cli: support `help <command>`

Support `help <command>` by re-invoking the command itself with the
`--help` argument.  This allows us to keep the help logic with the
commands itself.

src/cli/README.md

@@ -20,3 +20,7 @@ A git-compatible command-line interface that uses libgit2.
    entrypoint and a brief description that can be printed in `git help`.
    This is done because commands are linked into the main cli.
 
+3. Commands should accept a `--help` option that displays their help
+   information.  This will be shown when a user runs `<command> --help` and
+   when a user runs `help <command>`.
+

src/cli/cmd_help.c

@@ -45,7 +45,7 @@ static int print_commands(void)
 	printf("These are the %s commands available:\n\n", PROGRAM_NAME);
 
 	for (cmd = cli_cmds; cmd->name; cmd++)
-		printf("   %-8s  %s\n", cmd->name, cmd->desc);
+		printf("   %-11s  %s\n", cmd->name, cmd->desc);
 
 	printf("\nSee '%s help <command>' for more information on a specific command.\n", PROGRAM_NAME);
 
@@ -54,6 +54,8 @@ static int print_commands(void)
 
 int cmd_help(int argc, char **argv)
 {
+	char *fake_args[2];
+	const cli_cmd_spec *cmd;
 	cli_opt invalid_opt;
 
 	if (cli_opt_parse(&invalid_opt, opts, argv + 1, argc - 1, CLI_OPT_PARSE_GNU))
@@ -67,11 +69,18 @@ int cmd_help(int argc, char **argv)
 	if (!command)
 		return print_commands();
 
-	/* If the user asks for help with the help command */
-	if (strcmp(command, "help") == 0)
-		return print_help();
+	/*
+	 * If we were asked for help for a command (eg, `help <command>`),
+	 * delegate back to that command's `--help` option.  This lets
+	 * commands own their help.  Emulate the command-line arguments
+	 * that would invoke `<command> --help` and invoke that command.
+	 */
+	fake_args[0] = command;
+	fake_args[1] = "--help";
+
+	if ((cmd = cli_cmd_spec_byname(command)) == NULL)
+		return cli_error("'%s' is not a %s command. See '%s help'.",
+		                command, PROGRAM_NAME, PROGRAM_NAME);
 
-        fprintf(stderr, "%s: '%s' is not a %s command. See '%s help'.\n",
-            PROGRAM_NAME, command, PROGRAM_NAME, PROGRAM_NAME);
-	return CLI_EXIT_ERROR;
+	return cmd->fn(2, fake_args);
 }

src/cli/main.c

@@ -37,6 +37,8 @@ int main(int argc, char **argv)
 	const cli_cmd_spec *cmd;
 	cli_opt_parser optparser;
 	cli_opt opt;
+	char *help_args[3] = { NULL };
+	int help_args_len;
 	int args_len = 0;
 	int ret = 0;
 
@@ -72,10 +74,19 @@ int main(int argc, char **argv)
 		goto done;
 	}
 
-	/* If there was no command, we want to invoke "help" */
+	/*
+	 * If `--help <command>` is specified, delegate to that command's
+	 * `--help` option.  If no command is specified, run the `help`
+	 * command.  Do this by updating the args to emulate that behavior.
+	 */
 	if (!command || show_help) {
-		cli_opt_usage_fprint(stdout, PROGRAM_NAME, NULL, cli_common_opts);
-		goto done;
+		help_args[0] = command ? (char *)command : "help";
+		help_args[1] = command ? "--help" : NULL;
+		help_args_len = command ? 2 : 1;
+
+		command = help_args[0];
+		args = help_args;
+		args_len = help_args_len;
 	}
 
 	if ((cmd = cli_cmd_spec_byname(command)) == NULL) {