT_VAL_PARAM			521
+#define GETOPT_VAL_DRY_RUN			522
 
 #define ARGV0_BUF_SIZE	PATH_MAX
 
@@ -83,7 +84,7 @@ struct cmd_group;
 					"Global options:"
 
 #define HELPINFO_INSERT_FORMAT		"--format TYPE"
-
+#define HELPINFO_INSERT_DRY_RUN OPTLINE("--dry-run", "do not do any active/changing actions")
 #define HELPINFO_INSERT_VERBOSE	OPTLINE("-v|--verbose", "increase output verbosity")
 #define HELPINFO_INSERT_QUIET	OPTLINE("-q|--quiet", "print only errors")
 

common/utils.c

@@ -1000,6 +1000,16 @@ void bconf_save_param(const char *str)
 	}
 }
 
+void bconf_set_dry_run(void)
+{
+	pr_verbose(LOG_INFO, "Dry-run requested\n");
+	bconf.dry_run = 1;
+}
+
+bool bconf_is_dry_run(void)
+{
+	return bconf.dry_run == 1;
+}
 
 /* Returns total size of main memory in bytes, -1UL if error. */
 unsigned long total_memory(void)

common/utils.h

@@ -87,6 +87,8 @@ struct btrfs_config {
 	 *   > 0: verbose level
 	 */
 	int verbose;
+	/* Command line request to skip any modification actions. */
+	int dry_run;
 	struct list_head params;
 };
 extern struct btrfs_config bconf;
@@ -102,6 +104,8 @@ void bconf_be_verbose(void);
 void bconf_be_quiet(void);
 void bconf_add_param(const char *key, const char *value);
 void bconf_save_param(const char *str);
+void bconf_set_dry_run(void);
+bool bconf_is_dry_run(void);
 const char *bconf_param_value(const char *key);
 
 /* Pseudo random number generator wrappers */