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

[Patrick Steinhardt <ps@pks.im>]

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On Fri, Mar 27, 2020 at 02:25:34PM -0700, Junio C Hamano wrote:
> 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.
> 
> > +	{ "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;
> 	}

The reason why I moved those `skip_prefix` calls into each of the
respective commands is that this patch series introduces calls that do
not accept a trailing space at all. Thus we cannot handle the space
generically here, as that would was soon as we introduce the set of new
commands.

Initially I just had a trailing " " in the `command` array as you hint
at, but it felt a bit magical to me and might make readers wonder why
some commands are spelt "update " and why some are spelt "commit",
without a trailing space.

> 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.
> 
> > +		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?

I hope this makes more sense with the above background of commands
without trailing space. It would probably make sense to move this into
the command-matching loop though to be able to discern "option" and a
potentially new "options" command in the future. Instead of dying, we'd
then have to `continue` the loop and check whether any of the remaining
commands matches.

Patrick

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