Re: [PATCH 4/9] update-ref: organize commands in an array

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 27, 2020, 21:25 UTC
Message-ID: <xmqqh7y9fq29.fsf@gitster.c.googlers.com>
In-Reply-To: <50ffc263293571f8af71fd1d253ac238c6909229.1585129842.git.ps@pks.im>

Patrick Steinhardt <ps@pks.im> writes:

> +static const struct parse_cmd {
> +	const char *prefix;
> +	const char *(*fn)(struct ref_transaction *, struct strbuf *, const char *);
> +} commands[] = {

Do not call an array the represents a set of THINGs "type things[]"; instead call it "type thing[]", so that the 0th thing can be referred to as thing[0], not things[0].

One exception is when the set as a whole is referred to more often than individual element of an array, in which case "things" (without the [index]) becomes a sensible way to refer to the set.

Show 42 quoted lines

> +	{ "update", parse_cmd_update },
> +	{ "create", parse_cmd_create },
> +	{ "delete", parse_cmd_delete },
> +	{ "verify", parse_cmd_verify },
> +	{ "option", parse_cmd_option },
> +};
> +
>  static void update_refs_stdin(struct ref_transaction *transaction)
>  {
>  	struct strbuf input = STRBUF_INIT;
>  	const char *next;
> +	int i;
>  
>  	if (strbuf_read(&input, 0, 1000) < 0)
>  		die_errno("could not read from stdin");
>  	next = input.buf;
>  	/* Read each line dispatch its command */
>  	while (next < input.buf + input.len) {
> +		const struct parse_cmd *cmd = NULL;
> +
>  		if (*next == line_termination)
>  			die("empty command in input");
>  		else if (isspace(*next))
>  			die("whitespace before command: %s", next);
> -		else if (skip_prefix(next, "update ", &next))
> -			next = parse_cmd_update(transaction, &input, next);
> -		else if (skip_prefix(next, "create ", &next))
> -			next = parse_cmd_create(transaction, &input, next);
> -		else if (skip_prefix(next, "delete ", &next))
> -			next = parse_cmd_delete(transaction, &input, next);
> -		else if (skip_prefix(next, "verify ", &next))
> -			next = parse_cmd_verify(transaction, &input, next);
> -		else if (skip_prefix(next, "option ", &next))
> -			next = parse_cmd_option(&input, next);
> -		else
> +
> +		for (i = 0; i < ARRAY_SIZE(commands); i++) {
> +			if (!skip_prefix(next, commands[i].prefix , &next))
> +				continue;
> +			cmd = &commands[i];
> +			break;
> +		}

The only reason why you had to sprinkle

	if (!skip_prefix(next, " ", &next))
		die(_("%s: missing space after command"), cmd);

all over the place is because the table lacks the trailing SP (which makes sense---after all, you are making a table of commands). In other words, it's not like some command called from this dispatcher would require " " after the command name and some others would not.

So why not avoid touching the parse_cmd_<cmd>() at all (except for the "option" thing that now needs to take the transaction object for uniformity), and then verify the presence of " " here, perhaps like this:

	for (i = 0; i < ARRAY_SIZE(command); i++) {
		const char *eoc;
		if (!skip_prefix(next, commands[i].prefix, &eoc) ||
		    *eoc != ' ')
			continue;
		cmd = &command[i];
                next = eoc;
		break;
	}

Note that you cannot reuse &next here to future-proof the code; otherwise, you wouldn't be able to add a new command, e.g. "options", that sits next to the existing command "option", in the future.

Show 7 quoted lines

> +		if (!cmd)
>  			die("unknown command: %s", next);
>  
> +		if (input.buf[strlen(cmd->prefix)] != line_termination &&
> +		    input.buf[strlen(cmd->prefix)] != '\0' &&
> +		    input.buf[strlen(cmd->prefix)] != ' ')
> +			die("%s: no separator after command", cmd->prefix);

This part of your version does not make sense to me. If the input line began with "update" with some separator that is not a SP, the original would not have matched. But with this code, "update\0" would pass this code and cause cmd->fn() to be called, only to get the input rejected, because next is not pointing to SP. So why do you even need this if statement?

> +
> +		next = cmd->fn(transaction, &input, next);
>  		next++;
>  	}

Thanks.